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Unilateral High-load Resistance Training Influences Strength Changes In The Contralateral Arm Undergoing Low-load Training: 2095
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This review examines the experimental evidence regarding unilateral resistance training frequency, intensity, the type of training, training volume, and adjuvant therapies on the cross-education of strength. CINAHL, MEDLINE, APA PsycInfo, SPORTDiscus, and Web of Science were systematically searched with gray literature searches and pearling of references thereafter. Experiments were included in the review if they performed a unilateral resistance training intervention that directly compared the dose of a training variable on the cross-education response in healthy or clinical populations following a minimum of two weeks of training. Experiments must have reported maximal strength outcomes for the untrained limb. For each experiment, the study population, intervention methods, the dosage of the training variable being studied, and the outcomes for the untrained, contralateral limb were identified and collectively synthesized. The search returned a total of 912 articles, 57 of which qualified for inclusion. The results show that experimental trials have been conducted on resistance training frequency (n = 4), intensity (n = 7), the type of training (n = 26), training volume (n = 3), and adjuvant therapies (n = 17) on the cross-education of strength. This review maps the available evidence regarding exercise design and prescription strategies to promote the cross-education of strength. It appears that traditional resistance training frequencies (i.e., 2–3×/week) at high intensities are effective at promoting cross-education with eccentric muscle actions showing additive benefits. There is experimental evidence that neuromodulatory techniques can augment cross-education when layered with unilateral resistance training versus training alone. Registration identifier (osf.io/9sh5b).
Novelty
The cross-education of strength is moderated by exercise design and prescription in clinical and non-clinical populations.
This review synthesizes the available evidence regarding exercise design strategies for unilateral resistance training and provides evidence-based recommendations for the prescription of unilateral training to maximize the cross-education of strength.
Greater insights regarding the timing and effectiveness of cross-education interventions in clinical scenarios will strengthen the use of unilateral resistance training for individuals who may benefit from its use.
Considerable inter‐individual heterogeneity exists in the muscular adaptations to resistance training. It has been proposed that fast‐twitch fibres are more sensitive to hypertrophic stimuli and thus that variation in muscle fibre type composition is a contributing factor to the magnitude of training response. This study investigated if the inter‐individual variability in resistance training adaptations is determined by muscle typology and if the most appropriate weekly training frequency depends on muscle typology. In strength‐training novices, 11 slow (ST) and 10 fast typology (FT) individuals were selected by measuring muscle carnosine with proton magnetic resonance spectroscopy. Participants trained both upper arm and leg muscles to failure at 60% of one‐repetition maximum (1RM) for 10 weeks, whereby one arm and leg trained 3×/week and the contralateral arm and leg 2×/week. Muscle volume (MRI‐based 3D segmentation), maximal dynamic strength (1RM) and fibre type‐specific cross‐sectional area (vastus lateralis biopsies) were evaluated. The training response for total muscle volume (+3 to +14%), fibre size (−19 to +22%) and strength (+17 to +47%) showed considerable inter‐individual variability, but these could not be attributed to differences in muscle typology. However, ST individuals performed a significantly higher training volume to gain these similar adaptations than FT individuals. The limb that trained 3×/week had generally more pronounced hypertrophy than the limb that trained 2×/week, and there was no interaction with muscle typology. In conclusion, muscle typology cannot explain the high variability in resistance training adaptations when training is performed to failure at 60% of 1RM. image
Key points
This study investigated the influence of muscle typology (muscle fibre type composition) on the variability in resistance training adaptations and on its role in the individualization of resistance training frequency.
We demonstrate that an individual's muscle typology cannot explain the inter‐individual variability in resistance training‐induced increases in muscle volume, maximal dynamic strength and fibre cross‐sectional area when repetitions are performed to failure.
Importantly, slow typology individuals performed a significantly higher training volume to obtain similar adaptations compared to fast typology individuals.
Muscle typology does not determine the most appropriate resistance training frequency. However, regardless of muscle typology, an additional weekly training (3×/week vs. 2×/week) increases muscle hypertrophy but not maximal dynamic strength.
These findings expand on our understanding of the underlying mechanisms for the large inter‐individual variability in resistance training adaptations.
The purpose of this study was to evaluate differences in changes in muscle strength and muscle thickness (MT) of the plantar flexor muscles between traditional resistance training (RT) involving passive rest and RT combined with inter-set stretch in the calf raise exercise. Employing a within-subject design, 21 young, healthy men performed plantar flexion exercises twice per week in both a traditional RT (TRAD) format and combined with a 20-second inter-set stretch (STRETCH). One leg was randomly assigned to the TRAD condition and the contralateral leg performed the STRETCH condition throughout the 8-week study period. Dependent variables included MT of the lateral gastrocnemius (LG), medial gastrocnemius (MG) and the soleus (SOL), and isometric strength of the plantar flexors. Results indicated a potential beneficial hypertrophic effect of STRETCH compared to TRAD for the SOL [0.7 mm, CI90% = (0, 1.6)], while the LG had more ambiguous effects [0.4 mm (−0.4, 1.3)] and MG effects were equivocal [0 mm (−0.6, 0.7)]. In general, LG demonstrated greater standardized growth [z = 1.1 (1, 1.3)] as compared to MG [z = 0.3 (0.2, 0.5)] and SOL [z = 0.3 (0.2, 0.5)]. Measures of isometric strength showed a modest advantage to STRETCH. In conclusion, loaded inter-set stretch may enhance MT of the soleus but effects on the gastrocnemii appear uncertain or unlikely in untrained men; plantar flexor strength appears to be modestly enhanced by the interventional strategy.
The effect of resistance training with higher- and lower-loads on muscle mass and strength has been extensively studied, while changes in muscle endurance have received less attention. This trial aimed to assess the effect of training load on absolute muscle endurance (AME) and relative muscle endurance (RME). Sixteen untrained women (22.7 ± 3.3 yr: mean ± SD) had one arm and leg randomly assigned to train with higher loads (HL; 80–90% 1RM), and the contralateral limbs trained with lower loads (LL; 30–50% 1RM) thrice weekly to volitional fatigue for 10 weeks. Heavy and light load AME and RME, strength, and muscle mass were assessed pre- and post-training. Strength increased more in the HL compared to LL leg (P < 0.01), but similar increases in strength were observed between upper body conditions (P = 0.46). Lower body heavy and light load AME improved in both conditions, but HL training induced a larger improvement in heavy load AME (HL: 9.3 ± 4.3 vs. LL: 7.5 ± 7.1 repetitions, time × limb P < 0.01) and LL training induced a larger improvement in light load AME (LL: 24.7 ± 22.2 vs. HL: 15.2 ± 16.7 repetitions, time × limb P = 0.04). In the upper body, HL and LL training induced similar increases in both heavy (time × limb P = 0.99), and light load (time × limb P = 0.16) AME. Dual-energy X-ray absorptiometry showed no change in leg fat-and-bone-free mass (FBFM) for either condition, and an increase in only LL arm FBFM. AME improved in a manner specific to the training loads used. ClinicalTrials.gov (NCT04547972).
The purpose of this study was to determine the effects of training load (25% vs. 75% of one repetition maximum (1RM)) and fatigue (failure vs. non‐failure) during four weeks of unilateral knee extension resistance training (RT) on maximal voluntary force in the trained and the untrained knee extensors. Healthy young adults (n=42) were randomly assigned to control (CON, n=9, 24±4.3y), low‐load RT to failure (LLF, n=11, 21±1.3y, three sets to failure at 25% of 1RM), high‐load RT to failure (HLF, n=11, 21±1.4y, three sets to failure at 75% of 1RM), and high‐load RT without failure (HLNF, n=11, 22±1.5y, six sets of five repetitions at 75% of 1RM) groups. Before and after the four weeks of training, 1RM, maximal voluntary isometric force (MVIC) and corticospinal excitability (CSE) were measured. 1RM in the trained (20%, d=0.70, 15%, d=0.61) and the untrained knee extensors (5%, d=0.27, 6%, d=0.26) increased only in the HLF and HLNF groups, respectively. MVIC force increased only in the trained leg of the HLF (5%, d=0.35) and HLNF groups (12%, d=0.67). CSE decreased in the VL of both legs in the HLNF group (‐19%, d=0.44) and no changes occurred in the RF. In conclusion, high‐ but not low‐load RT improves maximal voluntary force in the trained and the untrained knee extensors and fatigue did not further enhance these adaptations. Voluntary force improvements were unrelated to CSE changes in both legs.
COVID-19 outbreak has a profound impact on almost every aspect of life. Universal masking is recommended as a means of source control. Routinely exercising in a safe environment is an important strategy for healthy living during this crisis. As sports clubs and public spaces may serve a source of viral transmission, masking may become an integral part of physical activity. This study aimed to assess the physiological effects of wearing surgical masks and N95 respirators during short term strenuous workout. This was a multiple cross-over trial of healthy volunteers. Using a standard cycle ergometry ramp protocol, each subject performed a maximal exercise test without a mask, with a surgical mask, and with an N95 respirator. Physiological parameters and time to exhaustion were compared. Each subject served his own control. Sixteen male volunteers (mean age and BMI of 34 ±4 years and 28.72 ±3.78 kg/m2, respectively) completed the protocol. Heart rate, respiratory rate, blood pressure, oxygen saturation, and time to exhaustion did not differ significantly. Exercising with N95 mask was associated with a significant increase in end-tidal carbon-dioxide (EtCO2 ) levels. The differences were more prominent as the load increased, reaching 8mmHg at exhaustion (none vs. N95, p=0.001). In conclusion, in healthy subjects, short term moderate-strenuous aerobic physical activity with a mask is feasible, safe, and associated with only minor changes in physiological parameters, particularly a mild increase in EtCO2 . Subjects suffering from lung diseases should have a cautious evaluation before attempting physical activity with any mask.
The objective of this study was to determine differences in 2 distinct resistance training protocols and if true variability can be detected after accounting for random error. Individuals (n = 151) were randomly assigned to 1 of 3 groups: (i) a traditional exercise group performing 4 sets to failure; (ii) a group performing a 1-repetition maximum (1RM) test; and (iii) a time-matched nonexercise control group. Both exercise groups performed 18 sessions of elbow flexion exercise over 6 weeks. While both training groups increased 1RM strength similarly (∼2.4 kg), true variability was only present in the traditional exercise group (true variability = 1.80 kg). Only the 1RM group increased untrained arm 1RM strength (1.5 kg), while only the traditional group increased ultrasound measured muscle thickness (∼0.23 cm). Despite these mean increases, no true variability was present for untrained arm strength or muscle hypertrophy in either training group. In conclusion, these findings demonstrate the importance of taking into consideration the magnitude of random error when classifying differential responders, as many studies may be classifying high and low responders as those who have the greatest amount of random error present. Additionally, our mean results demonstrate that strength is largely driven by task specificity, and the crossover effect of strength may be load dependent.
NoveltyMany studies examining differential responders to exercise do not account for random error.
True variability was present in 1RM strength gains, but the variability in muscle hypertrophy and isokinetic strength changes could not be distinguished from random error.
The crossover effect of strength may differ based on the protocol employed.
The purpose of this paper was to use a Bayesian approach to compare the relative change in muscle size between magnetic resonance imaging (MRI) and ultrasound measured muscle thickness (MTH) following 6 weeks of concentric and eccentric blood flow restricted exercise. Changes at each site were as follows: concentric 50% site (MRI: 10.2%, MTH: 8.7%), concentric 10 cm site (MRI: 12%, MTH: 4.5%), eccentric 50% site (MRI: −1.7%, MTH: 2.6%), and eccentric 10 cm site (MRI: 5.2%, MTH: 0.5%). When testing the difference between estimates using a default prior of 0.707, we provided evidence that the estimate at the 50% site of the concentric arm was similar between ultrasound and MRI [Median % (95% credible interval): −1.1 (−8.2, 5.8)]. However, evidence for other sites suggested differences or a degree of uncertainty. Both methods produce similar conclusions about the presence of growth but the magnitude of that change appears different at most sites.
PurposeThis study examined the time course of contralateral adaptations in maximal isometric strength (MVC), rate of force development (RFD), and rate of electromyographic (EMG) rise (RER) during 4 weeks of unilateral isometric strength training with the non-dominant elbow flexors.Methods
Twenty participants were allocated to strength training (n = 10, three female, two left hand dominant) or control (n = 10, three female, two left hand dominant) groups. Both groups completed testing at baseline and following each week of training to evaluate MVC strength, EMG amplitude, RFD and RER at early (RFD50, RER50) and late (RFD200, RER200) contraction phases for the dominant ‘untrained’ elbow flexors. The training group completed 11 unilateral isometric training sessions across 4 weeks.ResultsThe contralateral improvements for MVC strength (P < 0.01) and RFD200 (P = 0.017) were evidenced after 2 weeks, whereas RFD50 (P < 0.01) and RER50 (P = 0.02) showed significant improvements after 3 weeks. Each of the dependent variables was significantly (P < 0.05) greater than baseline values at the end of the training intervention for the trained arm. No changes in any of the variables were observed for the control group (P > 0.10).Conclusions
Unilateral isometric strength training for 2–3 weeks can produce substantial increases in isometric muscle strength and RFD for both the trained and untrained arms. These data have implications for rehabilitative exercise design and prescription.
Learning about hypothesis evaluation using the Bayes factor could enhance psychological research. The Bayes factor quantifies the support in the data for two competing hypotheses. These may be the traditional null and alternative hypotheses, but these may also be informative hypotheses like m1 > m2 > m3 and (m1 − m2) > (m2 − m3) where m1, m2, and m3 denote the means in three experimental groups. Bayesian hypotheses evaluation offers options such as quantifying evidence in favor of the null-hypothesis, simultaneous evaluation of multiple hypotheses, and Bayesian updating, that is, recomputation of the Bayes factor after additional data have been collected.
In this tutorial it is elaborated how researchers can use the Bayes factor for the analysis of their own data. The focus is completely applied and each topic discussed is illustrated using Bayes factors for the evaluation of hypotheses in the context of an ANOVA model, obtained using the R package bain. Readers can execute all the analyses presented while reading this tutorial if they download bain and the R-codes used from the bain website. It will be elaborated in a completely nontechnical manner: what the Bayes factor is, how it can be obtained, how Bayes factors should be interpreted, and what can be done with Bayes factors. After reading this tutorial and executing the associated code, researchers will be able to use their own data for the evaluation of hypotheses by means of the Bayes factor, not only in the context of ANOVA models, but also in the context of other statistical models.
Purpose:
Cross-education (CE) of strength is a well-known phenomenon whereby exercise of one limb can induce strength gains in the contralateral untrained limb. The only available meta-analyses on CE, which date back to a decade ago, estimated a modest 7.8% increase in contralateral strength following unilateral training. However, in recent years new evidences have outlined larger contralateral gains, which deserve to be systematically evaluated. Therefore, the aim of this meta-analysis was to appraise current data on CE and determine its overall magnitude of effect.
Methods:
Five databases were searched from inception to December 2016. All randomized controlled trials focusing on unilateral resistance training were carefully checked by two reviewers who also assessed the eligibility of the identified trials and extracted data independently. The risk of bias was assessed using the Cochrane Risk-of-Bias tool.
Results:
Thirty-one studies entered the meta-analysis. Data from 785 subjects were pooled and subgroup analyses by body region (upper/lower limb) and type of training (isometric/concentric/eccentric/isotonic-dynamic) were performed. The pooled estimate of CE was a significant 11.9% contralateral increase (95% CI 9.1-14.8; p < 0.00001; upper limb: + 9.4%, p < 0.00001; lower limb: + 16.4%, p < 0.00001). Significant CE effects were induced by isometric (8.2%; p = 0.0003), concentric (11.3%; p < 0.00001), eccentric (17.7%; p = 0.003) and isotonic-dynamic training (15.9%; p < 0.00001), although a high risk of bias was detected across the studies.
Conclusions:
Unilateral resistance training induces significant contraction type-dependent gains in the contralateral untrained limb. Methodological issues in the included studies are outlined to provide guidance for a reliable quantification of CE in future studies.
Aim:
Muscle thickness (MT) measured by ultrasound has been used to estimate cross-sectional area (measured by CT and MRI) at a single time-point. We tested whether MT could be used as a valid marker of MRI determined muscle anatomical cross-sectional area (ACSA) and volume changes following resistance training (RT).
Methods:
Nine healthy, young, male volunteers (24±2 y.o., BMI 24.1±2.8 kg/m(2) ) had vastus lateralis (VL) muscle volume (VOL) and ACSA mid (at 50% of femur length, FL) assessed by MRI, and VL MT measured by ultrasound at 50% FL. Measurements were taken at baseline and after 12 weeks of isokinetic RT. Differences between baseline and post-training were assessed by Student's paired t-test. The relationships between MRI and ultrasound measurements were tested by Pearson's correlation.
Results:
After RT, MT increased by 7.5±6.1% (p<0.001), ACSAmid by 5.2±5% (p<0.001) and VOL by 5.0±6.9% (p<0.05) (values: means±S.D.). Positive correlations were found, at baseline and 12 weeks, between MT and ACSAmid (r=0.82, p<0.001 and r=0.73, p<0.001, respectively), and between MT and VOL (r=0.76, p < 0.001 and r=0.73, p < 0.001, respectively). The % change in MT with training was correlated with % change in ACSAmid (r=0.69, p = 0.01), but not % change in VOL (r= 0.33, p>0.05).
Conclusions:
These data support evidence that MT is a reliable index of muscle ACSAmid and VOL at a single time-point. MT changes following RT are associated with parallel changes in muscle ACSAmid but not with the changes in VOL, highlighting the impact of RT on regional hypertrophy. This article is protected by copyright. All rights reserved.
Purpose:
To test the effects of 4 weeks of unilateral low-load resistance training (LLRT), with and without blood flow restriction (BFR), on maximal voluntary contraction (MVC), muscle thickness, volitional wave (V wave), and Hoffmann reflex (H reflex) of the soleus muscle.
Methods:
Twenty-two males were randomly distributed into three groups: a control group (CTR; n = 8); a low-load blood flow restriction resistance training group (BFR-LLRT; n = 7), who were an inflatable cuff to occlude blood flow; and a low-load resistance training group without blood flow restriction (LLRT; n = 7). The training consisted of four sets of unilateral isometric LLRT (25% of MVC) three times a week over 4 weeks.
Results:
MVC increased 33% (P < 0.001) and 22% (P < 0.01) in the trained leg of both BFR-LLRT and LLRT groups, respectively. The soleus thickness increased 9.5% (P < 0.001) and 6.5% (P < 0.01) in the trained leg of both BFR-LLRT and LLRT groups, respectively. However, neither MVC nor thickness changed in either of the legs tested in the CTR group (MVC -1 and -5%, and muscle thickness 1.9 and 1.2%, for the control and trained leg, respectively). Moreover, V wave and H reflex did not change significantly in all the groups studied (Vwave/M wave ratio -7.9 and -2.6%, and H max/M max ratio -3.8 and -4%, for the control and trained leg, respectively).
Conclusions:
Collectively, the present data suggest that in spite of the changes occurring in soleus strength and thickness, 4 weeks of low-load resistance training, with or without BFR, does not cause any change in neural drive or motoneuronal excitability.
Direct sampling of human skeletal muscle using the needle biopsy technique can facilitate insight into the biochemical and histological responses resulting from changes in exercise or feeding. However, the muscle biopsy procedure is invasive, and analyses are often expensive, which places pragmatic restraints on sample sizes. The unilateral exercise model can serve to increase statistical power and reduce the time and cost of a study. With this approach, 2 limbs of a participant are randomized to 1 of 2 treatments that can be applied almost concurrently or sequentially depending on the nature of the intervention. Similar to a typical repeated measures design, comparisons are made within participants, which increases statistical power by reducing the amount of between-person variability. A washout period is often unnecessary, reducing the time needed to complete the experiment and the influence of potential confounding variables such as habitual diet, activity, and sleep. Variations of the unilateral exercise model have been employed to investigate the influence of exercise, diet, and the interaction between the 2, on a wide range of variables including mitochondrial content, capillary density, and skeletal muscle hypertrophy. Like any model, unilateral exercise has some limitations: it cannot be used to study variables that potentially transfer across limbs, and it is generally limited to exercises that can be performed in pairs of treatments. Where appropriate, however, the unilateral exercise model can yield robust, well-controlled investigations of skeletal muscle responses to a wide range of interventions and conditions including exercise, dietary manipulation, and disuse or immobilization.
Limited data exist on the efficacy of low-load blood flow-restricted strength training (BFR), as compared directly to heavy-load strength training (HST). Here, we show that twelve weeks of twice-a-week unilateral BFR (30% of 1RM to exhaustion) and HST (6-10RM) of knee extensors provide similar increases in 1RM knee extension and cross sectional area of distal parts of m. quadriceps femoris in nine untrained women (age 22±1 years). The two protocols resulted in similar acute increases in serum levels of human growth hormone. On the cellular level, twelve weeks of BFR and HST resulted in similar shifts in muscle fiber composition in m. vastus lateralis, evident as increased MyHC2A proportions and decreased MyHC2X proportions. It also resulted in similar changes of the expression of 29 genes involved in skeletal muscle function, measured both in a rested-state following twelve weeks of training and subsequent to singular training sessions. Training had no effect on myonuclei proportions. Of particular interest; i) gross adaptations to BFR and HST were greater in individuals with higher proportions of type 2 fibers, ii) both BFR and HST resulted in ~4-fold increases in the expression of the novel exercise-responsive gene Syndecan-4, and iii) BFR provided lesser hypertrophy than HST in the proximal half of m. quadriceps femoris and also in CSApeak, potentially being a consequent of pressure from the tourniquet utilized to achieve blood flow restriction. In conclusion, BFR and HST of knee extensors resulted in similar adaptations in functional, physiological and cell biological parameters in untrained women.
Copyright © 2014, American Journal of Physiology - Regulatory, Integrative and Comparative Physiology.
Cross education is the process whereby training of one limb gives rise to enhancements in the performance of the opposite, untrained limb. Despite interest in this phenomenon having been sustained for more than a century, a comprehensive explanation of the mediating neural mechanisms remains elusive. With new evidence emerging that cross education may have therapeutic utility, the need to provide a principled evidential basis upon which to design interventions becomes ever more pressing. Generally, mechanistic accounts of cross education align with one of two explanatory frameworks. Models of the “cross activation” variety encapsulate the observation that unilateral execution of a movement task gives rise to bilateral increases in corticospinal excitability. The related conjecture is that such distributed activity, when present during unilateral practice, leads to simultaneous adaptations in neural circuits that project to the muscles of the untrained limb, thus facilitating subsequent performance of the task. Alternatively, “bilateral access” models entail that motor engrams formed during unilateral practice, may subsequently be utilized bilaterally—that is, by the neural circuitry that constitutes the control centers for movements of both limbs. At present there is a paucity of direct evidence that allows the corresponding neural processes to be delineated, or their relative contributions in different task contexts to be ascertained. In the current review we seek to synthesize and assimilate the fragmentary information that is available, including consideration of knowledge that has emerged as a result of technological advances in structural and functional brain imaging. An emphasis upon task dependency is maintained throughout, the conviction being that the neural mechanisms that mediate cross education may only be understood in this context.
Evidence that unilateral training increases contralateral strength is inconsistent, possibly because existing studies have design limitations such as lack of control groups, lack of randomization, and insufficient statistical power. This study sought to determine whether unilateral resistance training increases contralateral strength. Subjects (n = 115) were randomly assigned to a control group or one of the following four training groups that performed supervised elbow flexion contractions: 1) one set at high speed, 2) one set at low speed, 3) three sets at high speed, or 4) three sets at low speed. Training was 3 times/wk for 6 wk with a six- to eight-repetition maximum load. Control subjects attended sessions but did not exercise. Elbow flexor strength was measured with a one-repetition maximum arm curl before and after training. Training with one set at slow speed did not produce an increase in contralateral strength (mean effect of -1% or -0.07 kg; 95% confidence interval: -0.42-0.28 kg; P = 0.68). However, three sets increased strength of the untrained arm by a mean of 7% of initial strength (additional mean effect of 0.41 kg; 95% confidence interval: 0.06-0.75 kg; P = 0.022). There was a tendency for training with fast contractions to produce a greater increase in contralateral strength than slow training (additional mean effect of 5% or 0.31 kg; 95% confidence interval: -0.03-0.66 kg; P = 0.08), but there was no interaction between the number of sets and training speed. We conclude that three sets of unilateral resistance exercise produce small contralateral increases in strength.
If exercises are performed to increase muscle strength on one side of the body, voluntary strength can increase on the contralateral side. This effect, termed the contralateral strength training effect, is usually measured in homologous muscles. Although known for over a century, most studies have not been designed well enough to show a definitive transfer of strength that could not be explained by factors such as familiarity with the testing. However, an updated meta-analysis of 16 properly controlled studies (range 15-48 training sessions) shows that the size of the contralateral strength training effect is approximately 8% of initial strength or about half the increase in strength of the trained side. This estimate is similar to results of a large, randomized controlled study of training for the elbow flexors (contralateral effect of 7% initial strength or one-quarter of the effect on the trained side). This is likely to reflect increased motoneuron output rather than muscular adaptations, although most methods are insufficiently sensitive to detect small muscle contributions. Two classes of central mechanism are identified. One involves a "spillover" to the control system for the contralateral limb, and the other involves adaptations in the control system for the trained limb that can be accessed by the untrained limb. Cortical, subcortical and spinal levels are all likely to be involved in the "transfer," and none can be excluded with current data. Although the size of the effect is small and may not be clinically significant, study of the phenomenon provides insight into neural mechanisms associated with exercise and training.
The purpose was to determine if post activation performance enhancement is specific to the muscle being conditioned or if it is also observed within the homologous muscles of the contralateral limb (after accounting for the warm‐up and random error). We also investigated if this differed based on training status or muscle size. One hundred seven participants (75 untrained; 32 trained) participated in four sessions. Visit 1 included baseline measurements and familiarization. Visits 2–4 included the completion of one of the three experimental conditions: 1) control, 2) same side, and 3) cross over completed in a randomized order. The control condition completed all testing except for the conditioning contraction. The same side condition completed the conditioning contraction on the same side as the strength test. The cross‐over condition completed the conditioning on the arm opposite to the strength test. The variable of interest was the change from baseline in isokinetic strength. Our analysis indicated that of the hypotheses compared, the posterior probabilities (posterior probability of 0.506) favored the hypothesis that the effect was local and greatest in those who were resistance trained [mean (SD) of 1.4 (2.2) Nm over the control in those resistance trained]. We found no relationship between muscle size and post activation performance enhancement. In conclusion, there is an influence of training status pertaining to the post activation performance enhancement effect but no influence from baseline muscle size. It appears unlikely that the effect is due to a systemic mechanism.
Participation in resistance exercise is encouraged throughout the lifetime, offering such benefits as improved strength and muscle mass accretion. Considerable research has been completed on this topic within the past several decades, with the current narrative dictating that increased muscle size yields further increases in muscle strength. However, there remain unanswered questions relating to the observation that certain training interventions yield only one specific adaptation (strength or size). Studies investigating resistance training often include either bilateral or unilateral exercise programs. Unilateral exercise programs are often used as they allow for comparison between two separate training interventions within the same individual. This is viewed as an advantage, relating to statistical power, but a limitation insofar as one intervention could be confounded by the intervention within the opposing limb. For example, when only one limb is trained both limbs often get stronger (albeit to differing magnitudes), termed the cross-education effect. However, we propose that when both limbs are trained that the cross-education effect may not occur and that the adaptations produced are reflective of the contraction history of the muscle. Herein, we discuss ways to test the idea that strength change may be dictated by the contraction history of the muscle. If each limb responds only to the contraction history within each limb (as opposed to the opposite limb), then this would have immediate ramifications for research design. Furthermore, this would certainly be of importance among injured populations undergoing rehabilitation, seeking to find the most efficacious exercise regimens.
The purpose was to examine the acute skeletal muscle response to high load exercise and low‐load exercise with and without different levels of applied pressure (BFR). A total of 22 participants completed the following four conditions: elbow flexion exercise to failure using a traditional high load [70% 1RM, (7000)], low load [15% 1RM,(1500)], low load with moderate BFR [15%1RM+40%BFR(1540)] or low load with greater BFR [15% 1RM+80%BFR(1580)]. Torque and muscle thickness were measured prior to, immediately post, and 15 min postexercise. Muscle electromyography (EMG) amplitude was measured throughout. Immediately following exercise, the 7000 condition had lower muscle thickness [4·2(1·0)cm] compared to the 1500 [4·4 (1·1)cm], 1540 [4·4(1·1)cm] and 1580 [4·5(1·0)cm] conditions. This continued 15 min post. Immediately following exercise, torque was lower in the 1500 [31·8 (20) Nm], 1540 [28·3(16·9) Nm, P<0·001] and 1580 [29·5 (17) Nm] conditions compared to the 7000 condition [40 (19) Nm]. Fifteen minutes post, 1500 and 1540 conditions demonstrated lower torque compared to the 7000 condition. For the last three repetitions percentage EMG was greater in the 7000 compared to the 1580 condition. Very low‐load exercise (with or without BFR) appears to result in greater acute muscle swelling and greater muscular fatigue compared to high load exercise.
Introduction:
Large increases in 1-repetition maximum (1RM) strength have been demonstrated from repeated testing, but it is unknown whether these increases can be augmented by resistance training.
Methods:
Five trained individuals performed a 1RM test and maximal voluntary isometric contraction (MVC) for unilateral elbow flexion exercise on 1 arm (testing arm), while the other arm performed a 1RM test and MVC, in addition to 3 sets of exercise (70% 1RM) (training arm) for 21 straight days.
Results:
Although only the training arm had increased muscle thickness [mean: 0.28 (95% confidence interval: 0.22 - 0.33)] cm, 1RM strength increased similarly in the training [2.2 (95% confidence interval: 0.9 - 3.4) kg; P=0.008] and testing [1.9 (95% confidence interval: 0.5 - 3.2) kg; P=0.019] arms.
Discussion:
Increases in 1RM strength from resistance training are related to the specificity of exercise and are likely driven by mechanisms other than muscle growth. This article is protected by copyright. All rights reserved.
We have reported that the acute postexercise increases in muscle protein synthesis rates, with differing nutritional support, are predictive of longer-term training-induced muscle hypertrophy. Here, we aimed to test whether the same was true with acute exercise-mediated changes in muscle protein synthesis. Eighteen men (21 ± 1 yr, 22.6 ± 2.1 kg/m(2); means ± SE) had their legs randomly assigned to two of three training conditions that differed in contraction intensity [% of maximal strength (1 repetition maximum)] or contraction volume (1 or 3 sets of repetitions): 30%-3, 80%-1, and 80%-3. Subjects trained each leg with their assigned regime for a period of 10 wk, 3 times/wk. We made pre- and posttraining measures of strength, muscle volume by magnetic resonance (MR) scans, as well as pre- and posttraining biopsies of the vastus lateralis, and a single postexercise (1 h) biopsy following the first bout of exercise, to measure signaling proteins. Training-induced increases in MR-measured muscle volume were significant (P < 0.01), with no difference between groups: 30%-3 = 6.8 ± 1.8%, 80%-1 = 3.2 ± 0.8%, and 80%-3= 7.2 ± 1.9%, P = 0.18. Isotonic maximal strength gains were not different between 80%-1 and 80%-3, but were greater than 30%-3 (P = 0.04), whereas training-induced isometric strength gains were significant but not different between conditions (P = 0.92). Biopsies taken 1 h following the initial resistance exercise bout showed increased phosphorylation (P < 0.05) of p70S6K only in the 80%-1 and 80%-3 conditions. There was no correlation between phosphorylation of any signaling protein and hypertrophy. In accordance with our previous acute measurements of muscle protein synthetic rates a lower load lifted to failure resulted in similar hypertrophy as a heavy load lifted to failure.
The purpose of this study was to assess cortical activation associated with the cross-education effect to an immobilized limb, using functional magnetic resonance imaging.
Fourteen right-handed participants were assigned to two groups. One group (n = 7) wore a cast and strength trained the free arm (CAST-TRAIN). The second group (n = 7) wore a cast and did not strength train (CAST). Casts were applied to the nondominant (left) wrist and hand. Strength training was maximal isometric handgrip contractions (right hand) 5 d·wk(-1). Peak force (handgrip dynamometer), muscle thickness (ultrasound), EMG, and cortical activation (functional magnetic resonance imaging) were assessed before and after the intervention.
CAST-TRAIN improved right handgrip strength by 10.7% (P < 0.01) with no change in muscle thickness. There was a significant group × time interaction for strength of the immobilized arm (P < 0.05). Handgrip strength of the immobilized arm of CAST-TRAIN was maintained, whereas the immobilized arm of CAST significantly decreased by 11% (P < 0.05). Muscle thickness of the immobilized arm decreased by an average of 3.3% (P < 0.05) for all participants and was not different between groups after adjusting for baseline differences. There was a significant group × time interaction for EMG activation (P < 0.05), where CAST-TRAIN showed an increasing trend and CAST showed a decreasing trend, pooled across arms. For the immobilized arm of CAST-TRAIN, there was a significant increase in contralateral motor cortex activation after training (P < 0.05). For the immobilized arm of CAST, there was no change in motor cortex activation.
Handgrip strength training of the free limb attenuated strength loss during unilateral immobilization. The maintenance of strength in the immobilized limb via the cross-education effect may be associated with increased motor cortex activation.
The case for preferring analysis of covariance (ANCOVA) to the simple analysis of change scores (SACS) has often been made. Nevertheless, claims continue to be made that analysis of covariance is biased if the groups are not equal at baseline. If the required equality were in expectation only, this would permit the use of ANCOVA in randomized clinical trials but not in observational studies. The discussion is related to Lord's paradox. In this note, it is shown, however that it is not a necessary condition for groups to be equal at baseline, not even in expectation, for ANCOVA to provide unbiased estimates of treatment effects. It is also shown that although many situations can be envisaged where ANCOVA is biased it is very difficult to imagine circumstances under which SACS would then be unbiased and a causal interpretation could be made.
We aimed to gain insight into the role that the transitory increases in anabolic hormones play in muscle hypertrophy with unilateral resistance training. Ten healthy young male subjects (21.8 +/- 0.4 years, 1.78 +/- 0.04 m, 75.6 +/- 2.9 kg; mean +/- SE) engaged in unilateral resistance training for 8 week (3 days/week). Exercises were knee extension and leg press performed at 80-90% of the subject's single repetition maximum (1RM). Blood samples were collected in the acute period before and after the first training bout and following the last training bout and analyzed for total testosterone, free-testosterone, luteinizing hormone, sex hormone binding globulin, growth hormone, cortisol, and insulin-like growth factor-1. Thigh muscle cross sectional area (CSA) and muscle fibre CSA by biopsy (vastus lateralis) were measured pre- and post-training. Acutely, no changes in systemic hormone concentrations were observed in the 90 min period following exercise and there was no influence of training on these results. Training-induced increases were observed in type IIx and IIa muscle fibre CSA of 22 +/- 3 and 13 +/- 2% (both P < 0.001). No changes were observed in fibre CSA in the untrained leg (all P > 0.5). Whole muscle CSA increased by 5.4 +/- 0.9% in the trained leg (P < 0.001) and remained unchanged in the untrained leg (P = 0.76). Isotonic 1RM increased in the trained leg for leg press and for knee extension (P < 0.001). No changes were seen in the untrained leg. In conclusion, unilateral training induced local muscle hypertrophy only in the exercised limb, which occurred in the absence of changes in systemic hormones that ostensibly play a role in muscle hypertrophy.