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The Effects of Endurance, Strength, and Power Training on Muscle Fiber Type Shifting
Abstract
Muscle fibers are generally fractionated into type I, IIA, and IIX fibers. Type I fibers specialize in long duration contractile activities and are found in abundance in elite endurance athletes. Conversely type IIA and IIX fibers facilitate short-duration anaerobic activities and are proportionally higher in elite strength and power athletes. A central area of interest concerns the capacity of training to increase or decrease fiber types to enhance high-performance activities. Although interconversions between type IIA and IIX are well recognized in the literature, there are conflicting studies regarding the capacity of type I and II fibers to interconvert. Therefore, the purpose of this article is to analyze the effects of various forms of exercise on type I and type II interconversions. Possible variables that may increase type II fibers and decrease type I fibers are discussed, and these include high velocity isokinetic contractions; ballistic movements such as bench press throws and sprints. Conversely, a shift from type II to type I fibers may occur under longer duration, higher volume endurance type events. Special care is taken to provide practical applications for both the scientist and the athlete.
... Type I muscle fibers support long-duration contraction, such as endurance sports. While muscle fibers or type II support short-duration contraction activities (Jacob et al., 2012). ...
... Mitochondrial biogenesis increases the production of ATP and myoglobin in muscle (Lesmana, 2019;Liu et al., 2016). This increase in myoglobin levels and type I muscle fibers contributes to muscle specialization in long-term contractile activity and increased muscle endurance to physical stress (Jacob et al., 2012). ...
... Endurance to physical stress can be evaluated through the duration of mice running activity on a treadmill. Longer running duration indicates better ability to withstand physical stress (Hu et al., 2017;Jacob et al., 2012). ...
Introduction: The biological maturity period is the time or tempo of progress toward adulthood or maturity. The hormonal fluctuations play an important role in changing the characteristics of human body tissues during the biological maturity period. However, changes in tissue characteristics during biological maturity period have not been revealed. Skeletal muscle tissue is one of those believed to experience changes. Muscle fibers are thought to experience switching types from type 2 muscle fibers to type 1 muscle fibers. The transition of muscle fibers from type 2 to type 1 requires miR-499 activity from the expression of the Myh7b gene. MiR-499 directly inhibits FNIP-1. However, the biological maturity period of changing FNIP-1 expression has not been confirmed. Objective: This study aimed to analyse the effect of the biological maturity period on FNIP-1 expression and capillary density in the gastrocnemius muscle. Methodology: Thirty-six male mice were divided into mature groups aged eight weeks and immature groups aged four weeks. This study analyzed FNIP-1 and capillary density gastrocnemius muscle of mice using immunohistochemistry. Results: FNIP-1 expression test showed higher in immature mice than mature mice. In comparison, the capillary density test and endurance showed higher expression in mature mice than in immature mice. Conclusion: This study concludes maturation was characterized by a low distribution of FNIP-1 expression in the gastrocnemius muscle and a longer duration ability to run on the treadmill. Unfortunately the capillary density was not a specific mark to determine maturation in mice.
... Our findings substantiate the notion that a training program focused on fast-twitch muscle fibers can enhance the forearm flexion ability among older individuals when performing activities under forearm loading conditions. In accordance with Wilson et al. [41], high-intensity, lowvolume, and/or high-velocity training programs contribute to an increase in the proportion of fast-twitch muscle fibers. Therefore, engaging in brisk-paced dumbbell bicep curls with lightweight resistance may mitigate the mobility decrease induced by alterations in muscle fiber composition [41,42], aiding in preserving independence and autonomy in the daily lives of elderly individuals. ...
... In accordance with Wilson et al. [41], high-intensity, lowvolume, and/or high-velocity training programs contribute to an increase in the proportion of fast-twitch muscle fibers. Therefore, engaging in brisk-paced dumbbell bicep curls with lightweight resistance may mitigate the mobility decrease induced by alterations in muscle fiber composition [41,42], aiding in preserving independence and autonomy in the daily lives of elderly individuals. ...
... Our findings s stantiate the notion that a training program focused on fast-twitch muscle fibers can hance the forearm flexion ability among older individuals when performing activities der forearm loading conditions. In accordance with Wilson et al. [41], high-intensity, l volume, and/or high-velocity training programs contribute to an increase in the pro tion of fast-twitch muscle fibers. Therefore, engaging in brisk-paced dumbbell bicep c with lightweight resistance may mitigate the mobility decrease induced by alteration muscle fiber composition [41,42], aiding in preserving independence and autonomy in daily lives of elderly individuals. ...
Upper limb movement is vital in daily life. A biomechanical simulation of the forearm with consideration of the physiological characteristics of the muscles is instrumental in gaining deeper insights into the upper limb motion mechanisms. In this study, we established a finite element model of the forearm, including the radius, biceps brachii, and tendons. We simulated the motion of the forearm resulting from the contraction of the biceps brachii by using a Hill-type transversely isotropic hyperelastic muscle model. We adjusted the contraction velocity of the biceps brachii muscle in the simulation and found that a slower muscle contraction velocity facilitated forearm flexion. Then, we changed the percentage of fast-twitch fibers, the maximum muscle strength, and the neural excitation values of the biceps brachii muscle to investigate the forearm flexion of elderly individuals. Our results indicated that reduced fast-twitch fiber percentage, maximum muscle strength, and neural excitation contributed to the decline in forearm motion capability in elderly individuals. Additionally, there is a threshold for neural excitation, below which, motion capability sharply declines. Our model aids in understanding the role of the biceps brachii in forearm flexion and identifying the causes of upper limb movement disorders, which is able to provide guidance for enhancing upper limb performance.
... These findings somewhat support the theory proposed by Sandford et al. (34) regarding appropriate training prescription for athletes with different locomotor profiles. Those with a low ASR (i.e., endurance dominant) likely possess a higher proportion of type I muscle fibres (and higher mitochondrial density) (40) and may respond more favorably to endurancebased sessions such as long intervals HIIT (3,24). Players with a higher ASR likely have a higher proportion of fast twitch muscle fibers (i.e., type IIA and IIX), predisposing these players to be more responsive/tolerant of sprint-based training (23,34,40) which potentially explains the moderate relationship between ASR and HR during RST. ...
... Those with a low ASR (i.e., endurance dominant) likely possess a higher proportion of type I muscle fibres (and higher mitochondrial density) (40) and may respond more favorably to endurancebased sessions such as long intervals HIIT (3,24). Players with a higher ASR likely have a higher proportion of fast twitch muscle fibers (i.e., type IIA and IIX), predisposing these players to be more responsive/tolerant of sprint-based training (23,34,40) which potentially explains the moderate relationship between ASR and HR during RST. It is noteworthy that athletes with a higher proportion of fast twitch fibers require greater recovery time after fatiguing exercise (24) and the heightened acute physiological disturbances we observed should be considered when working with "speed-dominant" athletes (25). ...
Aspin, GL, Graham, M, Franklin, J, Hicks, KM, and Taylor, JM. The relationship between the anaerobic speed reserve and acute responses to high-intensity interval training in female soccer players. J Strength Cond Res XX(X): 000–000, 2024—The anaerobic speed reserve (ASR) is a popular method of profiling soccer players, often used to individualize training prescription. This study explored the reliability of ASR profiling, and the relationship between the ASR and acute physiological responses to high-intensity interval training (HIIT). Acute physiological responses to different HIIT types were also compared. Thirteen subelite female soccer players aged 20.2 ± 4.6 years completed 6 exercise sessions. In sessions 1–2, players completed a 40-m sprint to assess maximal sprint speed (MSS) and 1600-m time-trial to estimate maximal aerobic speed (MAS), which were used to calculate ASR and assess test–retest reliability. In sessions 3–6, players completed 4 HIIT sessions (repeated-sprint training, sprint interval training, long intervals, and short intervals HIIT). Intensities for long and short intervals HIIT were individualized according to MAS. Ratings of perceived exertion (RPE), heart rate (HR), and postsession blood lactates were recorded throughout. Relationships between the ASR and acute responses to HIIT, and between HIIT session comparisons in outcome measures were assessed. Anaerobic speed reserve (coefficient of variation ± 95% confidence limits; 3.1 ± 1.5%), MAS (1.8 ± 1.3%), and MSS (0.8 ± 0.6%) indicated acceptable reliability. Moderate correlations between ASR and RPE ( r = 0.33), postsession blood lactate ( r = 0.34), and HR ( r = 0.37) were observed during long intervals HIIT. A strong correlation was observed between ASR and RPE during SIT ( r = 0.50). Sprint interval training elicited higher RPE's and postsession blood lactate's than other HIIT sessions. Anaerobic speed reserve has good reliability and may influence acute physiological responses to HIIT in female soccer players.
... Sports training is widely recognized as a significant influencer of muscle plasticity changes. These alterations depend on many factors, such as the type of exercise, training intensity, and duration [2,3]. Degens et al. [4] noted that variations in physiological adaptations of muscles, such as aerobic and anaerobic power utilization, directly impact jumping performance across various groups, including endurance, power, and team athletes, as well as healthy individuals. ...
... Athletes who are involved in power training programs, whether or not they also participate in jumping sports, routinely perform explosive movements and short bursts of high-intensity effort [34]. This practice stimulates the fast-twitch muscle fibers [3], which are responsible for producing rapid and forceful contractions. This leads in turn to faster propagation of the action potentials through the facilitation of intracellular signaling pathways and changes in the structure of the contractile proteins, metabolic demand, and function [35,36]. ...
This study investigated hamstring activation in the frequency domain and jump-landing performance in a specialized-training athletic population and a healthy control group.
Thirty male athletes engaged in power training, both with and without jumping sports, or endurance training, together with ten healthy participants were recruited. Surface EMG electrodes were attached to the bellies of the lateral hamstring (LH) and medial hamstring (MH). The median EMG frequency was analyzed during takeoff, flight, before ground contact, after ground contact, and landing in countermovement jumps (CMJ) and drop-vertical jumps (DJ). Kinetic outcomes were also investigated.
The power-trained athletes (with and without jumping sports) exhibited a lower median EMG frequency in the MH during takeoff (p = 0.001 for DJ) and in the LH (p = 0.008 for DJ) and MH during landing (p = 0.004 for CMJ and 0.001 for DJ) compared with the endurance-trained or control groups. Furthermore, the power-trained group demonstrated greater jump heights (p = 0.009 for CMJ and p = 0.003 for DJ). All the athletic groups showed a lower landing force (p = 0.022) and loading rate (p = 0.043) in CMJ than the control group.
Training background differences influenced hamstring recruitment during jumping. Power-trained athletes exhibited a lower median EMG frequency and better jumping performance. All the athletes demonstrated a more effective landing strategy than the control group. These findings suggest the potential for enhancing athletic performance and aiding in landing strategy by exploiting different training styles.
... Skeletal muscle consists of various types of fibres with distinct contractile properties that are generally classified into type I (slow-twitch oxidative), IIa (fast-twitch oxidative-glycolytic) and IIx (fast-twitch glycolytic) (Wilson et al., 2012). Gray et al. (2008) further subdivided type II fibres into IIAX25 (1-25% IIX isoform), IIAX50 (26-50% IIX isoform), IIAX75 (51-75% IIX isoform) and IIAX100 (76-100% IIX isoform), whereas Karatzaferi et al. (2001b) classified type II fibres into IIAx and IIXa (which are defined as those containing less than or more than 50% of Myosin Heavy Chain IIx, respectively). ...
... A special component of speed-strength performance is the acute increase in strength, which is most pronounced in muscles with a high proportion of type II fibers and is detected minutes after high-intensity muscle contractions [19,20]. This phenomenon, known as Post-Activation Potentiation (PAP), enhances muscle contractile response following an intense voluntary contraction. ...
Background/objectives: The study aims to investigate potential differences in vertical jump performance between elite basketball and volleyball players before and after a standard training session, in comparison to a control group from the general population. The analysis focuses on the influence of selected gene polymorphisms that may contribute to variations in the assessed performance parameters. Aims: The aim was to investigate the influence of ACE (rs4646994), ACTN3 (rs1815739), PPARA rs4253778, HIF1A (rs11549465), and AMPD1 (rs17602729) genes polymorphisms on the combined effects of post-activation potentiation (PAP), post-activation performance enhancement (PAPE), and general adaptation syndrome (GAS), as reflected in vertical jump performance, in elite basketball and volleyball players compared to a control group from the general population. Methods: The effects of PAP at the beginning of the training load (acute exercise), and the combined influences of PAPE and GAS following the training load were evaluated using parameters measured by the OptoJump Next® system (Microgate, Bolzano, Italy). Results: A statistically significant (h, p < 0.05) negative effect of the CT genotype of the AMPD1 gene on jump height was observed in the group of athletes. The CT genotype of the AMPD1 gene negatively impacted on PAPE and GAS adaptive responses (ΔP, Δh, p < 0.001) also in the control group. A positive effect on the power during the active phase of the vertical jump was identified for the II genotype of the ACE gene and the Pro/Ser genotype of the HIF1A gene, both exclusively in the control group (ΔP, p < 0.05). Conclusion: Our findings demonstrate that different gene polymorphisms exert variable influences on the combined effects of PAPE and GAS, as reflected in vertical jump parameters, depending on the participants’ level of training adaptation.
... Generally, motor units in muscles that require fine manipulation contain fewer fibers, and motor units in muscles that require rough action are relatively large and contain more muscle fibers [5][6][7][8]. When the body is subjected to a large load, the structure of its muscle fibers changes accordingly, thus showing different degrees of functional status [9,10]. By effectively assessing and diagnosing the exercise state of muscles in an objective and scientific way, trainers are able to reasonably adjust the intensity of training and the state of moderate fatigue, which can enable muscles to achieve a better fatigue state, which is conducive to achieving a better effect of exercise training [11][12][13]. ...
Wushu, as a type of exercise nurtured in traditional Chinese culture, not only has profound cultural significance, but also possesses the physiological role of exercise. In order to investigate the effects of wushu exercise on human muscle fiber types and athletic ability, the study was based on the effects of exercise on skeletal muscle, and the cell biological mechanisms of skeletal muscle contraction during wushu exercise were investigated. The experimental subjects were modeled and grouped by designing experiments. The muscle fiber ratio, the ratio of fast and slow muscle fibers of gastrocnemius muscle and the cross-sectional area were collected by polyacrylamide gel electrophoresis, immunofluorescence staining and protein blotting in order to analyze the changes in the muscle fiber types of the subjects before and after the wushu exercise. Then, the effectiveness of martial arts exercise in enhancing athletic ability was investigated by comparing the intergroup and intragroup athletic ability before and after the experiment between the experimental group and the control group. Wushu exercise had an interaction effect on the ratio of fast and slow muscle fibers, the expression of PGC1α4 and PGC1α2/3, the expression of PPARδ, PDK4, and the protein expression of mitochondrial complex, and it did not have an interaction effect on the protein expression of P38MAPK/P38MAPK and P-AMPKα/AMPKα. Before the experiment, the motor ability of the experimental group and the control group was at the same level (P > 0.05), and after the experiment, the motor ability of the experimental group was much higher than that of the control group, and the P value of each dimension was less than 0.05. After the wushu experiment, the motor ability of the experimental group was greatly improved, and the P value of each dimension was less than 0.05, while the control group stayed at the same place, and the P value of each dimension was greater than 0.05. Wushu exercise could effectively improve the subject’s motor ability.
... Se aprecia un aumento de carga desplazada (kg) en las diversas máquinas, manteniendo percepciones RPE bajas a moderadas. Esto puede deberse a un incremento en la fuerza máxima, resultado tanto de la hipertrofia de las fibras musculares tipo IIA como de un mayor reclutamiento de unidades motoras que generan elevada fuerza en periodos cortos, aumentando la frecuencia estimulación y unidades motoras para un trabajo más eficiente (Wilson et al., 2012). ...
Las personas con Trastorno Espectro Autista (TEA) se manifiesta como un conjunto de afecciones heterogéneas del neurodesarrollo, caracterizadas por deterioros en la interacción social y las habilidades comunicativas, junto con la presencia de comportamientos repetitivos y patrones de interés restringidos y estereotipados. Se presento un participante masculino con TEA, con hipercifosis dorsal, donde fue evaluado en su calidad de vida con el cuestionario SF-36, para someterse a un programa de ejercicio terapéutico aeróbico y fuerza y estimulación nerviosa eléctrica transcutánea (TENS) combinado, para mejorar sus habilidades motrices básicas para observar el comportamiento de los canales de aprendizaje en relación a la intensidad y el tratamiento farmacológico. Los resultados muestran que el entrenamiento fue bien tolerado, sin embargo, el tratamiento farmacológico no fue el adecuado para la actividad física, los roles emocionales y sociales mejoraron considerablemente y el dolor disminuyó. La actividad física constante disminuye los comportamientos autolesivos, podría ser debido a una mejor expresión de desarrollo por la mioquina fractalquina/CX3CL1 y PGC1a, mejorando la fosforilación oxidativa. En conclusión, los programas de ejercicio terapéutico aeróbico y fuerza y TENS combinado, mejoran las respuestas y adaptaciones físicas emocionales en personas con TEA.
... MTORC1 plays a major role in mediating hypertrophic response to resistance exercise by upregulating muscle protein synthesis rates, ribosome biogenesis, and satellite cell recruitment (Wackerhage et al. 2019;Jin et al. 2019) (Fig. 1e, f). Endurance exercise elicits different adaptations in skeletal muscle, such as a shift from type II (fast twitch fiber) to type I (slow twitch fiber) muscle fibers (Wilson et al. 2012), mitochondrial adaptations and an increase in skeletal muscle capillary density (MacInnis and Gibala 2017). Importantly, recent research has shed light on the involvement of epigenetic factors in these adaptations. ...
Recovery methods, such as thermal interventions, have been developed to promote optimal recovery and maximize long-term training adaptations. However, the beneficial effects of these recovery strategies remain a source of controversy. This narrative review aims to provide a detailed understanding of how cold and heat interventions impact long-term training adaptations. Emphasis is placed on skeletal muscle adaptations, particularly the involvement of signaling pathways regulating protein turnover, ribosome and mitochondrial biogenesis, as well as the critical role of satellite cells in promoting myofiber regeneration following atrophy. The current literature suggests that cold interventions can blunt molecular adaptations (e.g., protein synthesis and satellite cell activation) and oxi-inflammatory responses after resistance exercise, resulting in diminished exercise-induced hypertrophy and lower gains in isometric strength during training protocols. Conversely, heat interventions appear promising for mitigating skeletal muscle degradation during immobilization and atrophy. Indeed, heat treatments (e.g., passive interventions such as sauna-bathing or diathermy) can enhance protein turnover and improve the maintenance of muscle mass in atrophic conditions, although their effects on uninjured skeletal muscles in both humans and rodents remain controversial. Nonetheless, heat treatment may serve as an important tool for attenuating atrophy and preserving mitochondrial function in immobilized or injured athletes. Finally, the potential interplay between exercise, thermal interventions and epigenetics is discussed. Future studies must be encouraged to clarify how repeated thermal interventions (heat and cold) affect long-term exercise training adaptations and to determine the optimal modalities (i.e., method of application, temperature, duration, relative humidity, and timing).
... By coordinating in vast ensembles, myosin motors drive macroscopic muscle dynamics, enabling a diverse set of mechanical tasks such as locomotion (walking, swimming, flight), internal regulation (stability, balance, fluid transport), and athletics (endurance, high-intensity). Myosin coordination on the organismal-scale is adaptable, either on long time scales due to selection pressures [1,2] or on short time scales due to training regimen [3,4,5]. In order to power cellular and organismal dynamics, myosin motors convert a stored energy source, in the form of adenosine triphosphate (ATP), into useful mechanical work. ...
Myosin motors are fundamental biological actuators, powering diverse mechanical tasks in eukaryotic cells via ATP hydrolysis. Recent work revealed that myosin's velocity-dependent detachment rate can bridge actomyosin dynamics to macroscale Hill muscle predictions. However, the influence of this microscale unbinding, which we characterize by a dimensionless parameter , on macroscale energetic flows-such as power consumption, output and efficiency-remains elusive. Here we develop an analytical model of myosin dynamics that relates unbinding rates to energetics. Our model agrees with published in-vivo muscle data and, furthermore, uncovers a performance-efficiency tradeoff governed by . To experimentally validate the tradeoff, we build HillBot, a robophysical model of Hill's muscle that mimics nonlinearity. Through HillBot, we decouple 's concurrent effect on performance and efficiency, demonstrating that nonlinearity drives efficiency. We compile 136 published measurements of in muscle and myoblasts to reveal a distribution centered at . Synthesizing data from our model and HillBot, we quantitatively show that corresponds to a class of generalist actuators that are both relatively powerful and efficient, suggesting that the performance-efficiency tradeoff underpins the prevalence of in nature. We leverage these insights and propose a nonlinear variable-impedance protocol to shift along a performance-efficiency axis in robotic applications.
... Muscular endurance is also influenced by the types of muscle fiber present [60]. These fibers are classified based on their contractile properties, metabolic characteristics, and resistance to fatigue [61,62]. ...
Muscular strength and endurance are vital for physical fitness. While mistletoe extract has shown efficacy in significantly increasing muscle strength and endurance, its accessibility is limited. This study explores combining mistletoe and apple peel extracts as an effective muscle health supplement. Analyses of histology, RNA, and protein in the combined extract-treated mouse group demonstrated significant enhancements in muscle strength and endurance, evidenced by larger muscle fibers, improved mitochondrial function, and a higher ratio of type I and IIa muscle fibers. Combining half doses of each extract resulted in greater improvements than using each extract separately, indicating a synergistic effect. Pathway analysis suggests that the observed synergy arises from complementary mechanisms, with a mistletoe extract-induced decrease in myostatin (MSTN) and an apple peel extract-induced increase in IGF1, leading to a sharp rise in AKT, S6K, and MuRF1, which promote myogenesis, along with a significant increase in PGC-1α, TFAM, and MEF2C, which are critical for mitochondrial biogenesis. This research provides practical insights into developing cost-effective, natural supplements to enhance muscle performance and endurance, with potential applications in athletic performance, improving muscle growth and endurance in children, and addressing age-related muscle decline.
... For example, increased mitochondrial biogenesis and oxidative capacity in skeletal muscle are readily observed during adaptation to regular endurance exercise (Hawley et al., 2014;Holloszy & Coyle, 1984;Schiaffino & Reggiani, 2011;Smith et al., 2023). Similarly, a shift in the contractile apparatus from fastto-slow is also observed in many animal species (Bassel-Duby & Olson, 2006;Schiaffino & Reggiani, 2011), but whether a similar contractility transition occurs in humans is still controversial (Hawley et al., 2014;Plotkin et al., 2021;Wilson et al., 2012). Conversely, inactivity, metabolic disorders, and muscular dystrophy often reduce oxidative capacity while also shifting the muscle contractile apparatus from slow-to-fast (Kuznetsov et al., 1998;Lexell, 1995;Schiaffino & Reggiani, 2011). ...
During skeletal muscle adaptation to physiological or pathophysiological signals, contractile apparatus and mitochondrial function are coordinated to alter muscle fiber type. Although recent studies have identified various factors involved in modifying contractile proteins and mitochondrial function, the molecular mechanisms coordinating contractile and metabolic functions during muscle fiber transition are not fully understood. Using a gene‐deficient mouse approach, our previous studies uncovered that vestigial‐like family member 2 (Vgll2), a skeletal muscle‐specific transcription cofactor activated by exercise, is essential for fast‐to‐slow adaptation of skeletal muscle. The current study provides evidence that Vgll2 plays a role in increasing muscle mitochondrial mass and oxidative capacity. Transgenic Vgll2 overexpression in mice altered muscle fiber composition toward the slow type and enhanced exercise endurance, which contradicted the outcomes observed with Vgll2 deficiency. Vgll2 expression was positively correlated with the expression of genes related to mitochondrial function in skeletal muscle, mitochondrial DNA content, and protein abundance of oxidative phosphorylation complexes. Additionally, Vgll2 overexpression significantly increased the maximal respiration of isolated muscle fibers and enhanced the suppressive effects of endurance training on weight gain. Notably, no additional alteration in expression of myosin heavy chain genes was observed after exercise, suggesting that Vgll2 plays a direct role in regulating mitochondrial function, independent of its effect on contractile components. The observed increase in exercise endurance and metabolic efficiency may be attributed to the acute upregulation of genes promoting fatty acid utilization as a direct consequence of Vgll2 activation facilitated by endurance exercise. Thus, the current study establishes that Vgll2 is an integrative regulator of mitochondrial function and contractility in skeletal muscle.
... Improvement in muscle function by exercise training is induced by an increase in the proportion of slow-twitch muscle fibers [26]. Moreover, endurance exercise is known to increase respiratory capacity and the ratio of slow-twitch muscle fibers [27,28]. Myricetin enhances anti-fatigue ability by switching fast-to slow-fiber type [29]. ...
6′-Sialyllactose (6′-SL), found in human breast milk, exhibits anti-inflammatory, immune function-enhancing, brain development-promoting, and gut health-improving effects. However, its effects on muscle fatigue remain unknown. Here, we aimed to investigate the effects of 6′-SL on blood lactate level, muscle fiber type, and oxidative phosphorylation protein complexes (OXPHOS) in muscle after exercise using C57BL/6J male mice. C57BL/6J mice were randomly assigned to control or 100 mg/kg 6′-SL. After 12 weeks of 6′-SL administration, the mice were made to perform treadmill exercise; their blood lactate and glucose levels were measured at the basal level (rest) and 0, 5, and 10 min after treadmill exercise. Results showed that 6′-SL treatment in C57BL/6J mice significantly reduced blood lactate level and improved blood glucose level. Moreover, 6′-SL increased the expression of slow-myosin heavy chain (MHC) and OXPHOS in gastrocnemius muscle. In addition, 6′-SL treatment for 12 weeks did not affect food intake, serum biomarkers of tissue injury, and lipid profiles compared with those of the controls. These findings indicate that non-toxic 6′-SL suppressed muscle fatigue during exercise by promoting protein expression of muscle fibers, especially slow-twitch muscle fibers characterized by abundant OXPHOS complexes and decreased blood lactate level. This study suggests that 6′-SL holds promise as a nutritional supplement in exercise and clinical settings, subject to further validation.
... In the current muscle impedance training, fast muscle fibers tend to be activated and developed through high-intensity and explosive exercises such as weightlifting and jumping, while slow muscle fibers are targeted for increased fiber thickness with lightload or low-intensity repetitive movements, such as walking and jogging [46]. For the elderly, the exercise of fast-twitch muscles is risky and impractical, so they may maintain and improve slow-twitch muscle fibers through low-intensity activities. ...
The six-minute walking test (6MWT) is an essential test for evaluating exercise tolerance in many respiratory and cardiovascular diseases. Frailty and sarcopenia can cause rapid aging of the cardiovascular system in elderly people. Early detection and evaluation of frailty and sarcopenia are crucial for determining the treatment method. We aimed to develop a wearable measuring system for the 6MWT and propose a method for identifying frailty and quantifying walking muscle strength (WMS). In this study, 60 elderly participants were asked to wear accelerometers behind their left and right ankles during the 6MWT. The gait data were collected by a computer or smartphone. We proposed a method for analyzing walking performance using the stride length (SL) and step cadence (SC) instead of gait speed directly. Four regions (Range I–IV) were divided by cutoff values of SC = 2.0 [step/s] and SL = 0.6 [m/step] for a quick view of the frail state. There were 62.5% of frail individuals distributed in Range III and 72.4% of non-frail individuals in Range I. A concept of a WMS score was proposed for estimating WMS quantitatively. We found that 62.5% of frail individuals were scored as WMS1 and 41.4% of the non-frail elderly as WMS4. The average walking distances corresponding to WMS1–4 were 207 m, 370 m, 432 m, and 462 m, respectively. The WMS score may be a useful tool for quantitatively estimating sarcopenia or frailty due to reduced cardiopulmonary function.
... Muscle fiber type can change from fast-to slowtwitch upon prolonged DEX exposure [36]. This change attenuates muscle strength and increases sensitivity to fatigue [37]. These findings suggest that KL-Biome administration effectively suppresses DEX-induced muscle function impairment. ...
Sarcopenia refers to an age-related decrease in muscle mass and strength. The gut–muscle axis has been proposed as a promising target to alleviate muscle atrophy. The effect of KL-Biome—a postbiotic preparation comprising heat-killed Lactiplantibacillus plantarum KM-2, its metabolites, and an excipient (soybean powder)—on muscle atrophy was evaluated using dexamethasone (DEX)-induced atrophic C2C12 myoblasts and C57BL/6J mice. KL-Biome significantly downregulated the expression of genes (Atrogin-1 and MuRF1) associated with skeletal muscle degradation but increased the anabolic phosphorylation of FoxO3a, Akt, and mTOR in C2C12 cells. Oral administration of KL-Biome (900 mg/kg) for 8 weeks significantly improved muscle mass, muscle function, and serum lactate dehydrogenase levels in DEX-treated mice. KL-Biome administration increased gut microbiome diversity and reversed DEX-mediated gut microbiota alterations. Furthermore, it significantly increased the relative abundances of the genera Subdologranulum, Alistipes, and Faecalibacterium prausnitzii, which are substantially involved in short-chain fatty acid production. These findings suggest that KL-Biome exerts beneficial effects on muscle atrophy by regulating gut microbiota.
... Previous research has shown that l-carnitine supplementation promotes the transition of muscle fibers from type II to type I in obese rats by activating genes encoding molecular regulators such as PGC-1α and PGC-1β 47 . Type I muscle fibers specialize in higher endurance and long-duration contraction 48 , indicating that an increase in the quantity of type I fibers may enhance fish muscle endurance. Additionally, laminopathy is characterized by cytoplasmic aggregation of nuclear envelope proteins, decreased autophagy, and downregulated expression of AMPKα. ...
Lamin A/C gene (LMNA) mutations contribute to severe striated muscle laminopathies, affecting cardiac and skeletal muscles, with limited treatment options. In this study, we delve into the investigations of five distinct LMNA mutations, including three novel variants and two pathogenic variants identified in patients with muscular laminopathy. Our approach employs zebrafish models to comprehensively study these variants. Transgenic zebrafish expressing wild-type LMNA and each mutation undergo extensive morphological profiling, swimming behavior assessments, muscle endurance evaluations, heartbeat measurement, and histopathological analysis of skeletal muscles. Additionally, these models serve as platform for focused drug screening. We explore the transcriptomic landscape through qPCR and RNAseq to unveil altered gene expression profiles in muscle tissues. Larvae of LMNA(L35P), LMNA(E358K), and LMNA(R453W) transgenic fish exhibit reduced swim speed compared to LMNA(WT) measured by DanioVision. All LMNA transgenic adult fish exhibit reduced swim speed compared to LMNA(WT) in T-maze. Moreover, all LMNA transgenic adult fish, except LMNA(E358K), display weaker muscle endurance than LMNA(WT) measured by swimming tunnel. Histochemical staining reveals decreased fiber size in all LMNA mutations transgenic fish, excluding LMNA(WT) fish. Interestingly, LMNA(A539V) and LMNA(E358K) exhibited elevated heartbeats. We recognize potential limitations with transgene overexpression and conducted association calculations to explore its effects on zebrafish phenotypes. Our results suggest lamin A/C overexpression may not directly impact mutant phenotypes, such as impaired swim speed, increased heart rates, or decreased muscle fiber diameter. Utilizing LMNA zebrafish models for drug screening, we identify l-carnitine treatment rescuing muscle endurance in LMNA(L35P) and creatine treatment reversing muscle endurance in LMNA(R453W) zebrafish models. Creatine activates AMPK and mTOR pathways, improving muscle endurance and swim speed in LMNA(R453W) fish. Transcriptomic profiling reveals upstream regulators and affected genes contributing to motor dysfunction, cardiac anomalies, and ion flux dysregulation in LMNA mutant transgenic fish. These findings faithfully mimic clinical manifestations of muscular laminopathies, including dysmorphism, early mortality, decreased fiber size, and muscle dysfunction in zebrafish. Furthermore, our drug screening results suggest l-carnitine and creatine treatments as potential rescuers of muscle endurance in LMNA(L35P) and LMNA(R453W) zebrafish models. Our study offers valuable insights into the future development of potential treatments for LMNA-related muscular laminopathy.
... Therefore, participants with moderate-to-high muscle strength and muscle endurance in our study were presumed to possess a high percentage of type I and type IIA muscle fibers. Endurance training alone or in combination with strength training increases the percentage of type I and type IIA muscle fibers [35][36][37] . In our study, many participants with moderate-to-high muscle strength and endurance engaged in various forms of active exercise, such as strength training, endurance training, soccer, ice hockey, and high-intensity cycling. ...
Peak oxygen uptake (VO2), evaluated as exercise tolerance, is a strong predictor of life prognosis regardless of health condition. Several previous studies have reported that peak VO2 is higher in those with a greater decrease in muscle oxygen saturation (SmO2) in the active muscles during incremental exercise. However, the skeletal muscle characteristics of individuals exhibiting a greater decrease in SmO2 during active muscle engagement in incremental exercise remain unclear. This study aimed to clarify the relationship among muscle strength, muscle endurance, and skeletal muscle oxygenation dynamics in active leg muscles during incremental exercise. Twenty-four healthy young men were included and categorized into the non-moderate-to-high muscular strength and endurance group (those with low leg muscle strength, endurance, or both; n = 11) and the moderate-to-high muscular strength and endurance group (those with both moderate-to-high leg muscle strength and endurance; n = 13). All participants underwent cardiopulmonary exercise testing combined with near-infrared spectroscopy to assess whole-body peak VO2 and the change in SmO2 at the lateral vastus lateralis from rest to each exercise stage as skeletal muscle oxygenation dynamics. A linear mixed-effects model, with the change in SmO2 from rest to each stage as the dependent variable, individual participants as random effects, and group and exercise load as fixed effects, revealed significant main effects for both group (P = 0.001) and exercise load (P < 0.001) as well as a significant interaction between the two factors (P < 0.001). Furthermore, multiple-comparison test results showed that the change in SmO2 from rest to 40%–100% peak VO2 was significantly higher in the moderate-to-high muscular strength and endurance group than in the non-moderate-to-high muscular strength and endurance group. Maintaining both muscle strength and endurance at moderate or higher levels contributes to high skeletal muscle oxygenation dynamics (i.e., greater decrease in SmO2) during moderate- or high-intensity exercise.
... In skeletal muscles, nuclear factors, such as the peroxisome proliferator-activated receptor (PPAR) and their co-regulators, sirtuin (SIRT) and adenosine monophosphate activated protein kinase (AMPK), play important roles in sensing energy homeostasis, coordinating metabolic flux and upregulation of genes involved in fatty acids and glucose uptake and oxidation [91]. ET induces PPARβ/δ overexpression in skeletal muscle resulting in muscular hyperplasia (type I, slow-twitch fibers), angiogenic response and a shift from type II fast-twitch fibers I towards oxidative type I muscle fibers [92]. In contrast, resistive exercise promotes muscle hypertrophy (type II, fast-twitch fibers) through the enhanced expression of Insulin Growth Factor (IGF-1) and may prevent muscle atrophy via an Aktand Foxo-1-dependent signaling pathway coupled with downregulation of MuRF-1 and pro-inflammatory mediators, such as TNF-α, IL-1β and IL-6 ( Figure 3) [93,94]. ...
Over the last two decades, the invasiveness of thoracic surgery has decreased along with technological advances and better diagnostic tools, whereas the patient’s comorbidities and frailty patterns have increased, as well as the number of early cancer stages that could benefit from curative resection. Poor aerobic fitness, nutritional defects, sarcopenia and “toxic” behaviors such as sedentary behavior, smoking and alcohol consumption are modifiable risk factors for major postoperative complications. The process of enhancing patients’ physiological reserve in anticipation for surgery is referred to as prehabilitation. Components of prehabilitation programs include optimization of medical treatment, prescription of structured exercise program, correction of nutritional deficits and patient’s education to adopt healthier behaviors. All patients may benefit from prehabilitation, which is part of the enhanced recovery after surgery (ERAS) programs. Faster functional recovery is expected in low-risk patients, whereas better clinical outcome and shorter hospital stay have been demonstrated in higher risk and physically unfit patients.
... Additionally, CT promotes divergent signaling pathways (AT: ubiquitin proteasome system; RT: mammalian target of rapamycin-mTOR) (Coffey and Hawley, 2007) and neuromuscular adaptations (Bell et al., 1997;Hakkinen et al., 2003;Hickson, 1980). Specifically, AT causes fiber-type interconversions toward type I, while RT facilitates interconversions toward type II (Wilson et al. 2012a). In general, if RT adaptations are the primary goal, performing AT violates the foundational principles of specificity. ...
The purpose of this investigation was to compare the effects of three different concurrent training (CT) programs and a resistance training (RT) program. Twenty-three resistance trained men (age: 24 ± 3 years) were randomized into four groups: concurrent RT and high intensity interval cycling (CTH, n = 6), concurrent RT and moderate intensity continuous cycling (CTM, n = 5), RT and barbell circuit training (RTC, n = 6), or RT only (RT, n = 6). Back squat and bench press strength, quadriceps, and pectoralis muscle thickness, VO2peak, and maximum workload (Wmax, Watts) were assessed. Squat strength gains were meaningful in all groups and comparable among CTH (16.88 kg [95% CrI: 11.15, 22.63]), CTM (25.54 kg [95% CrI: 19.24, 31.96]), RTC (17.5 kg [95% CrI: 11.66, 23.39]), and RT (20.36 kg [95% CrI: 15.29, 25.33]) groups. Bench press strength gains were meaningful in all groups and comparable among CTH (11.86 kg [95% CrI: 8.28, 15.47]), CTM (10.3 kg [95% CrI: 6.49, 14.13]), RTC (4.84 kg [95% CrI: 1.31, 8.47]), and RT (10.16 kg [95% CrI: 7.02, 13.22]) groups. Quadriceps hypertrophy was meaningful in all groups and comparable among CTH (2.29 mm [95% CrI: 0.84, 3.76]), CTM (3.41 mm [95% CrI: 1.88, 4.91]), RTC (2.6 mm [95% CrI: 1.17, 4.05]), and RT (2.83 mm [95% CrI: 1.55, 4.12]) groups. Pectoralis hypertrophy was meaningful in CTH (2.29 mm [95% CrI: −0.52, 5.1]), CTM (5.14 mm [95% CrI: 2.1, 8.15]), and RTC (7.19 mm [95% CrI: 4.26, 10.02]) groups, but not in the RT group (1 mm [95% CrI: −1.59, 3.59]); further, between-group contrasts indicated less pectoralis growth in the RT compared to the RTC group. Regarding cardiovascular outcomes, only the RTH and RTM groups experienced meaningful improvements in either measure (VO2peak or Wmax). These data suggest that the interference effect on maximal strength and hypertrophy can be avoided when the aerobic training is moderate intensity cycling, high intensity cycling, or a novel barbell circuit for ~one hour per week and on non-RT days. However, the barbell circuit failed to elicit meaningful cardiovascular adaptations.
... The distribution of muscle fibre type in the quadriceps of untrained individuals is estimated to be in the range of~40% type I, and~60% type II [44]. The distribution of fibre types within each muscle biopsy may influence gene expression since type I and type II have different modes of energy metabolism [45]. In a study of skeletal muscle transcriptomics in rats, high aerobic capacity muscle indicated enhanced tissue oxygenation and vascularization, unlike low-capacity muscle transcriptome, which indicated immune response and metabolic dysfunction relating to inflammation [46]. ...
Malignant hyperthermia (MH) is a pharmacogenetic condition of skeletal muscle that manifests in hypermetabolic responses upon exposure to volatile anaesthetics. This condition is caused primarily by pathogenic variants in the calcium-release channel RYR1, which disrupts calcium signalling in skeletal muscle. However, our understanding of MH genetics is incomplete, with no variant identified in a significant number of cases and considerable phenotype diversity. In this study, we applied a transcriptomic approach to investigate the genome-wide gene expression in MH-susceptible cases using muscle biopsies taken for diagnostic testing. Baseline comparisons between muscle from MH-susceptible individuals (MHS, n = 8) and non-susceptible controls (MHN, n = 4) identified 822 differentially expressed genes (203 upregulated and 619 downregulated) with significant enrichment in genes associated with oxidative phosphorylation (OXPHOS) and fatty acid metabolism. Investigations of 10 OXPHOS target genes in a larger cohort (MHN: n = 36; MHS: n = 36) validated the reduced expression of ATP5MD and COQ6 in MHS samples, but the remaining 8 selected were not statistically significant. Further analysis also identified evidence of a sex-linked effect in SDHB and UQCC3 expression, and a difference in ATP5MD expression across individuals with MH sub-phenotypes (trigger from in vitro halothane exposure only, MHS h (n = 4); trigger to both in vitro halothane and caffeine exposure, MHS hc (n = 4)). Our data support a link between MH-susceptibility and dysregulated gene expression associated with mitochondrial bioenergetics, which we speculate plays a role in the phenotypic variability observed within MH.
... A unique feature of the MST intervention is the inclusion of high-velocity, cyclic movement patterns performed under full weight-bearing conditions. The intent of MST is to increase central drive to muscles of the lower extremities, 39 facilitate high rates of muscle force development, 40,41 and mimic the coordinated lower limb joint actions and joint angular velocities encountered during rapid ambulation. These features may improve transferability of training effects by enhancing the ability, in the absence of body-weight support, to produce higher SFs during overground walking. ...
... In other words, the study participants in Mero's research were boys representing endurance and strength-speed disciplines. Achieving high results in a specific sports specialization depends largely on physiological predispositions as well as commitment to a systematic and targeted training process [42]. One of such physiological factors is the composition of muscle fiber types, which is associated with the production and utilization of lactate [41]. ...
Background: The purpose of this study was to analyze the course of changes in the blood lactate (BL) concentration in response to the graded exercise test (GXT) and the modified Wingate test (MWT). Methods: This study involved 23 male highly trained road cyclists (age: 16.2 ± 1.1 years; experience: 5.0 ± 2.1 years; VO2max 59.0 ± 3.5 mL × kg−1 × min−1). The analysis of BL concentration was conducted using an enzymatic–amperometric electrochemical technique. Results: Our study provided the following information: (i) peak BL concentration in response to GXT (12.86 ± 2.32 mmol × L−1) and MWT (12.85 ± 1.47 mmol × L−1) is expected around the third minute after the completion of the trial; (ii) 60 min is not a sufficient period for BL concentration to return to resting values after GXT; (iii) post-GXT BL removal during the 60 min period is unsteady (3–20 min: −2.6 ± −0.6% × min−1; 20–60 min: −1.6 ± −0.3% × min−1; p-value for comparison < 0.01), whereas post-MWT BL removal during the 12 min period appears to be constant (3–6 min: −2.4 ± −5.6% × min−1, 6–9 min: −2.6 ± −1.8 % × min−1; 9–12 min: −3.1 ± −2.1 % × min−1; p-value for all comparisons < 0.01). Conclusions: When aiming to obtain valuable data regarding the course of changes in BL concentration during the post-exertion period, it is essential to consider the number of measurements and the time points in sample collection for analysis.
... Depending on the type of muscle fiber, slow (type I) or fast (type II, which subdivides in type IIA, IIX, and IIB), different metabolic pathway strategies are employed. The former is enriched in mitochondria, displaying an oxidative metabolism, while fast fibers rely mainly in glycolytic metabolism 4 . Fast glycolytic fibers appear to be more prone to age-related alterations, and thus, their abundance declines over time [5][6][7] . ...
Skeletal muscle aging is characterized by the loss of muscle mass, strength and function, mainly attributed to the atrophy of glycolytic fibers. Underlying mechanisms driving the skeletal muscle functional impairment are yet to be elucidated. To unbiasedly uncover its molecular mechanisms, we recurred to gene expression and metabolite profiling in a glycolytic muscle, Extensor digitorum longus (EDL), from young and aged C57BL/6JRj mice. Employing multi-omics approaches we found that the main age-related changes are connected to mitochondria, exhibiting a downregulation in mitochondrial processes. Consistent is the altered mitochondrial morphology. We further compared our mouse EDL aging signature with human data from the GTEx database, reinforcing the idea that our model may recapitulate muscle loss in humans. We are able to show that age-related mitochondrial downregulation is likely to be detrimental, as gene expression signatures from commonly used lifespan extending interventions displayed the opposite direction compared to our EDL aging signature.
Purpose
This study explored neuromuscular adaptations following 11 weeks of concurrent training (CT) compared to resistance-only (R) and endurance-only (E) in trained men.
Methods
Thirty participants were randomized into three groups (E, R, CT), training three times per week. Neuromuscular assessments involved plantar-flexion maximal voluntary isometric contraction (MVIC), rate of torque development (RTD), evoked potentials (H reflex and V wave) and contractile properties at baseline, mid-training (week 5), and post-training. Resistance training for R and CT groups involved two phases: weeks 1–5 (maximal strength development) and weeks 6–11 (explosive/reactive strength and muscle power development). Endurance training for the E and CT groups involved 30-min of running on the heavy-intensity domain.
Results
MVIC increased similarly in the R and CT groups. The R group significantly improved peak and sequential RTD, soleus electromyographic (EMG) activity, V wave and contractile properties. Conversely, the CT group exhibited an interference effect during the first phase with no improvements in RTD or neuromuscular parameters. During the second phase, the CT group exhibited significant enhancements in sequential RTD, soleus EMG activity, H- and V-wave amplitude. The improvements in the E group were limited to H-reflex excitability.
Conclusion
These findings suggest that the magnitude of CT interference is modulated by the focus of resistance training (maximal vs explosive strength) being performed. However, it is relevant to note that contractile impairments seem to persist, likely due to endurance training in the heavy-intensity domain.
Exercise capacity of an individual describes the ability to perform physical activity. This exercise capacity is influenced by intrinsic factors such as genetic constitution and extrinsic factors such as exercise training. On the metabolic level exercise and metabolism are linked. As an important site of metabolism and the main source for ATP needed for muscle contraction, mitochondrial function can determine exercise capacity, and exercise inversely influences mitochondrial function. It has been suggested that exercise mediates many of its effects due to such metabolic changes. Although extrinsic factors affect exercise capacity, a major part of an individual's exercise capacity is genetically determined, and extrinsic factors can only improve on this baseline. Looking at the effect of exercise capacity on and with disease, the two go hand in hand. On one hand, disease is negatively affecting an individual's exercise capacity; on the other hand, exercise offers an effective treatment option. Combining these factors, exercise capacity is an often‐ignored prognostic variable for life expectancy as well as morbidity and mortality. In this review, we aim to provide the current knowledge on the links between inherited and acquired exercise capacity, as well as the mechanisms in which metabolism interacts with exercise capacity. © 2023 American Physiological Society. Compr Physiol 13:5115‐5155, 2023.
Selective androgen receptor modulators are currently not approved but are widely used in gyms. In the present study, the effects of ligandrol and its combination with endurance training on functional and clinically important parameters were studied in male healthy rats. Fourteen-week-old male rats were divided into four groups: two training (40 min, 5 times/week) and two non-training (5 min, 3 times/week). The velocity was 25 m/min at a track elevation of 5° for all groups. Ligandrol (0.4 mg/kg body weight, 5 times/week) was administered to one training and one non-training group and vehicle to the other groups (n = 10 per group) for 8 weeks. We conducted functional tests and examined morphometric, functional, hematological, hormonal, and clinical chemistry indicators in rats and histological and gene expression analyses in gastrocnemius muscle. Endurance training had a positive effect on all functional tests and increased vascular endothelial growth factor a (Vegf-a) gene expression. Ligandrol treatment reduced submaximal endurance, maximal oxygen consumption, concentrations of glucose, follicle-stimulating hormone, and testosterone. It increased grip strength, triglycerides, and total cholesterol concentrations and had no effect on maximal sprinting speed, maximal time to exhaustion, hematological and morphometric parameters, and gene expression of myostatin and insulin-like growth factor 1. The negative effects of ligandrol treatment outweighed its benefits in this study. Endurance training alone had favorable effects, and its combination with ligandrol did not seem to have an advantage. In the training group, ligandrol decreased Vegf-a gene expression and the size of muscle fibers type I and IIa.
This review examines the chemical properties of whey protein in protein powders and their impact on muscle growth in athletes. We explore the composition of whey protein, including its major fractions and amino acid profile, with a focus on the role of leucine and other branched-chain amino acids. The review discusses the different types of whey protein powders - concentrate, isolate, and hydrolysate - and their distinct characteristics. We analyze the mechanisms by which whey protein stimulates muscle protein synthesis, including its effects on cellular signaling pathways and hormonal responses. The paper evaluates current evidence on the effects of whey protein supplementation on lean body mass, muscle hypertrophy, strength, and athletic performance. Additionally, we consider optimal dosing strategies and timing of intake for maximizing the benefits of whey protein supplementation. This comprehensive review provides insights into the efficacy of whey protein as a dietary supplement for athletes and highlights areas for future research in sports nutrition.
Terapia ruchem, rozumiana jako specyficzny trening mięśniowy, jest kluczowym narzędziem w terapii aparatu ruchu, a więc również w obrębie miofunkcjonalnych działań logopedycznych. Termin ‘trening miofunkcjonalny’ wskazuje kierunek, w którym należy upatrywać trwałych rozwiązań problemów ruchowych. Niniejszy artykuł ma na celu zarysowanie podstaw fizjologii mięśni, stanowiąc bazę do rozwijania wiedzy terapeutów i stosowania zagadnień fizjologii wysiłku fizycznego w kontekście planowania terapii lub – precyzyjniej mówiąc – programowania treningu miofunkcjonalnego.
Background and Study Aim. Core stability is a fundamental aspect of gymnastics performance, essential for balance, flexibility, and overall strength. This study aims to investigate the effects of long-term circuit training with static and dynamic core stabilization on physical components in gymnasts. Material and Methods. This study used a randomized controlled trial (RCT) design. A total of 42 participants were randomly assigned to three groups: a control group (CTR, n=14), a static core stabilization group (CSS, n=14), and a dynamic core stabilization group (CSD, n=14). The intervention programs for static and dynamic core stabilization included six distinct exercises. These exercises were performed three times per week over an 8-week period. Data were collected at two time points: pretest and posttest. Physical components assessed included flexibility, balance, and the strength of the abdominal, back, leg, and arm muscles. Data analysis was conducted using a paired sample t-test with the significance level set at 5%. Results. The results showed significant improvements in flexibility (cm), balance (s), back muscle strength (kg), abdominal muscle strength (repetitions), and arm muscle strength (repetitions) in the core stabilization groups between the pretest and posttest (p ≤ 0.05). However, no significant changes were observed in leg muscle strength (kg) (p ≥ 0.05). When comparing groups, significant differences were noted in balance (s), back muscle strength (kg), abdominal muscle strength (repetitions), and arm muscle strength (repetitions) (p ≤ 0.05). Flexibility (cm) and leg muscle strength (kg) did not show significant differences between groups (p ≥ 0.05). Conclusions. The study highlights the importance of incorporating core stabilization exercises into circuit training programs for gymnasts. Both static and dynamic approaches to core stabilization provide valuable strategies for optimizing physical conditioning and enhancing athletic performance. These findings can inform the development of evidence-based training protocols aimed at improving key physical attributes critical for gymnastics.
Delphinidin, a plant anthocyanidin, suppresses disuse muscle atrophy in mice. However, its effect on muscle fiber type shift is unclear. To examine whether delphinidin affects skeletal muscle fiber type, differentiated C2C12 cells were treated with delphinidin. Results revealed that delphinidin upregulated the mRNA expression of myosin heavy chain type I (MyHCI), troponin C1, troponin I1, and MyHCIIx and increased slow MyHC protein level in C2C12 myotubes. Delphinidin also enhanced succinic dehydrogenase (SDH) activities and suppressed lactate dehydrogenase (LDH) activity. Adenosine monophosphate–activated protein kinase (AMPK) inhibition attenuated delphinidin-induced MyHCI upregulation and MyHCIIb downregulation. We investigated the effect of delphinidin on the upstream factors involved in AMPK activation. Delphinidin increased liver kinase B1 (LKB1) phosphorylation and nuclear respiratory factor 1 (NRF1) and calcium/calmodulin-dependent protein kinase 2 (CaMKK2) protein levels. In conclusion, delphinidin induced muscle fiber type conversion from fast-twitch to slow-twitch muscles through the AMPK signaling pathway.
Fontanetti, G, Barreto, RV, Junior, RC, Marangoni, V, Denadai, BS, Greco, CC, and Lima, LCR. The use of the self-selected rest interval method is as effective for optimizing postactivation performance enhancement in elite athletes as employing the best fixed rest interval. J Strength Cond Res 39(1): 10-15, 2025-This study investigated whether the adoption of a self-selected recovery interval (SSI) is more effective than the use of a fixed recovery interval (FRI) to elicit postactivation performance enhancement (PAPE). Ten male professional volleyball athletes (20.6 ± 1.5 years; 92.8 ± 4.9 kg; 195 ± 8 cm) participated in 3 experimental sessions, in random order, following familiarization with countermovement jump (CMJ) exercises and determination of the load for 5-repetition maximum (5RM: 146 ± 27 kg) squat exercise. The 3 experimental sessions consisted of (a) control session (CON), with measurement of CMJ height 4 minutes before and 2, 4, 6, 8, and 10 minutes after subjects rested while sitting in a chair without performing any preactivation (PA) protocol; (b) session with FRI, with measurement of CMJ height 2, 4, 6, 8, and 10 minutes after performing a PA exercise consisting of 5 squats with 5RM load; and (c) session with SSI, with evaluation of CMJ height once after performing the PA adopting a SSI based on a readiness scale. No significant differences were found between CMJ height in the CON session. Countermovement jump height was higher (p < 0.05) than the baseline measurement at minutes 4, 6, and 8 post-PA in the FRI session, for the best CMJ performance during the FRI session, and after the PA in the SSI session. No significant differences were found between the heights of the best CMJ in the FRI session (49.2 ± 6.8 cm) and post-PA in the SSI session (49.6 ± 6.6 cm). Therefore, adopting FRI and SSI is equally effective in eliciting PAPE in elite athletes. Despite being as effective as adopting FRI to elicit PAPE, SSI is more intuitive and easily applicable in contexts where PAPE can be implemented.
Resistance training (RT) triggers diverse morphological and physiological adaptations that are broadly considered beneficial for performance enhancement as well as injury risk reduction. Some athletes and coaches therefore engage in, or prescribe, substantial amounts of RT under the assumption that continued increments in maximal strength capacity and/or muscle mass will lead to improved sports performance. In contrast, others employ minimal or no RT under the assumption that RT may impair endurance or sprint performances. However, the morphological and physiological adaptations by which RT might impair physical performance, the likelihood of these being evoked, and the training program specifications that might promote such impairments, remain largely undefined. Here, we discuss how selected adaptations to RT may enhance or impair speed and endurance performances while also addressing the RT program variables under which these adaptations are likely to occur. Specifically, we argue that while some myofibrillar (muscle) hypertrophy can be beneficial for increasing maximum strength, substantial hypertrophy can lead to macro- and microscopic adaptations such as increases in body (or limb) mass and internal moment arms that might, under some conditions, impair both sprint and endurance performances. Further, we discuss how changes in muscle architecture, fiber typology, microscopic muscle structure, and intra- and intermuscular coordination with RT may maximize speed at the expense of endurance, or maximize strength at the expense of speed. The beneficial effect of RT for sprint and endurance sports can be further improved by considering the adaptive trade-offs and practical implications discussed in this review.
Graphical abstract
Skeletal muscle is a highly heterogeneous tissue, and its contractile proteins are composed of different isoforms, forming various types of muscle fiber, each of which has its own metabolic characteristics. It has been demonstrated that endurance exercise induces the transition of muscle fibers from fast-twitch to slow-twitch muscle fiber type. Herein, we discover a novel epigenetic mechanism for muscle contractile property tightly coupled to its metabolic capacity during muscle fiber type transition with exercise training. Our results show that an 8-week endurance exercise induces histone methylation remodeling of PGC-1α and myosin heavy chain (MHC) isoforms in the rat gastrocnemius muscle, accompanied by increased mitochondrial biogenesis and an elevated ratio of slow-twitch to fast-twitch fibers. Furthermore, to verify the roles of reactive oxygen species (ROS) and AMPK in exercise-regulated epigenetic modifications and muscle fiber type transitions, mouse C2C12 myotubes were used. It was shown that rotenone activates ROS/AMPK pathway and histone methylation enzymes, which then promote mitochondrial biogenesis and MHC slow isoform expression. Mitoquinone (MitoQ) partially blocking rotenone-treated model confirms the role of ROS in coupling mitochondrial biogenesis with muscle fiber type. In conclusion, endurance exercise couples mitochondrial biogenesis with MHC slow isoform by remodeling histone methylation, which in turn promotes the transition of fast-twitch to slow-twitch muscle fibers. The ROS/AMPK pathway may be involved in the regulation of histone methylation enzymes by endurance exercise.
Analyses of mitochondrial adaptations in human skeletal muscle have mostly used whole-muscle samples, where results may be confounded by the presence of a mixture of type I and II muscle fibres. Using our adapted mass spectrometry-based proteomics workflow, we provide insights into fibre-specific mitochondrial differences in the human skeletal muscle of men before and after training. Our findings challenge previous conclusions regarding the extent of fibre-type-specific remodelling of the mitochondrial proteome and suggest that most baseline differences in mitochondrial protein abundances between fibre types reported by us, and others, might be due to differences in total mitochondrial content or a consequence of adaptations to habitual physical activity (or inactivity). Most training-induced changes in different mitochondrial functional groups, in both fibre types, were no longer significant in our study when normalised to changes in markers of mitochondrial content.
Background
Fatigue and gastrointestinal (GI) distress are common among athletes with an estimated 30–90% of athletes participating in marathons, triathlons, or similar events experiencing GI complaints. Intense exercise can lead to increased intestinal permeability, potentially allowing members of the gut microbiota to permeate into the bloodstream, resulting in an inflammatory response and cascade of performance-limiting outcomes. Probiotics, through their capacity to regulate the composition of the gut microbiota, may act as an adjunctive therapy by enhancing GI and immune function while mitigating inflammatory responses. This review investigates the effectiveness of probiotic supplementation on fatigue, inflammatory markers, and exercise performance based on randomized controlled trials (RCTs).
Methods
This review follows the Preferred Reporting Items for Systematic Reviews and Meta-Analysis (PRISMA) guidelines and PICOS (Population, Intervention, Comparison, Outcome, Study design) framework. A comprehensive search was conducted in Sportdiscus, PubMed, and Scopus databases, and the screening of titles, abstracts, and full articles was performed based on pre-defined eligibility criteria. Of the 3505 records identified, 1884 were screened using titles and abstracts, of which 450 studies were selected for full-text screening. After final screening, 13 studies met the eligibility criteria and were included for review. The studies contained 513 participants, consisting of 351 males and 115 females, however, two studies failed to mention the sex of the participants. Among the participants, 246 were defined as athletes, while the remaining participants were classified as recreationally active (n = 267). All trials were fully described and employed a double- or triple-blind placebo-controlled intervention using either a single probiotic strain or a multi-strain synbiotic (containing both pro- and pre-biotics).
Results
This review assesses the effects of daily probiotic supplementation, ranging from 13 to 90 days, on physical performance and physiological markers in various exercise protocols. Ten studies reported improvements in various parameters, such as, enhanced endurance performance, improved anxiety and stress levels, decreased GI symptoms, and reduced upper respiratory tract infections (URTI). Moreover, despite no improvements in maximal oxygen uptake (VO2), several studies demonstrated that probiotic supplementation led to amelioration in lactate, creatine kinase (CK), and ammonia concentrations, suggesting beneficial effects on mitigating exercise-induced muscular stress and damage.
Conclusion
Probiotic supplementation, specifically at a minimum dosage of 15 billion CFUs daily for a duration of at least 28 days, may contribute to the reduction of perceived or actual fatigue.
Lung transplantation is an elective treatment option for end-stage respiratory diseases in which all medical therapy options have been exhausted. The current study aimed to identify updated information on the postoperative conditions that may impair rehabilitation after lung transplantation and to provide specific considerations of their clinical relevance during the recovery process. The present study is a systematic review conducted by searching three primary databases: the United States National Library of Medicine PubMed system, Scopus, and the Cochrane Library. The databases were searched for articles published from database inception until May 2024; at the end of the selection process, 27 documents were included in the final analysis. The retrieved material identified 19 conditions of rehabilitative interest that potentially affect the postoperative course: graft dysfunction, dysphagia, postsurgical pain, cognitive impairment, chronic lung allograft dysfunction-bronchiolitis obliterans syndrome, phrenic nerve injury, delayed extracorporeal membrane oxygenation weaning, airway clearance, refractory hypoxemia, mediastinitis, reduced oxidative capacity, sternal dehiscence, coronavirus disease 2019 (COVID-19), gastroparesis, ossification of the elbow, Takotsubo cardiomyopathy, airway dehiscence, recurrent pleural effusion, and scapular prolapse. Although some patients are not amenable to rehabilitation techniques, others can significantly improve with rehabilitation.
This study assessed muscle activity (root mean square, RMS, and median frequency, MDF) to evaluate the acute response to blood flow restriction (BFR) resistance exercise (RE) and conventional moderate intensity (MI) RE. We also performed exploratory analyses of differences based on sex and exercise‐induced hypoalgesia (EIH). Fourteen asymptomatic individuals performed four sets of unilateral leg press with their dominant leg to volitional fatigue under two exercise conditions: BFR RE and MI RE. Dominant side rectus femoris (RF) and vastus lateralis (VL) muscle activity were measured using surface electromyography (sEMG) through exercise. RMS and MDF were calculated and compared between conditions and timepoints using a linear mixed model. Pressure pain thresholds (PPT) were tested before and immediately after exercise and used to quantify EIH. Participants were then divided into EIH responders and nonresponders, and the differences on RMS and MDF were compared between the two groups using Hedges' g. RMS significantly increased over time (RF: p = 0.0039; VL: p = 0.001) but not between conditions (RF: p = 0.4; VL: p = 0.67). MDF decreased over time (RF: p = 0.042; VL: p < 0.001) but not between conditions (RF: p = 0.74; VL: p = 0.77). Consistently lower muscle activation was found in females compared with males (BRF, RF: g = 0.63; VL, g = 0.5. MI, RF: g = 0.72; VL: g = 1.56), with more heterogeneous findings in MDF changes. For BFR, EIH responders showed greater RMS changes (Δ RMS) (RF: g = 0.90; VL: g = 1.21) but similar MDF changes (Δ MDF) (RF: g = 0.45; VL: g = 0.28) compared to nonresponders. For MI, EIH responders demonstrated greater increase on Δ RMS (g = 0.61) and decrease on Δ MDF (g = 0.68) in RF but similar changes in VL (Δ RMS: g = 0.40; Δ MDF: g = 0.39). These results indicate that when exercising to fatigue, no statistically significant difference was observed between BFR RE and conventional MI RE in Δ RMS and Δ MDF. Lower muscle activity was noticed in females. While exercising to volitional fatigue, muscle activity may contribute to EIH.
PURPOSE This study aimed to identify ACTN3 gene polymorphisms amongprofessional ssireum players by weight class and between elite and non-elite players,and to select genotype that matches the characteristics of the sport. METHODS The subjects of this study were 148 male athletes currently working as professionalssireum players. Chi-square test cross-tabulation analysis was conducted to examinethe differences in ACTN3 genotypes between weight classes and between elite andnon-elite players. RESULTS There were no significant differences in allele or genotypebetween ssireum players, but there were significant differences in R-allele and XXgenotype. CONCLUSIONS Players with the R-allele type of the ACTN3 gene weremore often classified as elite. Using this marker as a basis for organizing a playerselection and training programs would be more effective in training those that matchthe characteristics of elite players of the game.
Skeletal muscle plays a crucial role in generating force to facilitate movement. Skeletal muscle is a heterogenous tissue composed of diverse fibers with distinct contractile and metabolic profiles. The intricate classification of skeletal muscle fibers exists on a continuum ranging from type I (slow-twitch, oxidative) to type II (fast-twitch, glycolytic). The heterogenous distribution and characteristics of fibers within and between skeletal muscles profoundly influences cellular signaling; however, this has not been broadly discussed as it relates to macroautophagy/autophagy. The growing interest in skeletal muscle autophagy research underscores the necessity of comprehending the interplay between autophagic responses among skeletal muscles and fibers with different contractile properties, metabolic profiles, and other related signaling processes. We recommend approaching the interpretation of autophagy findings with careful consideration for two key reasons: 1) the distinct behaviors and responses of different skeletal muscles or fibers to various perturbations, and 2) the potential impact of alterations in skeletal muscle fiber type or metabolic profile on observed autophagic outcomes. This review provides an overview of the autophagic profile and response in skeletal muscles/fibers of different types and metabolic profiles. Further, this review discusses autophagic findings in various conditions and diseases that may differentially affect skeletal muscle. Finally, we provide key points of consideration to better enable researchers to fine-tune the design and interpretation of skeletal muscle autophagy experiments.Abbreviation: AKT1: AKT serine/threonine kinase 1; AMPK: AMP-activated protein kinase; ATG: autophagy related; ATG4: autophagy related 4 cysteine peptidase; ATG5: autophagy related 5; ATG7: autophagy related 7; ATG12: autophagy related 12; BECN1: beclin 1; BNIP3: BCL2 interacting protein 3; CKD: chronic kidney disease; COPD: chronic obstructive pulmonary disease; CS: citrate synthase; DIA: diaphragm; EDL: extensor digitorum longus; FOXO3/FOXO3A: forkhead box O3; GAS; gastrocnemius; GP: gastrocnemius-plantaris complex; MAP1LC3/LC3: microtubule associated protein 1 light chain 3; MAPK: mitogen-activated protein kinase; MYH: myosin heavy chain; PINK1: PTEN induced kinase 1; PLANT: plantaris; PRKN: parkin RBR E3 ubiquitin protein ligase; QUAD: quadriceps; RA: rectus abdominis; RG: red gastrocnemius; RQ: red quadriceps; SOL: soleus; SQSTM1: sequestosome 1; TA: tibialis anterior; WG: white gastrocnemius; WQ: white quadriceps; WVL: white vastus lateralis; VL: vastus lateralis; ULK1: unc-51 like autophagy activating kinase 1.
Redox regulation is a fundamental physiological phenomenon related to oxygen-dependent metabolism, and skeletal muscle is mainly regarded as a primary site for oxidative phosphorylation. Several studies have revealed the importance of reactive oxygen and nitrogen species (RONS) in the signaling process relating to muscle adaptation during exercise. To date, improving knowledge of redox signaling in modulating exercise adaptation has been the subject of comprehensive work and scientific inquiry. The primary aim of this review is to elucidate the molecular and biochemical pathways aligned to RONS as activators of skeletal muscle adaptation and to further identify the interconnecting mechanisms controlling redox balance. We also discuss the RONS-mediated pathways during the muscle adaptive process, including mitochondrial biogenesis, muscle remodeling, vascular angiogenesis, neuron regeneration, and the role of exogenous antioxidants.
Background: Slips and falls are a serious health concern, particularly among older adults. Current physical therapy protocols strengthen the legs to improve balance. However, arm movements help maintain balance during a slip incident. Understanding how arm movements improve balance may help
clinicians develop more comprehensive fall-prevention protocols to improve patient outcomes.
Clinical question: What limitations exist in current fall prevention protocols for reducing falls in older adults during slip incidents, and what new strategies can enhance these outcomes?
Key results: Slip incidents often result in a sideways loss of balance, leading to hip fractures in older adults. During a slip, the legs do not produce sideways motion and are less effective in regaining balance in this direction. Contrary, the arms produce 100 + degrees of abduction and this motion reduces falls by
200%+ during a slip incident. Notably, older adults exhibit 35.7% decreased arm abduction acceleration responses compared to younger adults during a slip incident. This delay may be attributed to age-related decreases in type II fibers of the deltoid. High-velocity and ballistic training have been shown to improve the proportion and size of type II fibers as well as improve fall outcomes when focused on the lower extremities.
Clinical application: Therefore, I propose incorporating arm abductor training, alongside leg exercises, as a cost-effective and low-risk intervention to enhance the slip responses in older adults. In light of its minimal risk and considerable potential benefits, starting arm abductor exercises with older adults is a sensible move.
Lee-Confer J (2024) Strength in arms: empowering older adults against the risk of slipping and falling-a theoretical perspective. Front. Sports Act. Living 6:1371730.
Sportif performans ve değerlendirme aşamalarında motorik özellikler çok fazla ön plana çıkmaktadır. Sporcuların üstün performans gösterebilmeleri için bu özelliklerini mümkün olan en yüksek düzeyde geliştirmeleri gerekmektedir. Sportif başarı ancak bu özellikler spor dalına uygun olarak geliştirildiğinde mümkün olabilmektedir. Bu açıdan düşünüldüğünde sportif performans için motorik özelliklerin incelenmesi ve geliştirilmesine yönelik çalışmaların yapılması oldukça önemlidir.
Background: The global COVID-19 pandemic has significantly influenced athletes worldwide.
Objectives: This research aims to investigate the effects of a 4-week aerobic exercise program on athletic performance in both recovered and uninfected COVID-19 athletes during the post-COVID-19 period.
Methods: Fourteen male student-athletes aged 18 - 25 years from Imam Khomeini International University participated in this study. The participants comprised 7 recovered COVID-19 athletes and 7 athletes with no prior COVID-19 infection. The study employed a pre-test/post-test design conducted in 2 phases. During the pre and post-test phases, participants underwent baseline assessments of athletic performance, including maximum oxygen consumption (VO2 max) and anaerobic power. Subsequently, the participants engaged in a 4-week aerobic exercise intervention. Pre- and post-intervention outcomes within groups were compared using paired t-tests, while independent t-tests were utilized for comparisons between the recovered COVID-19 athlete group and the uninfected athlete group.
Results: Independent t-tests demonstrated significant increases in VO2 max and peak power after 4 weeks of aerobic exercise in both the recovered COVID-19 group (P = 0.001, P = 0.0001) and the uninfected COVID-19 group (P = 0.012, P = 0.001). However, dependent t-tests revealed a significant difference between the recovered COVID-19 group and the uninfected COVID-19 group in the post-test of VO2 max (P = 0.044) and peak power (P = 0.001).
Conclusions: This study indicates that a 4-week aerobic exercise regimen can improve athletic performance in both recovered and uninfected COVID-19 athletes. However, recovered COVID-19 athletes exhibited a notably slower rate of improvement compared to their uninfected counterparts. Therefore, it is recommended that, in addition to aerobic exercise, recovered athletes integrate supplementary strategies to optimize their return-to-sport timeline and maximize performance recovery.
Graham, MC, Thompson, KL, Hawk, GS, Fry, CS, and Noehren, B. Muscle fiber cross-sectional area is associated with quadriceps strength and rate of torque development after ACL injury. J Strength Cond Res XX(X): 000–000, 2023—The purpose of this study was to investigate the relationship between muscle fiber type–specific properties of the vastus lateralis and quadriceps muscle performance in individuals after an anterior cruciate ligament (ACL) tear. 26 subjects (22.0 ± 5.4 years) were included in this cross-sectional study, and all data were collected before ACL reconstruction. Quadriceps peak torque (QPT) and early (0–100 ms) and late (100–200 ms) rate of torque development (RTD) were obtained from maximal voluntary isometric quadriceps strength testing. Muscle fiber cross-sectional area (fCSA) and percent fiber type distribution (FT%) were evaluated through immunohistochemical analysis of a muscle biopsy. Between-limb differences in fiber characteristics were assessed using paired t-tests (with α-level 0.05). Relationships between fiber-specific properties and quadriceps muscle performance were determined using separate multiple linear regression analyses for ACL-injured and noninjured limbs. There were significant differences in fCSA between ACL-injured and noninjured limbs across all fiber types, but no differences in FT%. Type 1 fCSA, type 2a fCSA, and their interaction effect were the explanatory variables with the strongest relationship to all performance outcomes for the ACL-injured limb. The explanatory variables in the ACL-injured limb had a significant relationship to QPT and late RTD, but not early RTD. These findings suggest that QPT and late RTD are more heavily influenced by fCSA than FT% in ACL-injured limbs. This work serves as a foundation for the development of more specific rehabilitation strategies aimed at improving quadriceps muscle function before ACL reconstruction or for individuals electing nonsurgical management.
Scope
Endurance capacity is essential for endurance athletes’ achievement and individuals’ health. Nutritional supplements are a proven way to enhance endurance capacity. Previous studies have shown that ferulic acid (FA) enhances endurance capacity, but the underlying mechanism is unclear. The study is aimed to investigate the mechanism by which FA increases endurance capacity.
Methods and results
Forty mice are divided into control and 0.5% FA‐supplemented groups, and an exhaustive swimming test demonstrates increased endurance capacity with FA supplementation. This study investigates the underlying mechanism for this effect of FA. Firstly, RT‐PCR and western blot analysis find that FA increases the transformation from fast to slow muscle fiber. Additionally, adenosine triphosphate concentration, metabolic enzyme activity, and mitochondrial DNA analysis find that FA increases mitochondrial biogenesis and activates nuclear factor erythroid 2‐related factor (NRF)1 signaling pathway in muscle. Besides, through antioxidant capacity analysis, this study finds that FA activates NRF2 signaling pathway and improves the antioxidant capacity in muscle. Moreover, inhibiting NRF2 eliminates FA's effect on muscle fiber transformation in C2C12 cells.
Conclusion
Our results suggest that FA increases endurance capacity by promoting skeletal muscle oxidative phenotype, mitochondrial function, and antioxidant capacity, which may be related to the NRF1 and NRF2 signaling pathways.
Muscle biopsy samples were obtained from the gastrocnemius of 26 well-trained runners of widely varying ability. Portions of the sample were analyzed for succinate dehydrogenase (SDH) activity and for muscle fiber composition.
O2 max was determined during uphill treadmill running. Mean values for muscle SDH activity (14.6 U/g), fiber composition (55% slow twitch) and
O2 max (60.9 ml/kg×min−1) were lower than reported previously for groups of elite and sub-elite runners. The physiological data were consistent with the performance ability of the sample [5∶12, 11∶20 and 36∶40 (min∶s) for 1, 2 and 6 miles, respectively]. Within the sample, performance was most strongly related to
O2 max (r=−0.84, −0.87 and −0.88 for 1, 2, and 6 miles). There was little relationship between muscle SDH activity and either performance (r=−0.11, −0.14, −0.20 for 1, 2, and 6 miles) or
O2 max (r=0.23). The relationship between muscle fiber composition and performance was only modestly strong (r=−0.52, −0.54, −0.55 for 1, 2, and 6 miles). The results indicate that the primary determinant of cross-sectional differences in running performance is
O2 max. Skeletal muscle metabolism apparently contributes little to these cross-sectional differences and may be of much greater importance to variations in performance within an individual.
Thirty-five healthy men were matched and randomly assigned to one of four training groups that performed high-intensity strength and endurance training (C; n = 9), upper body only high-intensity strength and endurance training (UC; n = 9), high-intensity endurance training (E; n = 8), or high-intensity strength training (ST; n = 9). The C and ST groups significantly increased one-repetition maximum strength for all exercises (P < 0.05). Only the C, UC, and E groups demonstrated significant increases in treadmill maximal oxygen consumption. The ST group showed significant increases in power output. Hormonal responses to treadmill exercise demonstrated a differential response to the different training programs, indicating that the underlying physiological milieu differed with the training program. Significant changes in muscle fiber areas were as follows: types I, IIa, and IIc increased in the ST group; types I and IIc decreased in the E group; type IIa increased in the C group; and there were no changes in the UC group. Significant shifts in percentage from type IIb to type IIa were observed in all training groups, with the greatest shift in the groups in which resistance trained the thigh musculature. This investigation indicates that the combination of strength and endurance training results in an attenuation of the performance improvements and physiological adaptations typical of single-mode training.
We applied K-means cluster analysis to test the hypothesis that muscle-specific factors known to modulate protein synthesis and satellite cell activity would be differentially expressed during progressive resistance training (PRT, 16 wk) in 66 human subjects experiencing extreme, modest, and failed myofiber hypertrophy. Muscle mRNA expression of IGF-I isoform Ea (IGF-IEa), mechanogrowth factor (MGF, IGF-IEc), myogenin, and MyoD were assessed in muscle biopsies collected at baseline (T1) and 24 h after the first (T2) and last (T3) loading bouts from previously untrained subjects clustered as extreme responders (Xtr, n=17), modest responders (Mod, n=32), and nonresponders (Non, n=17) based on mean myofiber hypertrophy. Myofiber growth averaged 2,475 microm2 in Xtr, 1,111 microm2 in Mod, and -16 microm2 in Non. Main training effects revealed increases in all transcripts (46-83%, P<0.005). For the entire cohort, IGF-IEa, MGF, and myogenin mRNAs were upregulated by T2 (P<0.05), while MyoD did not increase significantly until T3 (P<0.001). Within clusters, MGF and myogenin upregulation was robust in Xtr (126% and 65%) and Mod (73% and 41%) vs. no changes in Non. While significant in all clusters by T3, IGF-IEa increased most in Xtr (105%) and least in Non (44%). Although MyoD expression increased overall, no changes within clusters were detected. We reveal for the first time that MGF and myogenin transcripts are differentially expressed in subjects experiencing varying degrees of PRT-mediated myofiber hypertrophy. The data strongly suggest the load-mediated induction of these genes may initiate important actions necessary to promote myofiber growth during PRT, while the role of MyoD is less clear.
While skeletal muscle protein accretion during resistance training (RT)-mediated myofiber hypertrophy is thought to result from upregulated translation initiation signaling, this concept is based on responses to a single bout of unaccustomed resistance exercise (RE) with no measure of hypertrophy across RT. Further, aging appears to affect acute responses to RE, but whether age differences in responsiveness persist during RT leading to impaired RT adaptation is unclear. We therefore tested whether muscle protein fractional synthesis rate (FSR) and Akt/mammalian target of rapamycin (mTOR) signaling in response to unaccustomed RE differed in old vs. young adults, and whether age differences in acute responsiveness were associated with differences in muscle hypertrophy after 16 wk of RT. Fifteen old and 21 young adult subjects completed the 16-wk study. The phosphorylation states of Akt, S6K1, ribosomal protein S6 (RPS6), eukaryotic initiation factor 4E (eIF4E) binding protein (4EBP1), eIF4E, and eIF4G were all elevated (23-199%) 24 h after a bout of unaccustomed RE. A concomitant 62% increase in FSR was found in a subset (6 old, 8 young). Age x time interaction was found only for RPS6 phosphorylation (+335% in old subjects only), while there was an interaction trend (P = 0.084) for FSR (+96% in young subjects only). After 16 wk of RT, gains in muscle mass, type II myofiber size, and voluntary strength were similar in young and old subjects. In conclusion, at the level of translational signaling, we found no evidence of impaired responsiveness among older adults, and for the first time, we show that changes in translational signaling after unaccustomed RE were associated with substantial muscle protein accretion (hypertrophy) during continued RT.
Muscle samples were obtained from the gastrocnemius of 17 female and 23 male track athletes, 10 untrained women, and 11 untrained men. Portions of the specimen were analyzed for total phosphorylase, lactic dehydrogenase (LDH), and succinate dehydrogenase (SDH) activities. Sections of the muscle were stained for myosin adenosine triphosphatase, NADH2 tetrazolium reductase, and alpha-glycerophosphate dehydrogenase. Maximal oxygen uptake (VO2max) was measured on a treadmill for 23 of the volunteers (6 female athletes, 11 male athletes, 10 untrained women, and 6 untrained men). These measurements confirm earlier reports which suggest that the athlete's preference for strength, speed, and/or endurance events is in part a matter of genetic endowment. Aside from differences in fiber composition and enzymes among middle-distance runners, the only distinction between the sexes was the larger fiber areas of the male athletes. SDH activity was found to correlate 0.79 with VO2max, while muscle LDH appeared to be a function of muscle fiber composition. While sprint- and endurance-trained athletes are characterized by distinct fiber compositions and enzyme activities, participants in strength events (e.g., shot-put) have relatively low muscle enzyme activities and a variety of fiber compositions.
These experiments explore the relationship between patterned impulse activity and contractile properties of skeletal muscles. Soleus (SOL) and extensor digitorum longus (EDL) muscles of adult rats were denervated and stimulated directly from 4 to 15 weeks with the same number of pulse trains at different intratrain pulse frequencies (1-500 Hz), with different numbers of pulse trains (864-4,320,000 pulses/d) at the same intratrain pulse frequencies, or with different combinations of pulse trains at 10 and 100 Hz. Chronic stimulation of the denervated SOL resulted in twitch times-to-peak and half-relaxation times that varied in a graded manner between values longer than those in the normal SOL to values as fast as those in the normal EDL, depending upon the pattern used. Increasing pulse frequencies (constant number) resulted in faster twitches, lower twitch/tetanus ratios, increasing post-tetanic potentiations, and larger tetanic tensions. Increasing pulse numbers (constant frequencies) resulted in slower twitches, lower twitch/tetanus ratios, post-tetanic depressions, and higher fatigue indices. The effect of varying the pulse number on twitch parameters was greater at low frequencies (10-20 Hz) than at high frequencies (100 Hz). SOL muscles receiving pulse trains at both 10 and 100 Hz became much faster than muscles receiving pulse trains at 10 Hz only, even in the experiments where the stimulation pattern contained 9 times as many pulses at 10 as at 100 Hz. Chronic stimulation of both the denervated and the innervated EDL with large numbers of pulses at 10 or 15 Hz resulted in twitches that were only half as slow as those induced in the SOL by the same "slow" patterns. In addition, these patterns led to a marked decrease in maximum tetanic tension and a marked increase in twitch/tetanus ratio. During stimulation with a small number of pulses at 150 Hz, on the other hand, twitch speed, twitch/tetanus ratio, and maximum tetanic tension remained normal or almost normal. We conclude that the isometric twitch and related properties of the rat SOL muscle can be graded within wide "adaptive ranges" by varying either the number or the frequency of pulses. In the EDL, the corresponding adaptive ranges appear much narrower, suggesting that the EDL and the SOL contain intrinsically different muscle fibers.
1. Changes in muscle strength, vastus lateralis fibre characteristics and myosin heavy-chain (MyoHC) gene expression were examined in 48 men and women following 3 weeks of knee immobilization and after 12 weeks of retraining with 1866 eccentric, concentric or mixed contractions. 2. Immobilization reduced eccentric, concentric and isometric strength by 47 %. After 2 weeks of spontaneous recovery there still was an average strength deficit of 11 %. With eccentric and mixed compared with concentric retraining the rate of strength recovery was faster and the eccentric and isometric strength gains greater. 3. Immobilization reduced type I, IIa and IIx muscle fibre areas by 13, 10 and 10 %, respectively and after 2 weeks of spontaneous recovery from immobilization these fibres were 5 % smaller than at baseline. Hypertrophy of type I, IIa and IIx fibres relative to baseline was 10, 16 and 16 % after eccentric and 11, 9 and 10 % after mixed training (all P < 0.05), exceeding the 4, 5 and 5 % gains after concentric training. Type IIa and IIx fibre enlargements were greatest after eccentric training. 4. Total RNA/wet muscle weight and ty I, IIa and IIx MyoHC mRNA levels did not change differently after immobilization and retraining. Immobilization downregulated the expression of type I MyoHC mRNA to 0.72-fold of baseline and exercise training upregulated it to 0.95 of baseline. No changes occurred in type IIa MyoHC mRNA. Immobilization and exercise training upregulated type IIx MyoHC mRNA 2.9-fold and 1.2-fold, respectively. For the immobilization segment, type I, IIa and IIx fibre area and type I, IIa and IIx MyoHC mRNA correlated (r = 0.66, r = 0.07 and r = -0.71, respectively). 5. The present data underscore the role muscle lengthening plays in human neuromuscular function and adaptation.
The aim of this study was to assess the relationships between human muscle fiber hypertrophy, protein isoform content, and maximal Ca(2+)-activated contractile function following a short-term period of resistance exercise training. Six male subjects (age 27 +/- 2 yr) participated in a 12-wk progressive resistance exercise training program that increased voluntary lower limb extension strength by >60%. Single chemically skinned fibers were prepared from pre- and posttraining vastus lateralis muscle biopsies. Training increased the cross-sectional area (CSA) and peak Ca(2+)-activated force (P(o)) of fibers containing type I, IIa, or IIa/IIx myosin heavy chain by 30-40% without affecting fiber-specific force (P(o)/CSA) or unloaded shortening velocity (V(o)). Absolute fiber peak power rose as a result of the increase in P(o), whereas power normalized to fiber volume was unchanged. At the level of the cross bridge, the effects of short-term resistance training were quantitative (fiber hypertrophy and proportional increases in fiber P(o) and absolute power) rather than qualitative (no change in P(o)/CSA, V(o), or power/fiber volume).
Previous research has indicated that 50 fiber measurements per individual for type I and II fibers would be sufficient to characterize the fiber areas. This study replicated the work of McCall et al. (1998) using the three major fiber types (I, IIA, and IIB) and sampling larger populations of fibers. Random blocks of fibers were also examined to investigate how well they correlated with the overall mean average fiber area. Using random blocks of 50 fibers provided an accurate reflection of the type IIB fibers (r = 0.96-0.98) but not for the type I (r = 0.85-0.94) or IIA fibers (r = 0.80-0.91). Type I fibers were consistently reflected by a random block of 150 fibers (r = 0.95-0.98) while type IIA fibers required random blocks of 200 fibers (r = 0.94-0.98), which appeared to provide an accurate reflection of the cross-sectional area. These results indicate that for a needle biopsy different numbers of fibers are needed depending on the fiber type to accurately characterize the mean fiber population.
Myosin heavy chain (MHC) isoform expression changes with physical training. This may be one of the mechanisms for muscular adaptation to exercise. We aimed to investigate the effects of different strength-training protocols on MHC isoform expression, bearing in mind that alpha- MHC(slow) (newly identified MHC isoform) mRNA may be upregulated in response to training. Twelve volunteers performed a 6-wk strength training with maximum contractions (Max group), and another 12 of similar age performed combination training of maximum contractions and ballistic and stretch-shortening movements (Combi group). Muscle samples were taken from triceps brachii before and after training. MHC isoform composition was determined by SDS-PAGE silver staining, and mRNA levels of MHC isoforms were determined by RT-PCR. In Max group, there was an increase in MHC(2A) (49.4 to 66.7%, P < 0.01) and a decrease in MHC(2X) (33.4 to 19.5%, P < 0.01) after training, although there was no significant change in MHC(slow). In Combi group, there was also an increase in MHC(2A) (47.7 to 62.7%, P < 0.05) and a decrease in MHC(slow) (18.2 to 9.2%, P < 0.05) but no significant change in MHC(2X). An upregulation of alpha-MHC(slow) mRNA was, therefore, found in both groups as a result of training. The strength training with maximum contractions led to a shift in MHC isoform composition from 2X to 2A, whereas the combined strength training produced an MHC isoform composition shift from slow to 2A.
Based on the growing body of evidence implicating an important role for myogenic regulatory factors (MRFs) in the adaptive responses of skeletal muscle to mechanical load, we tested the hypothesis that protein concentrations of MRFs as well as cell cycle proteins (i.e., cyclins and cyclin-dependent kinase inhibitors) would be altered after heavy leg resistance exercise (RE). Because we and others, however, have shown a blunted adaptive response to long-term resistance training in older (O) women [females (F)] compared with men (M), we also tested the hypothesis that these myogenic responses to RE would be influenced by age and gender. Twenty-two younger (Y) adults (20-35 yr, 11 YF, 11 YM) and 20 O adults (60-75 yr, 9 OF, 11 OM) consented to vastus lateralis muscle biopsy before and 24 h after a bout of RE using a regimen known to induce myofiber hypertrophy when performed 2-3 days/wk for several weeks (3 sets of 80% one-repetition maximum for squat, leg press, and knee extension). Protein concentrations of MRFs (MyoD, myogenin, myf-6), cyclin D1, cyclin B1, alpha-actin, and the cyclin-dependent kinase inhibitor p27kip were determined by immunoblotting. Data were analyzed by using age x gender x load repeated-measures ANOVA. Myogenin expression was 44% higher (P <0.05) in O compared with Y, and myf-6 tended to be higher in OF compared with YF (95%, P=0.059). A significant gender x load interaction indicated that, in F, RE led to a reduction in p27kip (20%; P<0.05), which was driven mainly by a 27% drop in OF. Levels of cyclin D1, cyclin B1, MyoD, myf-6, and alpha-actin were not influenced by age, gender, or loading. We report a novel finding in humans of markedly higher myogenin protein content in older sedentary muscle. The results do not, however, support the hypothesis that myogenic protein expression is altered 24 h after RE, irrespective of age or gender. Although the time point of postexercise muscle biopsy could be viewed as too early to capture maximal effects for most of these proteins, the significant decline in p27kip concentration found in OF suggests that mechanical load may provide one means of overcoming the inhibitory influence of p27kip.
Myostatin inhibits myoblast proliferation and differentiation in developing muscle. Mounting evidence suggests that myostatin also plays a limiting role in growth/repair/regeneration of differentiated adult muscle by inhibiting satellite cell activation. We tested the hypothesis that myostatin mRNA expression would decrease after resistance loading (RL) with a blunted response in older (O) females (F) who have shown minimal hypertrophy [vs. males (M)] after long-term RL. As myostatin is thought to modulate cell cycle activity, we also studied the response of gene transcripts key to stimulation (cyclin B1 and D1) and inhibition (p21cip and p27kip) of the cell cycle, along with the muscle-specific load-sensitive mitogen mechano-growth factor (MGF). Twenty young (Y; 20-35 yr, 10 YF, 10 YM) and 18 O (60-75 yr, 9 OF, 9 OM) consented to vastus lateralis biopsy before and 24 h after a bout of RL (3 sets x 8-12 repetitions to volitional fatigue of squat, leg press, knee extension). Gene expression levels were determined by relative RT-PCR with 18S as an internal standard and analyzed by age x gender x load repeated-measures ANOVA. A load effect was found for four transcripts (P < 0.005) including myostatin, cyclin D1, p27kip, and MGF as mRNA levels decreased for myostatin (-44%) and p27kip (-16%) and increased for cyclin D1 (34%) and MGF (49%). For myostatin, age x load and gender x load interactions (P < 0.05) were driven by a lack of change in OF, while marked declines were noted in YM (-56%), YF (-48%), and OM (-40%). Higher cyclin D1 levels in OF led to a main age effect (36%, O > Y) and an age x gender interaction (66%, OF > YF vs. 10%, OM > YM; P < 0.05). An age x gender x load interaction (P < 0.05) for cyclin D1 resulted from a 48% increase in OF. Post hoc testing within groups revealed a significant increase in MGF after RL in YM only (91%, P < 0.05). Higher levels of cyclin B1 in O (27%, O > Y) led to a main age effect (P < 0.05). An age x load interaction for cyclin B1 (P < 0.05) was driven by a 26% increase in Y with no change in O after RL. No age or gender differences, or load-mediated changes, were detected in levels of p21cip mRNA expression. These data clearly demonstrate that RL downregulates myostatin expression and alters genes key to cell cycle progression. However, failure to reduce myostatin expression may play a role in limiting RL-induced hypertrophy in OF.
Functional performance of lower limb muscles and contractile properties of chemically skinned single muscle fibers were evaluated before and after 8 wk of maximal effort stretch-shortening cycle (SSC) exercise training. Muscle biopsies were obtained from the vastus lateralis of eight men before and after the training period. Fibers were evaluated regarding their mechanical properties and subsequently classified according to their myosin heavy chain content (SDS-PAGE). After training, maximal leg extensor muscle force and vertical jump performance were improved 12% (P<0.01) and 13% (P<0.001), respectively. Single-fiber cross-sectional area increased 23% in type I (P<0.01), 22% in type IIa (P<0.001), and 30% in type IIa/IIx fibers (P<0.001). Peak force increased 19% in type I (P<0.01), 15% in type IIa (P<0.001), and 16% in type IIa/IIx fibers (P<0.001). When peak force was normalized with cross-sectional area, no changes were found for any fiber type. Maximal shortening velocity was increased 18, 29, and 22% in type I, IIa, and hybrid IIa/IIx fibers, respectively (P<0.001). Peak power was enhanced in all fiber types, and normalized peak power improved 9% in type IIa fibers (P<0.05). Fiber tension on passive stretch increased in IIa/IIx fibers only (P<0.05). In conclusion, short-term SSC exercise training enhanced single-fiber contraction performance via force and contraction velocity in type I, IIa, and IIa/IIx fibers. These results suggest that SSC exercises are an effective training approach to improve fiber force, contraction velocity, and therefore power.
A present debate in muscle biology is whether myonuclear addition is required during skeletal muscle hypertrophy. We utilized K-means cluster analysis to classify 66 humans after 16 wk of knee extensor resistance training as extreme (Xtr, n = 17), modest (Mod, n = 32), or nonresponders (Non, n = 17) based on myofiber hypertrophy, which averaged 58, 28, and 0%, respectively (Bamman MM, Petrella JK, Kim JS, Mayhew DL, Cross JM. J Appl Physiol 102: 2232-2239, 2007). We hypothesized that robust hypertrophy seen in Xtr was driven by superior satellite cell (SC) activation and myonuclear addition. Vastus lateralis biopsies were obtained at baseline and week 16. SCs were identified immunohistochemically by surface expression of neural cell adhesion molecule. At baseline, myofiber size did not differ among clusters; however, the SC population was greater in Xtr (P < 0.01) than both Mod and Non, suggesting superior basal myogenic potential. SC number increased robustly during training in Xtr only (117%; P < 0.001). Myonuclear addition occurred in Mod (9%; P < 0.05) and was most effectively accomplished in Xtr (26%; P < 0.001). After training, Xtr had more myonuclei per fiber than Non (23%; P < 0.05) and tended to have more than Mod (19%; P = 0.056). Both Xtr and Mod expanded the myonuclear domain to meet (Mod) or exceed (Xtr) 2,000 mum(2) per nucleus, possibly driving demand for myonuclear addition to support myofiber expansion. These findings strongly suggest myonuclear addition via SC recruitment may be required to achieve substantial myofiber hypertrophy in humans. Individuals with a greater basal presence of SCs demonstrated, with training, a remarkable ability to expand the SC pool, incorporate new nuclei, and achieve robust growth.
The soleus (SOL) or extensor digitorum longus (EDL) muscles of month-old rats were denervated for 14 days and then cross-transplanted so that the fast muscle was placed into the bed of the slow muscle and vice versa. At 17, 30, 60, and 90 days the transplants were tested for certain contractile and histochemical properties. By 90 days the cross-transplanted SOL showed complete conversion of the full contraction time and nearly complete conversion of the half relaxation time to those of the normal EDL. In contrast, the contraction and relaxation times of the cross-transplanted EDL became considerably slowed, but did not attain the values of the normal SOL. Histochemical staining for ATPase and SDH activity demonstrated similar transformations of fiber types. The degree of transformation of twitch and histochemical characteristics in cross-transplanted muscles was greater than the values reported after cross-innervation of the same muscles. The cross-transplantation model has certain advantages over nerve cross-union experiments because the cross-transplanted muscle is placed in the normal functional environment of the other muscle.
The aim of the present study was to throw further light on questions concerning fibre type transformation by studying long-term effects of different types of physical training on the percentages of different fibre types. Four long-distance runners participated in the study. Needle biopsies were taken from vastus lateralis after both training periods. After 'anaerobic' training, all subjects had a lower percentage type I and a higher percentage type II C fibres than after 'aerobic' training. Three subjects also had a higher percentage type II A + II B fibres and an increased II B/II A ratio after 'anaerobic' training. Thus, it seems reasonable to assume that a conversion of type I to type II C fibres had occurred in subjects 1 and 2 due to 'anaerobic' training and that a conversion in the opposite direction had occurred in subjects 3 and 4 due to 'aerobic' training. The type II C fibres might be fibres in transformation from type I to type II A or in the reversed direction.
Maximal oxygen uptake (Vo2 max) was determined in 138 male and 41 female human subjects and muscle fiber composition (gastrocnemius and vastus lateralis) in 53 of the males. Highest values for Vo2 max were 7.38 1 x min-1 and 4.341 x min-1 in males and females, respectively. In relation to body weight the highest values were 94 and 77 ml x (kg x min)-1. Athletes participating in endurance events had very high Vo2 max and predominantly slow twitch (ST) fiber populations whereas weight lifters attained rather low values for Vo2 max and had a higher percentage of fast twitch (FT) fibers. Among subjects with the same fiber composition, Vo2 max was higher in the athletes than in the moderately trained. All groups taken together demonstrated a positive relationship between Vo2 max and the relative number of ST fibers (r = 0.67). For endurance and strength athletes r = 0.72 and for the moderately trained r = 0.34, both correlation coefficients being significant.
We investigated the possible role of extracellular matrix in specifying the expression of superfast myosin during cat jaw muscle regeneration. Equal proportions of muscle tissue from jaw and limb were minced together after killing cellular elements from one source. We allowed the mince to regenerate in the bed of a fast limb muscle. Regenerates were analyzed immunocytochemically at 71 to 294 days after operation. Fibers in control regenerates containing live cells from both sources expressed fast, superfast or slow myosins, or a mixture of these myosins. In regenerates containing only one type of live cells, we detected only myosins appropriate to the live cells. Our results suggest that during regeneration the original extracellular matrix of jaw-closing or limb muscle is unable to specify the expression of superfast or fast myosins, respectively; they point to the cellular elements, probably the satellite cells, as determinants of muscle specificity during regeneration.
1. The slow-twitch soleus muscle and the fast-twitch extensor digitorum longus muscle (EDL) were denervated and stimulated directly with implanted electrodes for 33-82 days. Four different stimulation patterns were used in order to mimic important characteristics of the natural motor-unit activity in these muscles. In addition, to compare the effects of direct stimulation to other experimental models, some EDLs were stimulated through the nerve or cross-innervated by soleus axons. 2. After 33-82 days of stimulation the contractile properties were measured under isometric and isotonic conditions. 3. 'Native' stimulation patterns could maintain normal contractile speed in both EDL and soleus. In the EDL, normal isotonic shortening velocity was maintained only by a stimulation pattern consisting of very brief trains with an initial short interspike interval (doublet), and not by the other 'native' high-frequency patterns. 4. The contractile properties of both EDL and soleus muscles receiving a 'foreign' stimulation pattern were transformed in the direction of the muscle normally receiving that type of activity. The transformations were not complete, and soleus and EDL muscles stimulated with the same stimulation pattern remained different. This suggests that adult muscle fibres in rat EDL and soleus are irreversibly differentiated into different fibre types earlier in development. 5. The three high-frequency stimulation patterns used differed in their ability to change or maintain various contractile properties in the soleus and the EDL. The results indicate that the following qualities of a stimulation pattern might be of importance for the control of contractile properties: instantaneous frequency, total amount of stimulation, train length, interval between trains and presence of an initial doublet. 6. With the exception of the EDL shortening velocity, changes in contractile speed induced by a 'foreign' stimulation pattern were quantitatively similar to the effects of cross-innervation both in the EDL and the soleus. We thus suggest that the change in activity pattern is the mechanism behind most of the changes induced by cross-innervation.
Mitochondrial volume density and lipid droplet-volume density were stereologically determined from electron micrographs of muscle fibers from three hamster muscles: retractor, sartorius, and soleus. The number of capillaries around a fiber, length of capillary-fiber contact, and muscle fiber area were also measured. Glycolytic fibers of the retractor and sartorius had larger cross-sectional areas, lower mitochondrial-volume densities, fewer subsarcolemmal aggregates of mitochondria, and lower capillary-fiber contact length in comparison to oxidative fibers of the retractor and soleus. Values for mitochondrial volume density in the different muscles correlated well (r = 0.97) with resting O2 consumption. The mitochondrial volume densities for each muscle correlated well (r = 0.99) with O2 diffusion coefficients of these muscles. Our results indicate that an analysis of the adequacy of O2 supply to an individual muscle fiber must take into account an interplay among fiber size, percent of the muscle fiber perimeter in contact with a capillary (capillary-fiber contact), and O2 demand of the fiber estimated by mitochondrial volume density.
The purpose of this study was to assess the relationship between muscle fiber type distribution and enzymatic characteristics in sedentary male and female subjects. Muscle biopsy samples from the vastus lateralis muscle of 38 females and 37 males were analyzed to determine the fiber type composition (I, IIa, and IIb), the fiber size, and maximal activities of enzyme markers of energy metabolic pathways. Significant correlations were found (p less than 0.05) between percent fiber type I area and hexokinase (r = -0.39), phosphofructokinase (r = -0.39), lactate dehydrogenase (r = -0.41), and oxoglutarate dehydrogenase (r = 0.33) activities, whereas such correlations with total phosphorylase (r = -0.02), malate dehydrogenase (r = 0.12), and 3-hydroxyacyl CoA dehydrogenase (r = 0.12) activities were not significant. The results of the present study also suggest the presence of a significant but low covariation of less than 30% between the fiber type distribution and muscle enzyme activities. They confirm the presence of an important metabolic heterogeneity independent of the muscle fiber type distribution in sedentary male and female subjects. Moreover, these results indicate that sedentary males exhibit a lower mean value of percent fiber type I and higher glycolytic enzyme activities than sedentary females.
To investigate changes in the ultrastructure of the different muscle fiber types induced by endurance training ten sedentary subjects (five women and five men) were exercised on bicycle ergometers 5 times a week for 30 min. After 6 weeks of training there were significant changes in VO2max (+14%), in the percentage of type I (+12%) and type IIB fibers (-24%) as well as in the volume densities of mitochondria. The latter increased 35% in type I, 55% in type IIA and 35% in type IIB fibers. The relative increase in subsarcolemmal mitochondria was larger than in interfibrillar mitochondria in all fiber types. There was also a significant increase in the volume density of intracellular lipid in type II fibres. It is concluded that high intensity endurance training leads to an enhancement of the oxidative capacity in all muscle fiber types.
1. The characteristics of isometric twitch and tetanic contractions have been determined for normal (N‐EDL, N‐SOL), self‐innervated (S‐EDL, S‐SOL) and cross‐innervated (X‐EDL, X‐SOL) extensor digitorum longus (EDL) and soleus (SOL) muscles of the rat at 35° C. The muscles were then used for biochemical analyses of properties of myosin and actomyosin.
2. The ATPase activities of myosin and actomyosin of X‐EDL decreased to the level of those of N‐SOL or S‐SOL, and the ATPase activities of X‐SOL approached those of N‐EDL or S‐EDL. Of the various ATPase activities, the actin‐ and Mg ²⁺ ‐activated ATPase activity of myosin and the Mg ²⁺ ‐activated ATPase activity of actomyosin showed the highest degree of correlation with the intrinsic speed of shortening of the muscles.
3. Myosin of normal, self‐innervated, and cross‐innervated muscles combined with F‐actin superprecipitated at rates which were proportional to the speed of muscle contraction.
4. The pH profile curve and the ATP‐induced dinitrophenylation reaction revealed that the structure of myosin of X‐EDL was altered to that of N‐SOL or S‐SOL, and the structure of myosin of X‐SOL was modified to that of N‐EDL or S‐EDL.
5. No differences were found in the yield of myosin of normal, self‐innervated, and cross‐innervated extensor digitorum longus and soleus muscles.
A biochemical and histochemical approach was undertaken to study myosin ATPase in mixed human skeletal muscles with different fibre composition and the Ca++ activated ATPase was demonstrated to be related to the percentage of fast fibres.
1. The properties of self‐innervated (S‐EDL, S‐SOL) and cross‐innervated (X‐EDL, X‐SOL) extensor digitorum longus (EDL) and soleus (SOL) muscles have been determined at various times between 25 and 490 days after operations, and these are compared with the properties of normal muscles from unoperated animals of about the same age.
2. The muscle fibres of X‐SOL were 1·15 times longer than fibres of N‐SOL and S‐SOL at about 480 days after operations but the diameter of fibres was the same in the three muscles.
3. The length of muscle fibres was the same in X‐EDL, N‐EDL and S‐EDL 480 days after operations but the fibres of X‐EDL failed to grow in cross‐sectional area after innervation by soleus nerve fibres.
4. The twitch: tetanus ratio was altered transitorily in both X‐EDL and X‐SOL but returned to near normal values within about 300 days after operations.
5. The time courses of isometric contractions and the force: velocity properties were virtually the same in normal and self‐innervated muscles 480 days after operations. The isometric twitch contraction time was about 13 msec for N‐EDL, 25 msec for X‐EDL, 34 msec for N‐SOL and 15 msec for X‐SOL, and the intrinsic speed of shortening of sarcomeres was about 45·1 μ/sec for N‐EDL, 22·5 μ/sec for X‐EDL, 19·8 μ/sec for N‐SOL and 33·8 μ/sec for X‐SOL; in these respects there was incomplete transformation of EDL to a slow muscle and of SOL to a fast muscle.
6. There was a high degree of correlation between the intrinsic speed of shortening of sarcomeres and the isometric twitch contraction time but there was no correlation between isometric twitch contraction time and twitch: tetanus ratio of self‐innervated and cross‐innervated muscles 200–480 days after operations, and normal muscles from unoperated animals of the same age.
7. The relation between intrinsic speed of shortening of sarcomeres and isometric twitch contraction time was approximately hyperbolic for normal, self‐innervated and cross‐innervated EDL and SOL muscles 480 days after operations.
8. The maximum speed of shortening of whole muscle fibres was the same for X‐SOL, S‐EDL and N‐EDL, and the same for X‐EDL, S‐SOL and N‐SOL. The possibility that a neural influence determines the speed of shortening of whole fibres is discussed.
Muscle tissue samples were obtained from vastus lateralis muscle of elite weight/power lifters (WL/PL, n = 8), endurance athletes (EA, n = 8), and nonathletes (NA, n = 8). Histochemical stainings for myofibrillar ATPase, NADH-tetrazolium reductase, and amylase-periodic acid-Schiff, respectively, were undertaken to assess relative distribution of fast-twitch (FT) and slow-twitch (ST) muscle fiber types, fiber size, and capillary supply [capillaries per fiber (cap X fib-1) and capillaries per mm2 (cap X mm-2)]. Fiber type distribution in WL/PL, EA, and NA averaged 59 +/- 6 (SD), 40 +/- 11, and 61 +/- 10% FT. Values for mean fiber area and FT/ST area were significantly greater in WL/PL compared with values obtained in EA and NA. Similar values for cap X fib-1 were observed WL/PL (2.06 +/- 0.74) and NA (2.16 +/- 0.34). EA exhibited greater cap X fib-1 (3.11 +/- 0.73) than WL/PL (NS) and NA (P less than 0.01). However, cap X mm-2 in WL/PL (199 +/- 29) was lower than in EA (401 +/- 61, P less than 0.001) and NA (306 +/- 29, P less than 0.01). It is suggested that heavy resistance training in contrast to endurance training does not result in increased capillary density. Instead, as a consequence of fiber hypertrophy induced by muscle overloading, capillary density is decreased.
The myosin heavy chain (MHC) composition of single fibres from m. vastus lateralis of a group of male sprint athletes (n = 6) was analysed, before and after a three months period of intensive strength- and interval-training, using a sensitive gel electrophoretic technique. Significant improvements were observed after training in almost all of a series of performance tests. After training the sprinters revealed a decrease in fibres containing only MHC isoform I (52.0 +/- 3.0% vs. 41.2 +/- 4.7% (mean +/- SE) (P < 0.05)) and an increase in the amount of fibres containing only MHC isoform IIA (34.7 +/- 6.1% vs. 52.3 +/- 3.6% (P < 0.05)). Fibres showing co-existence of MHC isoforms IIA and IIB decreased with training (12.9 +/- 5.0% vs. 5.1 +/- 3.1% (P < 0.05)). Only one out of 1000 fibres analysed contained only MHC isoform IIB. In contrast, a higher amount of type IIB fibres (18.8 +/- 3.6% vs. 10.5 +/- 3.9%, (P < 0.05)) was observed with myofibrillar ATPase histochemistry. The majority of histochemically determined type IIB fibres of sprinters seems therefore to contain both MHC isoforms IIA and IIB. Sprint-training appears to induce an increased expression of MHC isoform IIA in skeletal muscles. This seems related to a bi-directional transformation from both MHC isoforms I and IIB towards MHC isoform IIA.
We recently reported that 19 wk of heavy resistance training caused a decrease in the percentage of type IIb and an increase in the percentage of type IIa fibers as determined by qualitative histochemical analyses of myofibrillar adenosinetriphosphatase activity of biopsies of musculus vastus lateralis (Hather et al. Acta Physiol. Scand. 143: 177-185, 1991). These data were interpreted to suggest that resistance training had caused transformation among the fast-twitch fiber subtypes. To more clearly establish the influence of resistance training on muscle fiber composition, biopsies from the original study were analyzed biochemically for myosin heavy chain (MHC) composition by use of sodium dodecyl sulfate-polyacrylamide gel electrophoresis and histochemically for fiber types by use of myofibrillar adenosinetriphosphatase activity. The results show that after training (n = 13), IIb MHC composition decreased (P < 0.05) from 19 +/- 4 to 7 +/- 1%. IIa MHC, in contrast, increased (P < 0.05) from 48 +/- 3 to 60 +/- 2%. These responses were essentially mirrored by alterations in fiber type distribution. The percentage of type IIb fibers decreased (P < 0.05) from 18 +/- 3 to 1 +/- 1%, whereas the percentage of type IIa fibers increased from 46 +/- 4 to 60 +/- 3% (P < 0.05). Neither I MHC composition nor type I fiber percentage changed with training. The control group (n = 4) showed no changes in MHC composition or fiber type distribution. These results suggest that heavy resistance training alters MHC composition in human skeletal muscle, presumably reflecting a change in genetic expression.
The contractile characteristics of three human muscle groups (triceps surae, quadriceps femoris and triceps brachii) of seven young male subjects were examined. The contractile properties were determined from electrically evoked isometric responses and compared with fibre type composition determined from needle biopsy samples. Fibre types were identified using myosin heavy chain (MHC) isoforms as molecular markers with gel electrophoresis (SDS-PAGE) and histochemical ATPase staining. Four contractile parameters (twitch time to peak torque, the maximal rate of torque development, frequency response and fatiguability) were found to be related to fibre type composition. From the biopsy samples, single muscle fibres were isolated and chemically skinned. Isometric tension (P
o) unloaded shortening velocity (V
o) and rate of tension rise (dP/dt) were determined. Each fibre was classified on the basis of its MHC isoform composition determined by SDS-PAGE. Fibres belonging to the same type showed identical contractile parameters regardless of the muscle of origin, except minor differences in P
o of the fast fibres and dP/dt of slow fibres. The results are in favour of the conclusion that fibre type composition, determined using MHC isoforms as markers, is the major determinant of the diversity of contractile properties among human muscle groups.
This brief review attempts to summarize a number of studies on the delineation, development, and distribution of human skeletal muscle fiber types. A total of seven fiber types can be identified in human limb and trunk musculature based on the pH stability/lability of myofibrillar adenosine triphosphatase (mATPase). For most human muscles, mATPase-based fiber types correlate with the myosin heavy chain (MHC) content. Thus, each histochemically identified fiber has a specific MHC profile. Although this categorization is useful, it must be realized that muscle fibers are highly adaptable and that innumerable fiber type transients exist. Also, some muscles contain specific MHC isoforms and/or combinations that do not permit routine mATPase-based fiber typing. Although the major populations of fast and slow are, for the most part, established shortly after birth, subtle alterations take place throughout life. These changes appear to relate to alterations in activity and/or hormonal levels, and perhaps later in life, total fiber number. Because large variations in fiber type distribution can be found within a muscle and between individuals, interpretation of data gathered from human muscle is often difficult.
Contractile phenotype of muscle fibres is strongly influenced by hormones, stretch and influences from the motor neurones, although cell lineage probably also plays a role. Motor neurones can affect muscle fibres by releasing neurotrophic substances and by evoking electrical activity in the muscle. For regulating contractile properties such as speed, strength and endurance it has been demonstrated that electrical activity is crucial, while the role of putative neurotrophic substances remains unclear. The signal to change is coded in the pattern of electrical activity. Thus, high amounts of activity lead to slow shortening velocity and myosin heavy chains, while low amounts of activity lead to a fast phenotype. For regulation of twitch duration frequency also plays a role, and for preventing atrophy in denervated muscles high frequency seems to be beneficial, particularly in fast muscles. Little is known about the excitation-adaptation pathway linking action potentials to expression of genes that are relevant for contractile properties. Muscle specific transcription factors of the helix-loop-helix family such as myoD and myogenin could be important for regulating genes related to metabolic profile and fibre size/strength, while their role in determining myosin heavy chain expression and classical fibre type is more uncertain.
The effects of two different duration space-flights on the extent of atrophy, fiber type composition, and myosin heavy chain (MHC) content of rat soleus muscles were compared. Adult male Fisher rats (n=12) were aboard flight STS-57 and exposed to 10 days of microgravity and adult ovariectomized female Spraque-Dawley rats (n=12) were aboard flight STS-62 for 14 days. Soleus muscles were bilaterally removed from the flight and control animals and frozen for subsequent analyses. Muscle wet weights, fiber types (I, IC, IIC, and IIA), cross-sectional area, and MHC content were determined. Although a significant difference was found between the soleus wet weights of the two ground-based control groups, they were similar with regard to MHC content (ca 90% MHCI and ca 10% MHCIIa) and fiber type composition. Unloading of the muscles caused slow-to-fast transformations which included a decrease in the percentage of type I fibers and MHCI, an increase in fibers classified as type IC, and the expression of two fast myosin heavy chains not found in the control rat soleus muscles (MHCIId and MHCIIb). Although the amount of atrophy (ca 26%) and the extent of slow-to-fast transformation (decrease in the percentage of MHCI from 90% to 82.5%) in the soleus muscles were similar between the two spaceflights, the percentages of the fast MHCs differed. After 14 days of spaceflight, the percentage of MHCIIa was significantly lower and the percentages of MHCIId and MHCIIb were significantly higher than the corresponding MHC content of the soleus muscles from the 10-day animals. Indeed, MHCIId became the predominant fast MHC after 14 days in space. These data suggest fast-to-faster transformations continued during the longer spaceflight.
Denervated fast extensor digitorum longus (EDL) muscles of adult rats were stimulated electrically for up to 4 months with a ‘slow’ pattern resembling the activity in soleus (Sol) motor units and examined with antibodies against myosin heavy chains (MHCs).
The normal EDL contained, on average, 45 % type IIB, 29 % type IIX, 23 % type IIA and 3 % type I fibres. All type IIB and almost all type IIX fibres disappeared during the first 3 weeks of stimulation. They were replaced by type IIA and type I fibres, whose percentages increased to about 75 and 15, respectively. Type IIA fibres remained at 75 % for nearly 2 months and were then gradually replaced by type I fibres during the next 2 months. The transformation occurred sequentially in the order IIB/IIX → IIA → I, the first step (IIB/IIX → IIA) occurring after a short delay (2 weeks) and the last step (IIA → I in originally IIB or IIX fibres) after a long delay (> 2 months). During the transformation coexpression of MHCs occurred.
It appears that the transformation to type I fibres occurred in pre‐existing type II fibres since no signs of fibre damage or regeneration were observed.
Normal EDL was also stimulated through an intact nerve with the same pattern for up to 37 days. The effects on fibre type distributions were identical to those observed in the denervated EDL. The result indicated that the Sol‐like pattern of evoked muscle activity, rather than nerve‐derived trophic influences or denervation per se , was primarily responsible for the fast to slow transformation.
The purpose of this study was to investigate the relationship between myosin heavy chain (MHC) composition and maximal contraction strength of the human quadriceps femoris muscle.
Muscle biopsies were obtained from m. vastus lateralis in your highly physical active males (N = 7). The MHC composition of muscle homogenates was determined by electrophoresis techniques (SDS-PAGE). Isokinetic peak torque and constant-angle torque (50 degrees knee flexion) were obtained during slow (30 degrees.s-1), medium (120 degrees.s-1), and fast (240 degrees.s-1) maximal concentric and eccentric quadriceps contractions and expressed relative to muscle volume.
The percentage of MHC II in the quadriceps muscle was positively correlated (rs = 0.61-0.93; P < 0.05-0.01) to maximal concentric quadriceps strength obtained at medium to high knee angular velocity. In contrast, no consistent pattern of correlation was observed for maximal eccentric quadriceps strength.
The relationship observed between muscular MHC composition and maximal contractile strength is suggested to appear as a consequence of MHC -related differences in contractile force-velocity characteristics and/or contractile Rate of Force Development (RFD).
Seventeen subjects performed resistance training of the leg extensor and flexor muscle groups two (2/wk) or three (3/wk) times per week. Changes in the relative myosin heavy chain (MHC) isoform contents (I, IIa and IIx) of the vastus lateralis and isometric, isokinetic and squat-lift one-repetition maximum (1 RM) strength were compared between conditions after both a common training period (6 weeks) and number of training sessions (18). After 6 weeks and 18 sessions (9 weeks for the 2/wk group), increments in 1RM strength for the 3/wk and 2/wk groups were similar [effect size (ES) differences approximately 0.3, 3/wk > 2/wk], whereas the 2/wk group presented greater isokinetic (ES differences = 0.3-1.2) and isometric (ES differences approximately 0.7) strength increases than the 3/wk condition. A significant (P < 0.05) increase in MHC IIa percentage was evident for the 2/wk group after 18 sessions. Both training groups exhibited a trend towards a reduction in the relative MHC IIx and an increase in MHC IIa contents (ES range = 0.5-1.24). However, correlations between changes in the strength and MHC profiles were weak (r2: 0.0-0.5). Thus, isometric and isokinetic strength responses to variations in training frequency differed from 1RM strength responses, and changes in strength were not strongly related to alterations in relative MHC content.
Skeletal muscle is an extremely heterogeneous tissue composed of a variety of fast and slow fiber types and subtypes. Moreover, muscle fibers are versatile entities capable of adjusting their phenotypic properties in response to altered functional demands. Major differences between muscle fiber types relate to their myosin complement, i.e., isoforms of myosin light and heavy chains. Myosin heavy chain (MHC) isoforms appear to represent the most appropriate markers for fiber type delineation. On this basis, pure fiber types are characterized by the expression of a single MHC isoform, whereas hybrid fiber type express two or more MHC isoforms. Hybrid fibers bridge the gap between the pure fiber types. The fiber population of skeletal muscles, thus, encompasses a continuum of pure and hybrid fiber types. Under certain conditions, changes can be induced in MHC isoform expression heading in the direction of either fast-to-slow or slow-to-fast. Increased neuromuscular activity, mechanical loading, and hypothyroidism are conditions that induce fast-to-slow transitions, whereas reduced neuromuscular activity, mechanical unloading, and hyperthyroidism cause transitions in the slow-to-fast direction.
The goal of this mini-review is to summarize findings concerning the role that different models of muscular activity and inactivity play in altering gene expression of the myosin heavy chain (MHC) family of motor proteins in mammalian cardiac and skeletal muscle. This was done in the context of examining parallel findings concerning the role that thyroid hormone (T(3), 3,5,3'-triiodothyronine) plays in MHC expression. Findings show that both cardiac and skeletal muscles of experimental animals are initially undifferentiated at birth and then undergo a marked level of growth and differentiation in attaining the adult MHC phenotype in a T(3)/activity level-dependent fashion. Cardiac MHC expression in small mammals is highly sensitive to thyroid deficiency, diabetes, energy deprivation, and hypertension; each of these interventions induces upregulation of the beta-MHC isoform, which functions to economize circulatory function in the face of altered energy demand. In skeletal muscle, hyperthyroidism, as well as interventions that unload or reduce the weight-bearing activity of the muscle, causes slow to fast MHC conversions. Fast to slow conversions, however, are seen under hypothyroidism or when the muscles either become chronically overloaded or subjected to intermittent loading as occurs during resistance training and endurance exercise. The regulation of MHC gene expression by T(3) or mechanical stimuli appears to be strongly regulated by transcriptional events, based on recent findings on transgenic models and animals transfected with promoter-reporter constructs. However, the mechanisms by which T(3) and mechanical stimuli exert their control on transcriptional processes appear to be different. Additional findings show that individual skeletal muscle fibers have the genetic machinery to express simultaneously all of the adult MHCs, e.g., slow type I and fast IIa, IIx, and IIb, in unique combinations under certain experimental conditions. This degree of heterogeneity among the individual fibers would ensure a large functional diversity in performing complex movement patterns. Future studies must now focus on 1) the signaling pathways and the underlying mechanisms governing the transcriptional/translational machinery that control this marked degree of plasticity and 2) the morphological organization and functional implications of the muscle fiber's capacity to express such a diversity of motor proteins.
Skeletal muscle is a complex, versatile tissue composed of a large variety of functionally diverse fiber types. The overall properties of a muscle largely result from a combination of the individual properties of its different fiber types and their proportions. Skeletal muscle fiber types, which can be delineated according to various parameters, for example, myofibrillar protein isoforms, metabolic enzyme profiles, and structural and contractile properties, are not fixed units but are capable of responding to altered functional demands and a variety of signals by changing their phenotypic profiles. This brief review summarizes our current understanding of the delineation of fiber types, modulations of their phenotypic profiles as induced under various conditions, and potential mechanisms involved in these transitions.
We compared changes in muscle fibre composition and muscle strength indices following a 10 week isokinetic resistance training programme consisting of fast (3.14 rad x s(-1)) or slow (0.52 rad x s(-1)) velocity eccentric muscle contractions. A group of 20 non-resistance trained subjects were assigned to a FAST (n = 7), SLOW (n = 6) or non-training CONTROL (n = 7) group. A unilateral training protocol targeted the elbow flexor muscle group and consisted of 24 maximal eccentric isokinetic contractions (four sets of six repetitions) performed three times a week for 10 weeks. Muscle biopsy samples were obtained from the belly of the biceps brachii. Isometric torque and concentric and eccentric torque at 0.52 and 3.14 rad x s(-1) were examined at 0, 5 and 10 weeks. After 10 weeks, the FAST group demonstrated significant [mean (SEM)] increases in eccentric [29.6 (6.4)%] and concentric torque [27.4 (7.3)%] at 3.14 rad x s(-1), isometric torque [21.3 (4.3)%] and eccentric torque [25.2 (7.2)%] at 0.52 rad x s(-1). The percentage of type I fibres in the FAST group decreased from [53.8 (6.6)% to 39.1 (4.4)%] while type IIb fibre percentage increased from [5.8 (1.9)% to 12.9 (3.3)%; P < 0.05]. In contrast, the SLOW group did not experience significant changes in muscle fibre type or muscle torque. We conclude that neuromuscular adaptations to eccentric training stimuli may be influenced by differences in the ability to cope with chronic exposure to relatively fast and slow eccentric contraction velocities. Possible mechanisms include greater cumulative damage to contractile tissues or stress induced by slow eccentric muscle contractions.
Human skeletal Muscle is composed of a heterogenous collection of muscle fiber types.(1-3) This range of muscle fiber types allows for the wide variety of capabilities that human muscles display. In addition, muscle fibers can adapt to changing demands by changing size or fiber type composition. This plasticity serves as the physiologic basis for numerous physical therapy interventions designed to increase a patient's force development or endurance. Changes in fiber type composition also may be partially responsible for some of the impairments and disabilities seen in patients who are deconditioned because of prolonged inactivity, limb immobilization, or muscle denervation.(2) Over the past several decades, the number of techniques available for classifying muscle fibers has increased, resulting in several classification systems. The objective of this update is to provide the basic knowledge necessary, to read and interpret research on human skeletal muscle. Muscle fiber types can be described using histochemical, biochemical, morphological, or physiologic characteristics; however, classifications of muscle fibers by different techniques do not always agree.(1) Therefore, Muscle fibers that may be grouped together by one classification technique may be placed in different categories using a different classification technique. A basic understanding of muscle structure and physiology is necessary to understand the muscle fiber classification techniques.
We performed gene screening of the ciliary neurotrophic factor receptor (CNTFR) gene and genotyped three newly identified polymorphisms: C-1703T in the 5' promoter region, T1069A in intron 5, and C174T in exon 9. We studied the association of these CNTFR variants with muscle strength, mass, and body composition in 465 men and women (20-90 yr) from the Baltimore Longitudinal Study of Aging. Only the C174T variant was significantly associated with muscle-related phenotypes. In the entire cohort, when corrected for age, sex, race, physical activity, and height, homozygotes for the common C allele at C174T (CC) exhibited lower total body mass and body mass index than carriers of the rare T allele, which appeared to be due to significant differences in total nonosseous fat-free mass (FFM) (48.0 +/- 0.4 vs. 50.0 +/- 0.7 kg; P = 0.011) and lower limb FFM (16.5 +/- 0.1 vs. 17.2 +/- 0.2 kg; P = 0.002). The CC group also exhibited significantly lower quadriceps concentric and eccentric isokinetic strength values at both 30 and 180 degrees /s than the T allele carriers (all P < 0.04), but these differences were no longer significant after adjustment for lower limb FFM. There were no significant sex-by-genotype interactions. The results indicate that the C174T polymorphism in exon 9 of CNTFR is significantly associated with FFM in men and women, with concomitant differences in muscular strength.
Biopsies fro the vastus lateralis muscle of male weightlifters (WL; n=6; X +/- SE, age=27.0 +/- 2.1 years), and non-weight-trained men (CON; n=7; age=27.0 +/- 2.0 years) were compared for fiber types, myosin heavy chain (MHC) and titin content, and fiber type-specific capillary density. Differences (p<0.05) were observed for percent fiber types IIC (WL=0.4 +/- 0.2, CON=2.4 +/- 0.8); IIA (WL=50.5 +/- 3.2, CON=26.9 +/- 3.7); and IIB (WL=1.7 +/- 1.4, CON=21.0 +/- 5.3), as well as percent MHC IIa (WL=65.3 +/- 2.4, CON=52.1 +/- 4.2) and percent MHC IIB (WL=0.9 +/- 0.9; CON=18.2 +/- 6.1). All WL exhibited only the titin-1 isoform. Capillary density (caps.mm(-2)) for all fiber types combined was greater for the CON subjects (WL=192.7 +/- 17.3; CON=262.9 +/- 26.3), due primarily to a greater capillary density in the IIA fibers. Weightlifting performances and vertical jump power were correlated with type II fiber characteristics. These results suggest that successful weightlifting performance is not dependent on IIB fibers, and that weightlifters exhibit large percentages of type IIA muscle fibers and MHC IIa isoform content.
Regenerative capacity appears to be impaired in sarcopenic muscle. As local growth factors and myogenic regulatory factors (MRFs) modulate repair/regeneration responses after overload, we hypothesized that resistance loading (RL)-induced expression of MRFs and muscle IGF-I-related genes would be blunted in older (O) males (M) and females (F) with demonstrable sarcopenia vs. young (Y) adults. Y (20-35 yr, 10 YF, 10 YM) and O (60-75 yr, 9 OF, 9 OM) underwent vastus lateralis biopsy before and 24 h after knee extensor RL. Sarcopenia was assessed by cross-sectional area of type I, IIa, and IIx myofibers. Transcript levels were assessed by relative RT-PCR and analyzed by age x gender x load repeated-measures ANOVA. O were sarcopenic based on type II atrophy with smaller type IIa (P < 0.05) and IIx (P < 0.001) myofibers. Within-gender cross-sectional area differences were more marked in F (OF < YF: IIa 21%, IIx 42%). Load effects (P < 0.05) were seen for four of seven mRNAs as IGF-IEa (34%), myogenin (53%), and MyoD (20%) increased, and myf-6 declined 10%. Increased IGF-IEa was driven by O (48%) and/or M (43%). An age x gender x load interaction was found for MyoD (P < 0.05). An age x load interaction for type 1 IGF receptor (P < 0.05) was driven by a small increase in O (16%, P < 0.05). A gender x load interaction (P < 0.05) was noted for IGF binding protein-4. Age effects (P < 0.05) resulted from higher MyoD (54%), myf-5 (21%), and IGF binding protein-4 (17%) in O and were primarily localized to F at baseline (OF > YF; MyoD 94%, myf-5 47%, P < 0.05). We conclude that RL acutely increases mRNA expression of IGF-IEa and myogenin, which may promote growth/regeneration in both Y and O. Higher resting levels of MRFs in OF vs. YF suggest elevated basal regenerative activity in sarcopenic muscle of OF.
Resistance training (RT) has shown the most promise in reducing/reversing effects of sarcopenia, although the optimum regime specific for older adults remains unclear. We hypothesized myofiber hypertrophy resulting from frequent (3 days/wk, 16 wk) RT would be impaired in older (O; 60-75 yr; 12 women, 13 men), sarcopenic adults compared with young (Y; 20-35 yr; 11 women, 13 men) due to slowed repair/regeneration processes. Myofiber-type distribution and cross-sectional area (CSA) were determined at 0 and 16 wk. Transcript and protein levels of myogenic regulatory factors (MRFs) were assessed as markers of regeneration at 0 and 24 h postexercise, and after 16 wk. Only Y increased type I CSA 18% (P < 0.001). O showed smaller type IIa (-16%) and type IIx (-24%) myofibers before training (P < 0.05), with differences most notable in women. Both age groups increased type IIa (O, 16%; Y, 25%) and mean type II (O, 23%; Y, 32%) size (P < 0.05). Growth was generally most favorable in young men. Percent change scores on fiber size revealed an age x gender interaction for type I fibers (P < 0.05) as growth among Y (25%) exceeded that of O (4%) men. Myogenin and myogenic differentiation factor D (MyoD) mRNAs increased (P < 0.05) in Y and O, whereas myogenic factor (myf)-5 mRNA increased in Y only (P < 0.05). Myf-6 protein increased (P < 0.05) in both Y and O. The results generally support our hypothesis as 3 days/wk training led to more robust hypertrophy in Y vs. O, particularly among men. However, this differential hypertrophy adaptation was not explained by age variation in MRF expression.