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# High Efficiency of Type I Muscle Fibers Improves Performance

Authors:
• Rutgers, The State University of New Jersey, USA - Harokopio University, Athens, Greece

## Abstract

We have recently demonstrated that people with a high percentage of Type I muscle fibers display a relatively high muscular efficiency when cycling. These individuals generate a relatively high muscular power output at a given steady-state level of oxygen consumption and caloric expenditure. The purpose of this study was to directly determine the extent to which differences in muscle fiber composition and efficiency influence endurance performance in competitive cyclists. The percentage of Type I and II muscle fibers was determined from several biopsies from the vastus lateralis which were histochemically stained for ATPase activity. During a laboratory performance test, 14 endurance trained cyclists (mean +/- SE; VO2max, 5.2 +/- 0.11/min; body weight, 74 +/- 1 kg) cycled an ergometer for 1 h at the highest work rate they could tolerate. VO2 and RER were simultaneously measured using open circuit spirometry for calculating caloric expenditure. Subjects were divided into two groups of seven according to their muscle fiber type composition: High % Type I Group (> 56% Type I fibers); Normal % Type I Group (38-55% Type I fibers). Each subject from High % Type I Group was paired with a subject from the Normal % Type I Group according to their similarity in VO2max, blood lactate threshold and average VO2 maintained during the 1 h performance test. Both groups averaged 4.5 +/- 0.11/min during the 1 h performance test (i.e., 86-88% VO2max).(ABSTRACT TRUNCATED AT 250 WORDS)
... This general principle in muscle physiology should also be key in prolonged, high-intensity cycling. Accordingly, there is strong evidence for differences in muscle morphology between high and low experienced cyclists [2][3][4][5]. World class cyclists have an exceptionally high economy of motion [6][7][8], which is usually determined by calculation of the gross efficiency (GE; S1 Fig), the ratio between mechanical output and energy expenditure [9]. Previous work also established an association between the proportion of type I fibers in leg muscles (e. g. ...
... Previous work also established an association between the proportion of type I fibers in leg muscles (e. g. VL) and GE in elite athletes [3,9], which is very likely a result of the lower energy demands of type I fibers compared to type II fibers [10]. On the other hand, oxygen consumption can be reduced by more effective transmission of force to the pedals, known as index of force effectiveness (IFE) [11]. ...
... Apparently, highly trained athletes can innervate their large hip flexors and extensors stronger and earlier in high-intensity cycling, resulting in reduced force load per muscle cross-section [5] and in adequate stimuli for the development of a muscle fiber structure optimized for endurance loads. These assumptions are supported by the findings that highly endurance-trained cyclists have a higher FT I fiber content in the VL [3,9] and that the six hip flexors and knee extensors (iliacus, psoas, RF, VI, VL, and VM) account for approximately 33% of the total lower limb muscle mass (35 major muscles) [53]. ...
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In cycling, propulsion is generated by the muscles of the lower limbs and hips. After the first reports of pedal/crank force measurements in the late 1960s, it has been assumed that highly trained athletes have better power transfer to the pedals than recreational cyclists. However, motor patterns indicating higher levels of performance are unknown. To compare leg muscle activation between trained (3.5–4.2 W/kgbw) and highly trained (4.3–5.1 W/kgbw) athletes we applied electromyography, lactate, and bi-pedal/crank force measurements during a maximal power test, an individual lactate threshold test and a constant power test. We show that specific activation patterns of the rectus femoris (RF) and vastus lateralis (VL) impact on individual performance during high-intensity cycling. In highly trained cyclists, we found a strong activation of the RF during hip flexion. This results in reduced negative force in the fourth quadrant of the pedal cycle. Furthermore, we discovered that pre-activation of the RF during hip flexion reduces force loss at the top dead center (TDC) and can improve force development during subsequent leg extension. Finally, we found that a higher performance level is associated with earlier and more intense coactivation of the RF and VL. This quadriceps femoris recruitment pattern improves force transmission and maintains propulsion at the TDC of the pedal cycle. Our results demonstrate neuromuscular adaptations in cycling that can be utilized to optimize training interventions in sports and rehabilitation.
... Gross efficiency (GE) is the ratio of work generated to the total metabolic energy cost (Ettema & Loras, 2009;Horowitz et al. 1994;Jobson et al. 2012) and has been reported to explain 30% of the variation in PO during a cycling time-trial (Jobson et al. 2012 When comparing time-trial performance in two groups with similar VȮ2max, Coyle et al. (1991) found that the cyclists with a higher LT were able to generate 11% more power during a 1-hour laboratory time-trial, which in turn correlated with a 10% higher velocity during an actual 40-km road time-trial. ...
... It was suggested the reason the cyclists were able to produce a higher PO was because they generated more power per pedal revolution for a lower metabolic cost. In accordance with the findings of Coyle et al. (1991), Horowitz et al. (1994) showed that cyclists with a higher GE could maintain a 9% higher PO during a 1hour time-trial, despite similar VȮ2 values. However, the effect of cycling position on GE was not examined in these studies (Coyle et al. 1991;Horowitz et al. 1994). ...
... In accordance with the findings of Coyle et al. (1991), Horowitz et al. (1994) showed that cyclists with a higher GE could maintain a 9% higher PO during a 1hour time-trial, despite similar VȮ2 values. However, the effect of cycling position on GE was not examined in these studies (Coyle et al. 1991;Horowitz et al. 1994). , GE was found to be significantly reduced following time-trials of varying durations. ...
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To investigate the physiological and metabolic effects of different torso angles (TA; while systematically controlling the aerodynamic time-trial position; AP), during submaximal exercise and self-paced time-trial efforts. Twelve participants completed four visits to the laboratory: Visit 1 being an incremental exercise test to identify power at maximal pulmonary oxygen uptake (PV?O2max) and Visits 2 to 4 being 20-minute time-trials with pre and post gross efficiency (GE) tests, performed at three different TAs (0o, 12o, 24o). GE was significantly reduced at the 0o TA, when compared to the 24o TA (P = 0.039). GE was significantly lower after the time-trials when compared to Pre GE (P < 0.001). There was no significant difference in the magnitude of decline in GE between TA. Combined data from all TA revealed a significant positive correlation between GE and mean time-trial power output (PO; R = 0.337; R2 = 0.114; P = 0.044). Mean time-trial PO was significantly higher at the 24o TA, when compared to the 12o TA (P = 0.012) and 0o TA (P = 0.007). There was a significant positive correlation between relative TA and mean time-trial PO (R = 0.374; R2 = 0.140; P = 0.025). GE declines during time-trial exercise, while lower TAs do not further exacerbate the magnitude of decline in GE. Lowering TA results in a reduction in physiological and metabolic performance at submaximal and time-trial intensity. There remains a trade-off between physiological functioning and aerodynamic drag.
... Viral infections have been shown to lower the thresholds in the dorsal horns of the spinal column (Melzack & Wall 1965), creating a shift from the use of Type-1 muscle fibers to Type-2 muscle fibers (Horowitz et al 1994). Since ‗Long COVID' is seen primarily as a disease of blood vessels and with little or no data to reference, it seems reasonable to suggest there could be a relation between the shift from Type-1 muscle fibers towards Type-2 fibers since Type-1 fibers depend more on blood supply (RIZIV 2021). ...
... Teaching the movement sequences from the YESBODY® program is based on optimizing the readiness potentials as described by Benjamin Libet (Libet 1985) and Daniel Wegner (Wegner 2003). This means that a specific tool, imagining (Nicholson et al 2019), is important to prime the energy-efficient Type-1 muscle fibers (Horowitz et al 1994). ...
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A similarity of symptoms and dysfunction is clinically observed between Long COVID-19 patients and patients with autonomic dysfunctions, chronic fatigue syndrome and fibromyalgia. The content of the proposed bio-psychosocial case management rehabilitation program is based on the muscle fiber type composition, a novel policy aimed at priming Type-1 muscle fibers, the energy-saving function of the ligamentum nuchae, and the influence of viruses on movement patterns. The start of the program addresses how to move safely, utilizing strategies involving neuromuscular efficiency, while later, moving more. If needed, getting stronger is the third step. These three steps are aimed at structural recovery, improving quality of life and social cohesion. Getting back to work and normal lifestyle is always the end goal that is kept in mind, depending on the age of the patient. Patients are re-evaluated using physical functions of some global, regional, and local anatomical areas. The inclusion and exclusion criteria at the intake are based on objectification of physical functions and some questionnaires. The program can be offered to individuals or in a group, either by direct interaction or by video call.
... While mechanical power output is a valuable measure, ultimately, endurance cycling performance is determined by metabolic power (Horowitz et al. 1994;Passfield and Doust 2000). The ratio between metabolic power and mechanical power, i.e., gross efficiency (GE) is not constant as it depends (among other factors) on cadence. ...
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Purpose With few cycling races on the calendar in 2020 due to COVID-19, Everesting became a popular challenge: you select one hill and cycle up and down it until you reach the accumulated elevation of Mt. Everest (8,848 m or 29,029ft). With an almost infinite number of different hills across the world, the question arises what the optimal hill for Everesting would be. Here, we address the biomechanics and energetics of up- and downhill cycling to determine the characteristics of this optimal hill. Methods During uphill cycling, the mechanical power output equals the power necessary to overcome air resistance, rolling resistance, and work against gravity, and for a fast Everesting time, one should maximize this latter term. To determine the optimal section length (i.e., number of repetitions), we applied the critical power concept and assumed that the U-turn associated with an additional repetition comes with a 6 s time penalty. Results To use most mechanical power to overcoming gravity, slopes of at least 12% are most suitable, especially since gross efficiency seems only minimally diminished on steeper slopes. Next, we found 24 repetitions to be optimal, yet this number slightly depends on the assumptions made. Finally, we discuss other factors (fueling, altitude, fatigue) not incorporated in the model but also affecting Everesting performances. Conclusion For a fast Everesting time, our model suggests to select a hill climb which preferably starts at (or close to) sea level, with a slope of 12–20% and length of 2–3 km.
... However, current research is not clear, and there are mixed findings between muscle fiber type and RE (Bosco et al., 1987;Kaneko, 1990;Heikki Kyrolainen et al., 2003;Williams & Cavanagh, 1987). The relationship between fiber type and efficiency seems to be clearer in cycling, with studies showing strong correlations between greater efficiency and greater percentage of type I muscle fibers (Coyle et al., 1992;Horowitz et al., 1994). RE can be improved through several training interventions (Barnes & Kilding, 2015a). ...
Thesis
The objectives of this thesis were to investigate the performance determinants of trail running, and to evaluate the changes in running economy following prolonged endurance running exercise. First, we tested elite road and trail runners for differences in performance factors. Our results showed that elite trail runners are stronger than road runners, but they have greater cost of running when running on flat ground. In the second study, we evaluated the performance factors that predicted performance in trail running races of different distances, ranging from 40 to 170 km. We found that maximal aerobic capacity was a determinant factor of performance for races up to 100 km. Performance in shorter races, up to approximately 55 km, was also predicted by lipid utilization at slow speed, while performance in the 100 km race was also predicted by maximal strength and body fat percentage. The most important factors of performance for races longer than 100 km are still debated. We also tested the effects of trail running race distance on cost of locomotion, finding that cost of running increased after races up to 55 km, but not after races of 100-170 km. Finally, we tested the. effects of two different exercise modalities, cycling and running, on cost of locomotion, after 3 hours of intensity-matched exercise. Cost of locomotion increased more following cycling than running, and the change in cost of locomotion was related to changes in cadence and loss of force production capacity.
... However, it was interesting to note that NAC may have lessened the slope of the decline in gross efficiency during the fixed-intensity cycling bout. While not clearly different from a statistical standpoint, with the 95% CI just including zero (i.e., 0.001), the difference in gross efficiency between NAC and Placebo after 40 min of cycling was 0.48% (i.e., 0.012% × 40 min), which could equate to meaningful performance improvement during a longer task, such as a 2-h performance test (Horowitz et al. 1994). Indeed, the bike dimensions, crank length, cadence and other environmental factors (e.g., temperature) that can alter the metabolic cost of cycling were controlled for within the study, thus supporting this to be a supplement effect. ...
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Purpose To compare physiological responses to submaximal cycling and sprint cycling performance in women using oral contraceptives (WomenOC) and naturally cycling women (WomenNC) and to determine whether N-acetylcysteine (NAC) supplementation mediates these responses. Methods Twenty recreationally trained women completed five exercise trials (i.e., an incremental cycling test, a familiarisation trial, a baseline performance trial and two double-blind crossover intervention trials). During the intervention trials participants supplemented with NAC or a placebo 1 h before exercise. Cardiopulmonary parameters and blood biochemistry were assessed during 40 min of fixed-intensity cycling at 105% of gas-exchange threshold and after 1-km cycling time-trial. Results WomenOC had higher ventilation (β [95% CI] = 0.07 L·min⁻¹ [0.01, 0.14]), malondialdehydes (β = 12.00 mmol·L⁻¹ [6.82, 17.17]) and C-reactive protein (1.53 mg·L⁻¹ [0.76, 2.30]), whereas glutathione peroxidase was lower (β = 22.62 mU·mL⁻¹ [− 41.32, − 3.91]) compared to WomenNC during fixed-intensity cycling. Plasma thiols were higher at all timepoints after NAC ingestion compared to placebo, irrespective of group (all p < 0.001; d = 1.45 to 2.34). For WomenNC but not WomenOC, the exercise-induced increase in malondialdehyde observed in the placebo trial was blunted after NAC ingestion, with lower values at 40 min (p = 0.018; d = 0.73). NAC did not affect cycling time-trial performance. Conclusions Blood biomarkers relating to oxidative stress and inflammation are elevated in WomenOC during exercise. There may be an increased strain on the endogenous antioxidant system during exercise, since NAC supplementation in WomenOC did not dampen the exercise-induced increase in malondialdehyde. Future investigations should explore the impact of elevated oxidative stress on exercise adaptations or recovery from exercise in WomenOC.
... The positive effects of PBMT on muscle endurance performance in single-joint exercises and time to exhaustion performance in cycling may be explained by a decreased rate of muscle fatigue development [4,18]. Muscle endurance and time to exhaustion performance are strictly related to the recruitment of slow-twitch motor units (i.e., type I muscle fibres) [64,65]. In these muscle fibres, the oxidative pathway is the main ATP source for the processes involved in tissue contraction [66]. ...
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Background Photobiomodulation therapy (PBMT) is defined as non-thermal electromagnetic irradiation through laser or light-emitting diodes sources. In recent decades, PBMT has attracted attention as a potential pre-conditioning method. The current meta-analysis was conducted to assess the effectiveness of PBMT in improving mode-specific exercise performance in healthy young adults. Methods A computerized literature search was conducted, ending on 15 May 2022. The databases searched were PubMed, Cochrane Central Register of Controlled Trials, Embase, SPORTDiscus, and Physiotherapy Evidence Database. Inclusion/exclusion criteria limited articles to crossover, double-blind, placebo-controlled studies investigating the PBMT effects as a pre-conditioning method. The included trials were synthesised according to exercise mode (single-joint, cycling, running, and swimming). All results were combined with the standardized mean differences (SMDs) method and the 95% confidence intervals (95% CI) were described. Results A total of 37 individual studies, employing 78 exercise performance measurements in 586 participants, were included in the analyses. In single-joint exercises, PBMT improved muscle endurance performance (SMD = 0.27, 95% CI = 0.12 to 0.41; p < 0.01) but not muscle strength performance (p = 0.92). In cycling, PBMT improved time to exhaustion performance (SMD = 0.35; 95% CI = 0.10 to 0.59; p < 0.01), but had no effect on all-out sprint performance (p = 0.96). Similarly, PBMT had no effect on time to exhaustion (p = 0.10), time-trial (p = 0.61), or repeated-sprint (p = 0.37) performance in running, and no effect on time-trial performance in swimming (p = 0.81). Conclusion PBMT improves muscle endurance performance in single-joint exercises and time to exhaustion performance in cycling, but it is not effective for muscle strength performance in single-joint exercises, running, or swimming performance metrics.
... low MFO). A worse fiber type I/type II ratio is indeed related to poorer efficiency (Horowitz et al., 1994). Nevertheless, the more powerful and active old women in our study, as reflected by the association between muscle power and IPAQ, got also the greater CHOox peak , what could point to a larger preservation of type II fibers in these active women (Phu et al., 2015). ...
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Purpose: Aging deteriorates metabolic flexibility (MF). Moreover, recent studies show that glycolysis is barely increased despite impoverished lipid metabolism, in addition to increased relevance of muscle power in older adults. This study aims to analyze MF, i.e., fat and carbohydrates oxidation rates (FATox and CHOox), and the point of maximal fat oxidation (MFO), in a group of active women over-60. It also aims to delve into the role of power production and mechanical efficiency regarding MF. This will help to decipher their metabolic behavior in response to increasing intensity. Methods: Twenty-nine women (66.13 ± 5.62 years) performed a submaximal graded cycling test, increasing 10 W each 3-min15-s, from 30 W to the second ventilatory threshold (VT 2 ). Muscle power was adjusted with a Saris-H3 roller, together with a continuous gas analysis by indirect calorimetry (Cosmed K4b2). Pre and post-test blood lactate (BLa) samples were included. Frayn’s equations, MFO and CHOox peak (mg/min/kg FFM) were considered for MF analysis (accounting for average VO 2 and VCO 2 in each last 60-s), whilst delta and gross efficiencies (DE%, GE%), and exercise economy (EC), were added for Mechanical Efficiency. Mean comparisons regarding intensities 60, 80 and 100% at VT 2 , completed the study together with correlation analysis among the main variables. Results: MFO and CHOox peak were small (6.35 ± 3.59 and 72.79 ± 34.76 g/min/kgFFM respectively) for a reduced muscle power (78.21 ± 15.84 W). Notwithstanding, GE% and EC increased significantly ( p < 0.01) with exercise intensity. Importantly, coefficients of variation were very large confirming heterogeneity. Whilst muscle power outcomes correlated significantly ( p < 0.01) with MFO ( r = 0.66) and age ( r = −0.62), these latter failed to be associated. Only GE% correlated to CHOox peak ( r = −0.61, p < 0.01) regarding mechanical efficiency. Conclusions: Despite being active, women over-60 confirmed impaired substrates switching in response to exercise, from both FAT and CHO pathways. This limits their power production affecting exercise capacity. Our data suggest that decreased power with age has a key role above age per se in this metabolic inflexibility. Vice versa , increasing power seems to protect from mitochondrial dysfunction with aging. New studies will confirm if this higher efficiency when coming close to VT 2 , where GE is the more informative variable, might be a protective compensatory mechanism.
... Pedaling rate or pedal speed (ie, linear speed of the pedal spindle) and mechanical power output account for 99% of metabolic cost during submaximal cycling. [1][2][3] This association of pedaling rate with metabolic cost suggests that manipulation of pedal speed within the pedal cycle might facilitate reduced metabolic cost, thereby improving cycling efficiency and performance. Noncircular chainrings with their eccentricity (ratio of major-tominor axes) ( Figure 1) can alter instantaneous crank angular velocity 4 and thereby pedal speed. ...
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Pedal speed and mechanical power output account for 99% of metabolic cost during submaximal cycling. Noncircular chainrings can alter instantaneous crank angular velocity and thereby pedal speed. Reducing pedal speed during the portion of the cycle in which most power is produced could reduce metabolic cost and increase metabolic efficiency. Purpose: To determine the separate contributions of pedal speed and chainring shape/eccentricity to the metabolic cost of producing power and evaluate joint-specific kinematics and kinetics during submaximal cycling across 3 chainring eccentricities (CON = 1.0; LOW = 1.13; HIGH = 1.24). Methods: Eight cyclists performed submaximal cycling at power outputs eliciting 30%, 60%, and 90% of their individual lactate threshold at pedaling rates of 80 rpm under each chainring condition (CON80rpm; LOW80rpm; HIGH80rpm) and at pedaling rates for the CON chainring chosen to match pedal speeds of the noncircular chainrings (CON78rpm to LOW80rpm; CON75rpm to HIGH80rpm). Physiological measures, metabolic cost, and gross efficiency were determined by indirect calorimetry. Pedal and joint-specific powers were determined using pedal forces and limb kinematics. Results: Physiological and metabolic measures were not influenced by eccentricity and pedal speed (all Ps > .05). Angular velocities produced during knee and hip extension were lower with the HIGH80rpm condition compared with the CON80rpm condition (all Ps < .05), while angular velocity produced during ankle plantar flexion remained unchanged. Conclusions: Despite the noncircular chainrings imposing their eccentricity on joint angular kinematics, they did not reduce metabolic cost or increase gross efficiency. Our results suggest that noncircular chainrings neither improve nor compromise submaximal cycling performance in trained cyclists.
Chapter
Cycling and running are aerobic activities that represent two different means of exercising, rarely combined. Both cycling and running are essential for overall health, proper function of the cardiovascular system, and weight balance, with few differences between them. Although not widely known, running can contribute to a cyclist’s training and performance. Running and cycling seem to result in different physiological and metabolic responses having different physiological effects. There are certain benefits of running for cyclists, though. These include an increase in bone density, an increased cardiovascular and muscular strength, better exercise tolerance, and psychological benefits as well. Nevertheless, drawbacks also exist. Running in cyclists has been correlated to an increased risk for injury. More specifically, a combination of running and cycling without a proper periodization of the training can lead to overload and injury. The perfect merge of these two activities, running and cycling, is cyclo-cross.
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The purpose of the study was to determine the relationship between running economy and distance running performance in highly trained and experienced distance runners of comparable ability. Oxygen uptake (Vo2) during steady-state and maximal aerobic power (Vo2max) were measured during treadmill running using the open-circuit method. Distance running performance was determined in a nationally prominent 10 km race; all subjects (12 males) placed among the top 19 finishers. The subjects averaged 32.1 min on the 10 km run, 71.7 ml.kg-1.min-1 for Vo2max, and 44.7, 50.3, and 55.9 ml.kg-1.min-1 for steady-state Vo2 at three running paces (241, 268, and 295 m.min-1). The relationship between Vo2max and distance running performance was r = -0.12 (p = 0.35). The relationship between steady-state Vo2 at 241, 268 and 295 m.min-1 and 10 km time were r = 0.83, 0.82, and 0.79 (p < 0.01), respectively. Within this elite cluster of finishers, 65.4% of the variation observed in race performance time on the 10 km run could be explained by variation in running economy. It was concluded that among highly trained and experienced runners of comparable ability and similar Vo2max, running economy accounts for a large and significant amount of the variation observed in performance on a 10 km race.
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To evaluate the effect of extreme endurance training on muscle fibre composition and activities of oxidative enzymes in different fibre types biopsies were taken from vastus lateralis, gastrocnemius and deltoideus of elite orienteers. Comparisons were made between the (trained) leg muscles and the (relatively untrained) arm muscles, and with leg muscles of 16--18 years old boys. The orienteers had the same percentage type I fibres and vastus lateralis and gastrocnemius as in deltoideus, but higher percentage type I fibres in vastus lateralis compared with the controls. The similarity between trained and untrained muscle in the orienteers suggests that training had not caused the high percentage type I fibres which rather might be the result of selection of individuals with the best prerequisites for high oxidative capacity. However, the distribution of type II subgroups in the leg muscles of the orienteers differed from both their own deltoideus and leg muscles of the controls, the relationship IIA/IIB being altered in favour of the more oxidative IIA. The leg muscles of the orienteers also showed an increased occurrence of the normally IIC fibre. These latter findings point at the possibility of a training induced alteration in the subgroup pattern. Unlike in the controls there was no significant difference in succinate dehydrogenase activity, measured in single fibres, between type I and II fibres in gastrocnemius of the orienteers. Thus, type II fibres have the ability metabolically to adapt to high oxidative demands. This might to some extent be mediated by a conversion from IIB to IIA form.
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The present study confirms earlier observations that the musculature of elite distance runners is characterized by a high predominance of ST fibers. Although the percent ST fibers effectively discriminates between good and elite distance runners, fiber composition alone is a poor predictor of distance running success within the group of elite runners. Muscle enzyme measurements suggest that the 11 to 20 miles (17.7 to 32.2 km) of daily training performed by the elite runners produced a significantly greater increase in muscle SDH activity than was observed in the good distance runners, who were running 7 to 11 miles (11.3 to 17.7 km) per day, Although such endurance training enhances the oxidative capacity of the muscle, it apparently has little influence on the enzymes of glycogenolysis.
The running economy of seventeen athletes was studied during running at a low speed (3.3 m X s-1) on a motor-driven treadmill. The net energetic cost during running expressed in kJ X kg-1 X km-1 was on average 4.06. As expected, a positive relationship was found between the energetic cost and the percentage of fast twitch fibres (r = 0.60, n = 17, p less than 0.01). In addition, the mechanical efficiency during two different series of jumps performed with and without prestretch was measured in thirteen subjects. The effect of prestretch on muscle economy was represented by the ratio between the efficiency of muscular work performed during prestretch jumps and the corresponding value calculated in no prestretch conditions. This ratio demonstrated a statistically significant relationship with energy expenditure during running (r = -0.66, n = 13, P less than 0.01), suggesting that the elastic behaviour of leg extensor muscles is similar in running and jumping if the speeds of muscular contraction during eccentric and concentric work are of similar magnitudes.