Strength training the free limb attenuates strength loss during unilateral immobilization

Mitigating skeletal muscle wasting in unloading and augmenting subsequent recovery

Skeletal muscle wasting is the hallmark pathophysiological adaptation to unloading or disuse that demonstrates the dependency on frequent mechanical stimulation (e.g. muscle activation and subsequent loading) for homeostasis of normally load-bearing muscles. In the absence of mitigation strategies, no mammalian organism is resistant to muscle atrophy driven by unloading. Given the profound impact of unloading-induced muscle wasting on physical capacity, metabolic health and immune function; mitigation strategies during unloading and/or augmentation approaches during recovery have broad healthcare implications in settings of bed-bound hospitalization, cast immobilization and spaceflight. This topical review aims to: (1) provide a succinct, state-of-the-field summary of seminal and recent findings regarding the mechanisms of unloading-induced skeletal muscle wasting; (2) discuss unsuccessful vs. promising mitigation and recovery augmentation strategies; and (3) identify knowledge gaps ripe for future research. We focus on the rapid muscle atrophy driven by relatively short-term mechanical unloading/disuse, which is in many ways mechanistically distinct from both hypermetabolic muscle wasting and denervation-induced muscle atrophy. By restricting this discussion to mechanical unloading during which all components of the nervous system remain intact (e.g. without denervation models), mechanical loading requiring motor and sensory neural circuits in muscle remain viable targets for both mitigation and recovery augmentation. We emphasize findings in humans with comparative discussions of studies in rodents which enable elaboration of key mechanisms. We also discuss what is currently known about the effects of age and sex as biological factors, and both are highlighted as knowledge gaps and novel future directions due to limited research.

The role of muscle disuse in muscular and cardiovascular fitness: A systematic review and meta-regression

We aimed to assess the effects of muscle disuse on muscle strength (MS), muscle mass (MM) and cardiovascular fitness. Databases were scrutinized to identify human studies assessing the effects of muscle disuse on both (1) MM and (2) maximal oxygen uptake (VO2max) and/or MS. Random-effects meta-analysis and meta-regression with initial physical fitness and length of the protocol as a priori determined moderators were performed. We quantitatively analyzed 51 different studies, and the level of significance was set at p < 0.05. Data from the participants in 14 studies showed a decline in both VO2max (SMD: -0.93; 95% CI: -1.27 to -0.58) and MM (SMD: -0.34; 95% CI: -0.57 to -0.10). Data from 47 studies showed a decline in strength (-0.88; 95% CI: -1.04 to -0.73) and mass (SMD: -0.47; 95% CI: -0.58 to -0.36). MS loss was twice as high as MM loss, but differences existed between anatomical regions. Notably, meta-regression analysis revealed that initial MS was inversely associated with MS decline. VO2max and MS decline to a higher extent than MM during muscle disuse. We reported a more profound strength loss in subjects with high muscular strength. This is physiologically relevant for athletes because their required muscular strength can profoundly decline during a period of muscle disuse. It should however be noted that a period of muscle disuse can have devastating consequences in old subjects with low muscular strength.

Ipsilateral transfer of motor skill from lower to upper limb in healthy adults: A randomized controlled trial

BACKGROUND AND PURPOSE

Whereas motor skills of the untrained upper limb (UL) can improve following practice with the other UL, it has yet to be determined if an UL motor skill can improve following practice of that skill with the lower limb (LL).

METHODS

Forty-five healthy subjects randomly participated in a 10-minute single-session intervention of (1) practicing 50 reaching movement (RM) sequences with the non-dominant left LL toward light switches (LL group); or (2) observing the identical 50 light switches sequences (Switches Observation (SO) group); or (3) observing nature films (Nature Observation (NO) group). RM sequence performance with the left UL toward the light switches was tested before and immediately after the intervention and retested after 24 h.

RESULTS

Reaching response time improved in the LL group more than in the SO and NO groups in the posttest (pBonferroni = 0.038 and pBonferroni < 0.001, respectively), and improved in the LL group more than in the NO group in the retest (pBonferroni = 0.004). Percentage of fails did not differ between groups across the timepoints.

CONCLUSIONS

It appears that the actual practice of the RM sequence skill with the UL together with the cognitive element embedded in the observation of the RM sequences contributes to ipsilateral transfer from LL to UL.

Effect of leg dominance on ipsilateral and contralateral limb training adaptation in middle-aged women after unilateral sensorimotor and resistance exercise training

The aim was to examine the directionality of global training effects in middle-aged women after unilateral training.

Thirty-nine middle-aged female volunteers (59.4 ± 5.4 years) were randomly assigned to one of three groups: 1. Unilateral Dominant Lower Limb Training (UDLT); 2. Unilateral Non-Dominant Lower Limb Training (UNDLT) or 3. Control group. Outcome measures assessing isometric strength, static and dynamic balance were recorded at baseline, and 1 week after 12 weeks (post-test) of training or no-intervention.

The net cross education adaptation changes of the contralateral quadriceps isometric maximum voluntary (MVC) force (F2,34 = 4.33; p = 0.022), Stork balance score (F2,34 = 4.26; p = 0.023) and the Star Excursion Balance test score (F2,34 = 11.80; p = 0.001) were asymmetrical in the UNDLT group and on average, exceeded the UDLT group.

The results demonstrated asymmetrical cross education training adaptations with unilateral training of non-dominant leg (UNDLT) to contralateral homologous and heterologous muscles, with the exception of knee flexor MVC. The results of this study provide a novel exercise or rehabilitation strategy that can be employed when one of the limbs is affected.

Relative Cross-Education Training Effects of Male Youth Exceed Male Adults

ABSTRACT

Ben Othman, A, Anvar, SH, Aragão-Santos, JC, Behm, DG, and Chaouachi, A. Relative cross-education training effects of male youth exceed male adults. J Strength Cond Res 38(5): 881-890, 2024-Cross-education has been studied extensively with adults, examining the training effects on contralateral homologous muscles. There is less information on the cross-education effects on contralateral heterologous muscles and scant information comparing these responses between adults and youth. The objective was to compare cross-education training effects in male youth and adults to contralateral homologous and heterologous muscles. Forty-two male children (10-13-years) and 42 adults (18-21-years) were tested before and following an 8-week unilateral, dominant or nondominant arm, chest press (CP) training program or control group (14 subjects each). Unilateral testing assessed dominant and nondominant limb strength with leg press and CP 1 repetition maximum (1RM), knee extensors, elbow extensors (EE), elbow flexors, and handgrip maximum voluntary isometric contraction (MVIC) strength and shot put distance and countermovement jump height. Upper-body tests demonstrated large magnitude increases, with children overall exceeding adults ( p = 0.05- p < 0.0001, η2 : 0.51, 10.4 ± 11.1%). The dominant trained limb showed significantly higher training adaptations than the nondominant limb for the adults with CP 1RM ( p = 0.03, η2 : 0.26, 6.7 ± 11.5%) and EE ( p = 0.008, η2 : 0.27, 8.8 ± 10.3%) MVIC force. Unilateral CP training induced significantly greater training adaptations with the ipsilateral vs. contralateral limb ( p = 0.008, η2 : 0.93, 27.8 ± 12.7%). In conclusion, children demonstrated greater training adaptations than adults, upper-body strength increased with no significant lower-body improvements, and ipsilateral training effects were greater than contralateral training in adults.

Exercise prescription and strategies to promote the cross-education of strength: a scoping review

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.

Cerebral Hemodynamic Changes during Unaffected Handgrip Exercises in Stroke Patients: An fNIRS Study

This study aimed to assess the effect of the altered strength of the sound limb on the hemodynamics in the affected brain of stroke patients. We recruited 20 stroke patients to detect changes in the HbO concentrations in the bilateral prefrontal cortex (PFC), sensorimotor cortex (SMC), and occipital lobe (OL). We performed functional near-infrared spectroscopy (fNIRS) to detect changes in oxyhemoglobin (HbO) concentrations in regions of interest (ROIs) in the bilateral cerebral hemispheres of stroke patients while they performed 20%, 50%, and 80% maximal voluntary contraction (MVC) levels of handgrip tasks with the unaffected hands. The results suggest that when patients performed handgrip tasks with 50% of the MVC force, SMC in the affected cerebral hemisphere was strongly activated and the change in the HbO concentration was similar to that of the handgrip with 80% of MVC. When the force was 50% of MVC, the SMC in the affected hemisphere showed a more proportional activation than that at 80% MVC. Overall, this research suggests that stroke patients with a poor upper limb function should perform motor training with their sound hands at 50% of the MVC grip task to activate the ipsilesional hemisphere.

Bilateral Improvements Following Unilateral Home-Based Training in Plantar Flexors: A Potential for Cross-Education in Rehabilitation

CONTEXT

Cross-education (CE) refers to neuromuscular gains in the untrained limb upon contralateral limb training. To date, only laboratory-based exercise programs have demonstrated CE. Home-based exercise prescription eliciting CE could have greater clinical applicability.

OBJECTIVE

To determine the effect of an 8-week, home-based unilateral strength training intervention on isokinetic muscle strength, muscular excitation, and power in trained and untrained plantar flexors.

DESIGN

Randomized controlled trial.

METHODS

Thirty-four healthy participants were randomized to intervention (n = 20) or control (n = 14). The intervention group completed 3 sets of 12 repetitions of progressively loaded unilateral calf raises 3 days per week. Concentric and eccentric peak torque were measured using isokinetic dynamometry at 30°/s and 120°/s. Maximal electromyogram amplitude was simultaneously measured. Power was measured using a jump mat. All variables were measured at preintervention, midintervention, and postintervention.

RESULTS

Strength significantly increased bilaterally pre-post at both velocities concentrically and eccentrically in intervention group participants. Maximal electromyogram amplitude significantly increased pre-post bilaterally at both velocities in the medial gastrocnemii of the intervention group. Power significantly increased bilaterally pre-post in the intervention group, with a dose-response effect demonstrated in the untrained plantar flexors. The CE effects of strength, power, and electromyogram activation were 23.4%, 14.6%, and 25.3%, respectively. All control group values were unchanged pre-post.

CONCLUSION

This study shows that a simple at-home unilateral plantar flexor exercise protocol induces significant increases in contralateral strength, muscular excitation, and power. These results suggest the applicability of CE in home rehabilitation programs aiming to restore or maintain neuromuscular function in inactive individuals or immobilized ankles.

The Effect of Contralateral Knee Neuromuscular Exercises on Static and Dynamic Balance, Knee Function, and Pain in Athletes Who Underwent Anterior Cruciate Ligament Reconstruction: A Single-Blind Randomized Controlled Trial

CONTEXT

Contralateral training in the early stages after surgery can improve the balance of the reconstructed knee, which is impaired following anterior cruciate ligament reconstruction (ACLR). However, little is known about the neuromuscular cross exercise after ACLR.

OBJECTIVE

To investigate the effects of an 8-week cross exercise on balance and function of the reconstructed knee following ACLR.

DESIGN

A single-blind randomized clinical trial.

PARTICIPANTS

Thirty athletic males who underwent ACLR were randomly divided into intervention (n = 15) and control groups (n = 15).

INTERVENTION

The intervention and control groups received a routine physiotherapy program. In addition, the intervention group performed neuromuscular exercises on the nonoperated limb.

OUTCOME MEASURES

Before and 9 weeks after ACLR, dynamic and static balance, function, and pain in the reconstructed knee were measured by Star Excursion Balance Test (SEBT), stork balance stand test, balance error scoring system (BESS), Lysholm questionnaire, and visual analog scale. Data were analyzed by SPSS using 2-independent sample t test, paired t test, and analysis of covariance.

RESULTS

Between-group comparison showed that, contralateral knee neuromuscular exercises significantly increased in the reaching distance in SEBT in the anterior (P < .001), posterior (P < .001), posteromedial (P = .010), and posterolateral directions (P = .007), decreased the number of errors in 4 stance positions of BESS including single stance on the firm (P ≤ .001) and foam surface (P ≤ .001), and tandem stance on the firm (P = .028) and foam surface (P ≤ .001). It also increased the time of standing of the stork stand test (P = .044) and decreased the pain intensity (P = .014).

CONCLUSION

Neuromuscular exercise of the nonsurgical knee could improve the dynamic and static balance, and pain in the early stages following ACLR in the surgical leg. These findings may be potentially valuable for current rehabilitation protocols.

Does wing use and disuse cause behavioural and musculoskeletal changes in domestic fowl (Gallus gallus domesticus)?

Domestic chickens may live in environments which restrict wing muscle usage. Notably, reduced wing activity and accompanying muscle weakness are hypothesized risk factors for keel bone fractures and deviations. We used radio-frequency identification (RFID) to measure duration spent at elevated resources (feeders, nest-boxes), ultrasonography to measure muscle thickness (breast and lower leg) changes, radiography and palpation to determine fractures and deviations, respectively, following no, partial (one-sided wing sling) and full (cage) immobilization in white- and brown-feathered birds. We hypothesized partially immobilized hens would reduce elevated resource usage and that both immobilization groups would show decreased pectoralis thickness (disuse) and increased prevalence of fractures and deviations. Elevated nest-box usage was 42% lower following five weeks of partial immobilization for brown-feathered hens but no change in resource usage in white-feathered birds was observed. Fully immobilized, white-feathered hens showed a 17% reduction in pectoralis thickness, while the brown-feathered counterparts showed no change. Lastly, fractures and deviations were not affected in either strain or form of wing immobilization; however, overall low numbers of birds presented with these issues. Altogether, this study shows a profound difference between white- and brown-feathered hens in response to wing immobilization and associated muscle physiology.

Motor effects of movement representation techniques and cross-education: a systematic review and meta-analysis

INTRODUCTION

The objective was to assess the impact of movement representation techniques (MRT) through motor imagery (MI), action observation (AO) and visual mirror feedback (VMF) and cross-education training (CE) on strength, range of motion (ROM), speed, functional state and balance during experimental immobilization processes in healthy individuals, in patients with injuries that did not require surgery and in those with surgical processes that did or did not require immobilization.

EVIDENCE ACQUISITION

MEDLINE, EMBASE, CINAHL and Google Scholar were searched. Thirteen meta-analyses were conducted.

EVIDENCE SYNTHESIS

Regarding the immobilized participants, in the healthy individuals, MI showed significant results regarding maintenance of strength and ROM, with low-quality evidence. Regarding the process with no immobilization, VMF and MI techniques showed significant changes in maintaining ROM in patients with injury without surgery, with very low-quality evidence. Results had shown that MI demonstrated significantly higher maintenance of strength and speed in patients undergoing surgery, with low-quality evidence. No significant results were found in ROM. Low-quality evidence showed better results in AO plus usual care compared with usual treatment in isolation with respect to maintenance of functional state and balance. CE training demonstrated maintenance of strength in patients undergoing surgery, with moderate evidence; however, not in healthy experimentally immobilized individuals. VMF did not show significant results in maintaining ROM after surgery without immobilization, nor did MI in maintaining strength after surgery and immobilization.

CONCLUSIONS

MRT and CE training have been shown to have a significant impact on the improvement of various motor variables and on physical maintenance in general.

The Impact of Lower Limb Immobilization and Rehabilitation on Angiogenic Proteins and Capillarization in Skeletal Muscle

PURPOSE

Skeletal muscle vascularization is important for tissue regeneration after injury and immobilization. We examined whether complete immobilization influences capillarization and oxygen delivery to the muscle and assessed the efficacy of rehabilitation by aerobic exercise training.

METHODS

Young healthy males had one leg immobilized for 14 d and subsequently completed 4 wk of intense aerobic exercise training. Biopsies were obtained from musculus vastus lateralis, and arteriovenous blood sampling for assessment of oxygen extraction and leg blood flow during exercise was done before and after immobilization and training. Muscle capillarization, muscle and platelet content of vascular endothelial growth factor (VEGF), and muscle thrombospondin-1 were determined.

RESULTS

Immobilization did not have a significant impact on capillary per fiber ratio or capillary density. The content of VEGF protein in muscle samples was reduced by 36% (P = 0.024), and VEGF to thrombospondin-1 ratio was 94% lower (P = 0.046). The subsequent 4-wk training period increased the muscle VEGF content and normalized the muscle VEGF to thrombospondin-1 ratio but did not influence capillarization. Platelet VEGF content followed the trend of muscle VEGF. At the functional level, oxygen extraction, blood flow, and oxygen delivery at rest and during submaximal exercise were not affected by immobilization or training.

CONCLUSIONS

The results demonstrate that just 2 wk of leg immobilization leads to a strongly reduced angiogenic potential as evidenced by reduced muscle and platelet VEGF content and a reduced muscle VEGF to thrombospondin-1 ratio. Moreover, a subsequent period of intensive aerobic exercise training fails to increase capillarization in the previously immobilized leg, possibly because of the angiostatic condition caused by immobilization.

Set Configuration in Strength Training Programs Modulates the Cross Education Phenomenon

ABSTRACT

Fariñas, J, Mayo, X, Giraldez-García, MA, Carballeira, E, Fernandez-Del-Olmo, M, Rial-Vazquez, J, Kingsley, JD, and Iglesias-Soler, E. Set configuration in strength training programs modulates the cross education phenomenon. J Strength Cond Res 35(9): 2414-2420, 2021-This study aimed to compare the strength gains in the nontrained arm after 2 independent unilateral training programs differing in the set configuration. Thirty-five subjects were randomly assigned to 3 groups: traditional training (TT; n = 12), cluster training (CT; n = 11), or control (CON; n = 12). The experimental groups performed a 5-week training program of a unilateral biceps curl exercise with the dominant limb using the 10 repetition maximum (10RM) load. Traditional training performed 5 sets of 6 repetitions and 135 seconds of rest between sets. Cluster training completed 30 repetitions with 18.5 seconds of rest between each repetition. Anthropometry (ANT), muscle thickness (MT), 1RM, the number of repetitions with 10RM (n10RM), and isometric maximal voluntary contraction (MVC) were measured before and after the intervention. Regarding the nontrained arm, TT improved 1RM (7.3%, p < 0.001). No changes were observed in CT. Regarding the trained arm, TT improved 1RM (9.1%, p < 0.001), n10RM (p = 0.005), and MVC (p = 0.011), whereas CT only showed a trend for improvement of 1RM (3.4%, p = 0.052). These results suggest that when total volume and repetition-to-rest ratio are equated, a more fatiguing set configuration causes a higher effect on the non-trained limb.

Relationship of Barbell and Dumbbell Repetitions With One Repetition Maximum Bench Press in College Football Players

ABSTRACT

Heinecke, ML, Mauldin, ML, Hunter, ML, Mann, JB, and Mayhew, JL. Relationship of barbell and dumbbell repetitions with one repetition maximum bench press in college football players. J Strength Cond Res 35(2S): S66-S71, 2021-Dumbbell training to augment barbell training is gaining popularity. However, information is lacking that details the compatibility of strength and endurance between dumbbell and barbell performances in the same exercise. Therefore, the purposes of this study were to compare the similarity of muscular endurance performance between dumbbell and barbell exercises and to assess the accuracy of predicting one repetition maximum (1RM) barbell bench press from barbell and dumbbell repetitions to fatigue (RTF). College football players (n = 40) performed 1RM barbell bench press and RTF with a 90.9-kg barbell. On separate days, unilateral (45.5 kg) and bilateral dumbbell (90.9 kg) RTF were performed. Barbell RTF (13.8 ± 9.2) were significantly greater (effect size [ES] = 0.14) than bilateral dumbbell RTF (12.5 ± 9.5) but highly correlated (r = 0.96). Unilateral dumbbell RTF were significantly greater (ES = 0.13) for dominant hand (10.8 ± 10.1) than nondominant hand (9.5 ± 9.7) but highly correlated (r = 0.97). Prediction of 1RM barbell bench press was equally effective using a constant weight barbell (r = 0.90) or equivalent weight bilateral dumbbells (r = 0.87) with total errors of 7.3 and 8.2%, respectively. Barbell and dumbbell repetitions with equivalent weights place a similar demand on the upper-body musculature for training and testing purposes in football athletes.

Massage as a Mechanotherapy for Skeletal Muscle

Massage is anecdotally associated with many health benefits, but physiological and clinically relevant mechanisms recently have begun to be investigated in a controlled manner. Herein, we describe research supporting our hypothesis that massage can be used as a mechanotherapy imparting biologically relevant adaptations in skeletal muscle and improving muscle properties.

Symmetric interlimb transfer of newly acquired skilled movements

In this study, we aimed to examine features of interlimb generalization or "transfer" of newly acquired motor skills, with a broader goal of better understanding the mechanisms mediating skill learning. Right-handed participants (n = 36) learned a motor task that required them to make very rapid but accurate reaches to one of eight randomly presented targets, thus bettering the typical speed-accuracy tradeoff. Subjects were divided into an "RL" group that first trained with the right arm and was then tested on the left and an "LR" group that trained with the left arm and was subsequently tested on the right. We found significant interlimb transfer in both groups. Remarkably, we also observed that participants learned faster with their left arm compared with the right. We hypothesized that this could be due to a previously suggested left arm/right hemisphere advantage for movements under variable task conditions. To corroborate this, we recruited two additional groups of participants (n = 22) that practiced the same task under a single target condition. This removal of task level variability eliminated learning rate differences between the arms, yet interlimb transfer remained robust and symmetric, as in the first experiment. Additionally, the strategy used to reduce errors during learning, albeit heterogeneous across subjects particularly in our second experiment, was adopted by the untrained arm. These findings may be best explained as the outcome of the operation of cognitive strategies during the early stages of motor skill learning.NEW & NOTEWORTHY How newly acquired motor skills generalize across effectors is not well understood. Here, we show that newly learned skilled actions transfer symmetrically across the arms and that task-level variability influences learning rate but not transfer magnitude or direction. Interestingly, strategies developed during learning with one arm transfer to the untrained arm. This likely reflects the outcome of learning driven by cognitive mechanisms during the initial stages of motor skill acquisition.

Contralateral effects of eccentric resistance training on immobilized arm

This study compared the effects of contralateral eccentric-only (ECC) and concentric-/eccentric-coupled resistance training (CON-ECC) of the elbow flexors on immobilized arm. Thirty healthy participants (18-34 y) were randomly allocated to immobilization only (CTRL; n = 10), immobilization and ECC (n = 10), or immobilization and CON-ECC group (n = 10). The non-dominant arms of all participants were immobilized (8 h·day^-1 ) for 4 weeks, during which ECC and CON-ECC were performed by the dominant (non-immobilized) arm 3 times a week (3-6 sets of 10 repetitions per session) with an 80%-120% and 60%-90% of one concentric repetition maximum (1-RM) load, respectively, matching the total training volume. Arm circumference, 1-RM and maximal voluntary isometric contraction (MVIC) strength, biceps brachii surface electromyogram amplitude (sEMG_RMS ), rate of force development (RFD), and joint position sense (JPS) were measured for both arms before and after immobilization. CTRL showed decreases (P < .05) in MVIC (-21.7%), sEMG_RMS (-35.2%), RFD (-26.0%), 1-RM (-14.4%), JPS (-87.4%), and arm circumference (-5.1%) of the immobilized arm. These deficits were attenuated or eliminated by ECC and CON-ECC, with greater effect sizes for ECC than CON-ECC in MVIC (0.29: +12.1%, vs -0.18: -0.1%) and sEMG_RMS (0.31:17.5% vs -0.15: -5.9%). For the trained arm, ECC showed greater effect size for MVIC than CON-ECC (0.47 vs 0.29), and increased arm circumference (+2.9%), sEMG_RMS (+77.9%), and RDF (+31.8%) greater (P < .05) than CON-ECC (+0.6%, +15.1%, and + 15.8%, respectively). The eccentric-only resistance training of the contralateral arm was more effective to counteract the negative immobilization effects than the concentric-eccentric training.

The Impact of Disuse and High-Fat Overfeeding on Forearm Muscle Amino Acid Metabolism in Humans

CONTEXT

Anabolic resistance is mechanistically implicated in muscle disuse atrophy.

OBJECTIVE

The objective of this study is to assess whether anabolic resistance is associated with reduced postprandial amino acid uptake or exacerbated by excess lipid availability.

DESIGN, SETTING, PARTICIPANTS, AND INTERVENTIONS

Twenty men underwent 7 days of forearm immobilization while consuming a eucaloric (CON; n = 11) or high-fat overfeeding (HFD; n = 9; 50% excess energy as fat) diet (parallel design) within our Nutritional Physiology Research Unit.

MAIN OUTCOME MEASURES

Preimmobilization and postimmobilization we measured forearm muscle cross-sectional area (aCSA), and postabsorptive and postprandial (3-hour postingestion of a liquid, protein-rich, mixed meal) forearm amino acid metabolism using the arterialized venous-deep venous balance method and infusions of L-[ring-2H5]phenylalanine and L-[1-13C]leucine.

RESULTS

Immobilization did not affect forearm muscle aCSA in either group, but tended to reduce postabsorptive phenylalanine (P = .07) and leucine (P = .05) net balances equivalently in CON and HFD. Mixed-meal ingestion switched phenylalanine and leucine net balances from negative to positive (P < .05), an effect blunted by immobilization (P < .05) and to a greater extent in HFD than CON (P < .05). Preimmobilization, meal ingestion increased leucine rates of disappearance (Rd; P < .05), with values peaking at 191% (from 87 ± 38 to 254 ± 60 µmol·min-1·100 mL forearm volume-1) and 183% (from 141 ± 24 to 339 ± 51 µmol·min-1·100 mL-1) above postabsorptive rates in CON and HFD, respectively, with meal-induced increases not evident postimmobilization in either group (P > .05).

CONCLUSIONS

Disuse impairs the ability of a protein-rich meal to promote positive muscle amino acid balance, which is aggravated by dietary lipid oversupply. Moreover, disuse reduced postprandial forearm amino acid uptake; however, this is not worsened under high-fat conditions.

Effect of Immobilisation on Neuromuscular Function In Vivo in Humans: A Systematic Review

BACKGROUND

Muscle strength loss following immobilisation has been predominantly attributed to rapid muscle atrophy. However, this cannot fully explain the magnitude of muscle strength loss, so changes in neuromuscular function (NMF) may be involved.

OBJECTIVES

We systematically reviewed literature that quantified changes in muscle strength, size and NMF following periods of limb immobilisation in vivo in humans.

METHODS

Studies were identified following systematic searches, assessed for inclusion, data extracted and quality appraised by two reviewers. Data were tabulated and reported narratively.

RESULTS

Forty eligible studies were included, 22 immobilised lower and 18 immobilised upper limbs. Limb immobilisation ranged from 12 h to 56 days. Isometric muscle strength and muscle size declined following immobilisation; however, change magnitude was greater for strength than size. Evoked resting twitch force decreased for lower but increased for upper limbs. Rate of force development either remained unchanged or slowed for lower and typically slowed for upper limbs. Twitch relaxation rate slowed for both lower and upper limbs. Central motor drive typically decreased for both locations, while electromyography amplitude during maximum voluntary contractions decreased for the lower and presented mixed findings for the upper limbs. Trends imply faster rates of NMF loss relative to size earlier in immobilisation periods for all outcomes.

CONCLUSIONS

Limb immobilisation results in non-uniform loss of isometric muscle strength, size and NMF over time. Different outcomes between upper and lower limbs could be attributed to higher degrees of central neural control of upper limb musculature. Future research should focus on muscle function losses and mechanisms following acute immobilisation.

REGISTRATION

PROSPERO reference: CRD42016033692.

Biomechanical Evaluation of Preoperative Rehabilitation in Patients of Anterior Cruciate Ligament Injury

Objectives

To investigate the biomechanical characteristics of patients with anterior cruciate ligament (ACL) injury by gait analysis, surface electromyography (SEMG), and proprioception test, and provide rehabilitation suggestions according to the results.

Methods

In this retrospective cohort study, 90 adults with unilateral ACL injury, ranging in age from 19 to 45 years (66 men and 24 women, average age: 30.03 ± 7.91) were recruited for this study form May 2018 to July 2019. They were divided into three groups according to the time after the injury: group A (3‐week to 1.5‐month), group B (1.5‐month to 1 year), and group C (more than 1 year). The SEMG signals were collected from the bilateral rectus femoris (RF), vastus medialis (VM), and vastus lateralis (VL) and the root mean square (RMS) were used to assess muscular activity. SEMG were used to analyze muscles function, gait analysis was used to evaluate the walking stability, balance and location assessment were used to analyze the proprioception.

Results

Through the comparison between bilateral limbs, all muscles strength shown decreased (RF: 239.94 ± 129.70 vs 364.81 ± 148.98, P = 0.001; VM: 298.88 ± 175.41 vs 515.79 ± 272.49, P = 0.001; VL:389.54 ± 157.97 vs 594.28 ± 220.31, P < 0.001) and the division of proprioception became larger (tandem position: 7.79 ± 1.57 vs 6.33 ± 1.49, P = 0.001; stance with one foot: 8.13 ± 0.84 vs 7.1 ± 0.57, P = 0.003; variance of 30°: 6.96 ± 3.15 vs 4.45 ± 1.67, P = 0.03; variance of 60°: 4.64 ± 3.38 vs 2.75 ± 1.98, P = 0.044) in the injured side when compared to the non‐injured and 26 gait parameters were shown difference in group A. In group B, the muscle strength of VL shown decreased (VL: 381.23 ± 142.07 vs 603.9 ± 192.72, P < 0.001) and the division of location of 30° became larger (7.62 ± 4.98 vs 4.33 ± 3.24, P = 0.028) in the injured side when compared to the non‐injured side and there were eight gait parameters that showed differences. In group C, the muscle strength and proprioception showed no differences and only 16 gait parameters showed differences between the bilateral limbs.

Conclusion

The results proved the deterioration of proprioception in 30° of injured side will not recover and non‐injury side and will become worse after 1 year from the injury; among the VL, VM, and RF, the recovery rate of VL is the slowest and bilateral straight leg raising (SLR) (30°) is the best way to train it; the gait stability will be worse after 1 year from the injury. Therefore, we suggest that the training for proprioception in 30° and VL are important for the rehabilitation, and the ACL reconstruction should be performed within 1 year.

The contraction history of the muscle and strength change: lessons learned from unilateral training models

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 time course of cross-education during short-term isometric strength training

PURPOSE

This 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.

RESULTS

The 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.

Dominant and nondominant leg press training induce similar contralateral and ipsilateral limb training adaptations with children

Cross-education has been extensively investigated with adults. Adult studies report asymmetrical cross-education adaptations predominately after dominant limb training. The objective of the study was to examine unilateral leg press (LP) training of the dominant or nondominant leg on contralateral and ipsilateral strength and balance measures. Forty-two youth (10-13 years) were placed (random allocation) into a dominant (n = 15) or nondominant (n = 14) leg press training group or nontraining control (n = 13). Experimental groups trained 3 times per week for 8 weeks and were tested pre-/post-training for ipsilateral and contralateral 1-repetition maximum (RM) horizontal LP, maximum voluntary isometric contraction (MVIC) of knee extensors (KE) and flexors (KF), countermovement jump (CMJ), triple hop test (THT), MVIC strength of elbow flexors (EF) and handgrip, as well as the stork and Y balance tests. Both dominant and nondominant LP training significantly (p < 0.05) increased both ipsilateral and contralateral lower body strength (LP 1RM (dominant: 59.6%-81.8%; nondominant: 59.5%-96.3%), KE MVIC (dominant: 12.4%-18.3%; nondominant: 8.6%-18.6%), KF MVIC (dominant: 7.9%-22.3%; nondominant: nonsignificant-3.8%), and power (CMJ: dominant: 11.1%-18.1%; nondominant: 7.7%-16.6%)). The exception was that nondominant LP training demonstrated a nonsignificant change with the contralateral KF MVIC. Other significant improvements were with nondominant LP training on ipsilateral EF 1RM (6.2%) and THT (9.6%). There were no significant changes with EF and handgrip MVIC. The contralateral leg stork balance test was impaired following dominant LP training. KF MVIC exhibited the only significant relative post-training to pretraining (post-test/pre-test) ratio differences between dominant versus nondominant LP cross-education training effects. In conclusion, children exhibit symmetrical cross-education or global training adaptations with unilateral training of dominant or nondominant upper leg.

Change in skeletal muscle associated with unplanned hospital admissions in adult patients: A systematic review and meta-analysis

OBJECTIVES

The primary objective of the review was to describe change that occurs in skeletal muscle during periods of unplanned hospitalisation in adult patients. The secondary objective was to examine the relationship between both physical activity and inflammation with the change in skeletal muscle. A further objective was to investigate the effect of interventions on change in skeletal muscle during periods of unplanned hospitalisation.

DESIGN

A systematic review and meta-analyses. Embase, MEDLINE, CINAHL, AMED, PEDro and the Cochrane Library were searched for studies that included any measures of skeletal muscle (excluding pulmonary function) at two time points during unplanned hospitalisation. Studies that were set in critical care, or included patients with acute or progressive neurological illness, were excluded.

RESULTS

Our search returned 27,809 unique articles, of which 35 met the inclusion criteria. Meta-analyses of change between baseline and follow-up in random effects models suggested that grip strength had an average increase: standardised mean difference (SMD) = 0.10 (95% CI: 0.03; 0.16); knee extension strength had an average reduction: SMD = -0.24 (95% CI: -0.33; -0.14); and mid-arm muscle circumference had an average reduction: SMD = -0.17 (95% CI: -0.22; -0.11). Inflammation appeared to be associated with greater loss of muscle strength. There was inconclusive evidence that the level of physical activity affects change in skeletal muscle. In regard to the effect of interventions, only exercise interventions were consistently associated with improved skeletal muscle outcomes.

CONCLUSIONS

Adult patients who undergo an unplanned hospital admission may experience a small reduction in knee extension strength and mid-arm muscle mass. Prospective research is needed to clarify the contribution of confounding factors underlying the observations made in this review, with particular attention to levels of physical activity, and possible contributions from environmental factors and processes of hospital care.

Determining the potential sites of neural adaptation to cross-education: implications for the cross-education of muscle strength

Cross-education describes the strength gain in the opposite, untrained limb following a unilateral strength training program. Since its discovery in 1894, several studies now confirm the existence of cross-education in contexts that involve voluntary dynamic contractions, eccentric contraction, electrical stimulation, whole-body vibration and, more recently, following mirror feedback training. Although many aspects of cross-education have been established, the mediating neural mechanisms remain unclear. Overall, the findings of this review show that the neural adaptations to cross-education of muscle strength most likely represent a continuum of change within the central nervous system that involves both structural and functional changes within cortical motor and non-motor regions. Such changes are likely to be the result of more subtle changes along the entire neuroaxis which include, increased corticospinal excitability, reduced cortical inhibition, reduced interhemispheric inhibition, changes in voluntary activation and new regions of cortical activation. However, there is a need to widen the breadth of research by employing several neurophysiological techniques (together) to better understand the potential mechanisms mediating cross-education. This fundamental step is required in order to better prescribe targeted and effective guidelines for the clinical practice of cross-education. There is a need to determine whether similar cortical responses also occur in clinical populations where, perhaps, the benefits of cross-education could be best observed.

Cross-education does not accelerate the rehabilitation of neuromuscular functions after ACL reconstruction: a randomized controlled clinical trial

PURPOSE

Cross-education reduces quadriceps weakness 8 weeks after anterior cruciate ligament (ACL) surgery, but the long-term effects are unknown. We investigated whether cross-education, as an adjuvant to the standard rehabilitation, would accelerate recovery of quadriceps strength and neuromuscular function up to 26 weeks post-surgery.

METHODS

Group allocation was randomized. The experimental (n = 22) and control (n = 21) group received standard rehabilitation. In addition, the experimental group strength trained the quadriceps of the non-injured leg in weeks 1-12 post-surgery (i.e., cross-education). Primary and secondary outcomes were measured in both legs 29 ± 23 days prior to surgery and at 5, 12, and 26 weeks post-surgery.

RESULTS

The primary outcome showed time and cross-education effects. Maximal quadriceps strength in the reconstructed leg decreased 35% and 12% at, respectively, 5 and 12 weeks post-surgery and improved 11% at 26 weeks post-surgery, where strength of the non-injured leg showed a gradual increase post-surgery up to 14% (all p ≤ 0.015). Limb symmetry deteriorated 9-10% more for the experimental than control group at 5 and 12 weeks post-surgery (both p ≤ 0.030). One of 34 secondary outcomes revealed a cross-education effect: Voluntary quadriceps activation of the reconstructed leg was 6% reduced for the experimental vs. control group at 12 weeks post-surgery (p = 0.023). Both legs improved force control (22-34%) and dynamic balance (6-7%) at 26 weeks post-surgery (all p ≤ 0.043). Knee joint proprioception and static balance remained unchanged.

CONCLUSION

Standard rehabilitation improved maximal quadriceps strength, force control, and dynamic balance in both legs relative to pre-surgery but adding cross-education did not accelerate recovery following ACL reconstruction.

Contralateral effects of unilateral training: sparing of muscle strength and size after immobilization

The contralateral effects of unilateral strength training, known as cross-education of strength, date back well over a century. In the last decade, a limited number of studies have emerged demonstrating the preservation or "sparing" effects of cross-education during immobilization. Recently published evidence reveals that the sparing effects of cross-education show muscle site specificity and involve preservation of muscle cross-sectional area. The new research also demonstrates utility of training with eccentric contractions as a potent stimulus to preserve immobilized limb strength across multiple modes of contraction. The cumulative data in nonclinical settings suggest that cross-education can completely abolish expected declines in strength and muscle size in the range of ∼13% and ∼4%, respectively, after 3-4 weeks of immobilization of a healthy arm. The evidence hints towards the possibility that unique mechanisms may be involved in preservation effects of cross-education, as compared with those that lead to functional improvements under normal conditions. Cross-education effects after strength training appear to be larger in clinical settings, but there is still only 1 randomized clinical trial demonstrating the potential utility of cross-education in addition to standard treatment. More work is necessary in both controlled and clinical settings to understand the potential interaction of neural and muscle adaptations involved in the observed sparing effects, but there is growing evidence to advocate for the clinical utility of cross-education.

Unilateral strength training leads to muscle-specific sparing effects during opposite homologous limb immobilization

Cross education (CE) occurs after unilateral training whereby performance of the untrained contralateral limb is enhanced. A few studies have shown that CE can preserve or "spare" strength and size of an opposite immobilized limb, but the specificity (i.e., trained homologous muscle and contraction type) of these effects is unknown. The purpose was to investigate specificity of CE "sparing" effects with immobilization. The nondominant forearm of 16 participants was immobilized with a cast, and participants were randomly assigned to a resistance training (eccentric wrist flexion, 3 times/week) or control group for 4 weeks. Pre- and posttesting involved wrist flexors and extensors eccentric, concentric and isometric maximal voluntary contractions (via dynamometer), muscle thickness (via ultrasound), and forearm muscle cross-sectional area (MCSA; via peripheral quantitative computed tomography). Only the training group showed strength preservation across all contractions in the wrist flexors of the immobilized limb (training: -2.4% vs. control: -21.6%; P = 0.04), and increased wrist flexors strength of the nonimmobilized limb (training: 30.8% vs. control: -7.4%; P = 0.04). Immobilized arm MCSA was preserved for the training group only (training: 1.3% vs. control: -2.3%; P = 0.01). Muscle thickness differed between groups for the immobilized (training: 2.8% vs. control: -3.2%; P = 0.01) and nonimmobilized wrist flexors (training: 7.1% vs. control: -3.7%; P = 0.02). Strength preservation was nonspecific to contraction type ( P = 0.69, [Formula: see text] = 0.03) yet specific to the trained flexors muscle. These findings suggest that eccentric training of the nonimmobilized limb can preserve size of the immobilized contralateral homologous muscle and strength across multiple contraction types. NEW & NOTEWORTHY Unilateral strength training preserves strength, muscle thickness, and muscle cross-sectional area in an opposite immobilized limb. The preservation of size and strength was confined to the trained homologous muscle group. However, strength was preserved across multiple contraction types.

Cross-education of muscular strength following unilateral resistance training: a meta-analysis

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.

Ipsilateral corticomotor responses are confined to the homologous muscle following cross-education of muscular strength

Cross-education of strength occurs when strength-training 1 limb increases the strength of the untrained limb and is restricted to the untrained homologous muscle. Cortical circuits located ipsilateral to the trained limb might be involved. We used transcranial magnetic stimulation (TMS) to determine the corticomotor responses from the untrained homologous (biceps brachii) and nonhomologous (flexor carpi radialis) muscle following strength-training of the right elbow flexors. Motor evoked potentials were recorded from the untrained left biceps brachii and flexor carpi radialis during a submaximal contraction from 20 individuals (10 women, 10 men; aged 18-35 years; training group, n = 10; control group, n = 10) before and after 3 weeks of strength-training the right biceps brachii at 80% of 1-repetition maximum. Recruitment-curves for corticomotor excitability and inhibition of the untrained homologous and nonhomologous muscle were constructed and assessed by examining the area under the recruitment curve. Strength-training increased strength of the trained elbow flexors (29%), resulting in an 18% increase in contralateral strength of the untrained elbow flexors (P < 0.0001). The trained wrist flexors increased by 19%, resulting in a 12% increase in strength of the untrained wrist flexors (P = 0.005). TMS showed increased corticomotor excitability and decreased corticomotor inhibition for the untrained homologous muscle (P < 0.05); however, there were no changes in the untrained nonhomologous muscle (P > 0.05). These findings show that the cross-education of muscular strength is spatially distributed; however, the neural adaptations are confined to the motor pathway ipsilateral to the untrained homologous agonist.

The Cross-Education Phenomenon: Brain and Beyond

Objectives: Unilateral resistance training produces strength gains in the untrained homologous muscle group, an effect termed “cross-education.” The observed strength transfer has traditionally been considered a phenomenon of the nervous system, with few studies examining the contribution of factors beyond the brain and spinal cord. In this hypothesis and theory article, we aim to discuss further evidence for structural and functional adaptations occurring within the nervous, muscle, and endocrine systems in response to unilateral resistance training. The limitations of existing cross-education studies will be explored, and novel potential stakeholders that may contribute to the cross-education effect will be identified.

Design: Critical review of the literature.

Method: Search of online databases.

Results: Studies have provided evidence that functional reorganization of the motor cortex facilitates, at least in part, the effects of cross-education. Cross-activation of the “untrained” motor cortex, ipsilateral to the trained limb, plays an important role. While many studies report little or no gains in muscle mass in the untrained limb, most experimental designs have not allowed for sensitive or comprehensive investigation of structural changes in the muscle.

Conclusions: Increased neural drive originating from the “untrained” motor cortex contributes to the cross-education effect. Adaptive changes within the muscle fiber, as well as systemic and hormonal factors require further investigation. An increased understanding of the physiological mechanisms contributing to cross-education will enable to more effectively explore its effects and potential applications in rehabilitation of unilateral movement disorders or injury.

Cross-education after high-frequency versus low-frequency volume-matched handgrip training

INTRODUCTION

Cross-education training programs cause interlimb asymmetry of strength and hypertrophy. We examined the cross-education effects from a high-frequency (HF) versus a low-frequency (LF) volume-matched handgrip training program on interlimb asymmetry.

METHODS

Right-handed participants completed either HF (n = 10; 2 × 6 repetitions 10 times per week) or LF (n = 9; 5 × 8 repetitions 3 times per week) training. Testing occurred twice before and once after 4 weeks of right-handed isometric handgrip training totaling 120 weekly repetitions. Measures were maximal isometric handgrip and wrist flexion torque, muscle thickness, and muscle activation (electromyography; EMG).

RESULTS

Grip strength was greater in both limbs posttraining, pooled across groups (P < 0.001). Trained limb muscle thickness increased in both groups (P < 0.05; untrained, P = 0.897). EMG and wrist flexion torque did not change (all P > 0.103).

DISCUSSION

Both LF and HF induced cross-education of grip strength to the untrained limb, but HF did not reduce asymmetry. These findings have implications for injury rehabilitation. Muscle Nerve 56: 689-695, 2017.

Therapy and Rehabilitation for Upper Extremity Injuries in Athletes

The approach to rehabilitation of upper extremity injuries in athletes differs from traditional rehabilitation protocols. In general, athletes have higher functional demands and wish to return to competitive sport in a timely manner. Comprehensive rehabilitation must therefore be balanced with a timely and safe return to sport. Several rehabilitation programs and adjunctive therapies are available to hasten convalescence while minimizing the athlete's risks of reinjury. Here, we review techniques for soft tissue mobilization and strength training in athletic populations. We also discuss orthotics, taping, and alternative therapies used in rehabilitation and evaluate the evidence in support of these modalities.

Investigating human skeletal muscle physiology with unilateral exercise models: when one limb is more powerful than two

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.

Long-term progressive motor skill training enhances corticospinal excitability for the ipsilateral hemisphere and motor performance of the untrained hand

It is well established that unilateral motor practice can lead to increased performance in the opposite non-trained hand. Here, we test the hypothesis that progressively increasing task difficulty during long-term skill training with the dominant right hand increase performance and corticomotor excitability of the left non-trained hand. Subjects practiced a visuomotor tracking task engaging right digit V for 6 weeks with either progressively increasing task difficulty (PT) or no progression (NPT). Corticospinal excitability (CSE) was evaluated from the resting motor threshold (rMT) and recruitment curve parameters following application of transcranial magnetic stimulation (TMS) to the ipsilateral primary motor cortex (iM1) hotspot of the left abductor digiti minimi muscle (ADM). PT led to significant improvements in left-hand motor performance immediately after 6 weeks of training (63 ± 18%, P < 0.001) and 8 days later (76 ± 14%, P < 0.001). In addition, PT led to better task performance compared to NPT (19 ± 15%, P = 0.024 and 27 ± 15%, P = 0.016). Following the initial training session, CSE increased across all subjects. After 6 weeks of training and 8 days later, only PT was accompanied by increased CSE demonstrated by a left and upwards shift in the recruitment curves, e.g. indicated by increased MEP_max (P = 0.012). Eight days after training similar effects were observed, but 14 months later motor performance and CSE were similar between groups. We suggest that progressively adjusting demands for timing and accuracy to individual proficiency promotes motor skill learning and drives the iM1-CSE resulting in enhanced performance of the non-trained hand. The results underline the importance of increasing task difficulty progressively and individually in skill learning and rehabilitation training.

Mirror Visual Feedback Training Improves Intermanual Transfer in a Sport-Specific Task: A Comparison between Different Skill Levels

Mirror training therapy is a promising tool to initiate neural plasticity and facilitate the recovery process of motor skills after diseases such as stroke or hemiparesis by improving the intermanual transfer of fine motor skills in healthy people as well as in patients. This study evaluated whether these augmented performance improvements by mirror visual feedback (MVF) could be used for learning a sport-specific skill and if the effects are modulated by skill level. A sample of 39 young, healthy, and experienced basketball and handball players and 41 novices performed a stationary basketball dribble task at a mirror box in a standing position and received either MVF or direct feedback. After four training days using only the right hand, performance of both hands improved from pre- to posttest measurements. Only the left hand (untrained) performance of the experienced participants receiving MVF was more pronounced than for the control group. This indicates that intermanual motor transfer can be improved by MVF in a sport-specific task. However, this effect cannot be generalized to motor learning per se since it is modulated by individuals' skill level, a factor that might be considered in mirror therapy research.

Low-load resistance training during step-reduction attenuates declines in muscle mass and strength and enhances anabolic sensitivity in older men

Step-reduction (SR) in older adults results in muscle atrophy and an attenuated rise in postprandial muscle protein synthesis (MPS): anabolic resistance. Knowing that resistance exercise (RT) can enhance MPS, we examined whether RT could enhance MPS following 2 weeks of SR. In addition, as we postulated that SR may impair feeding-induced vasodilation limiting nutrient delivery to muscle, we also examined whether citrulline (CIT), as an arginine and nitric oxide precursor, could attenuate muscle anabolic resistance accompanying SR. We used a unilateral leg model to compare older subjects’ who had undergone SR to a loaded condition of SR plus RT (SR + RT). Thirty older men (70 ± 1 years) underwent 14 days of SR (<1500 steps/day) with supplementation of either 5 g/day CIT or glycine placebo. Throughout SR, subjects performed unilateral low-load RT thrice weekly. We assessed muscle protein synthesis in the postabsorptive and postprandial state (20 g whey isolate plus 15 g glycine or as micellar-whey with 5 g CIT or 15 g glycine, n = 10/group). As MPS was similar after ingestion of either whey isolate, micellar-whey, or micellar-whey + CIT data related to different dietary groups were collapsed to compare SR and SR + RT legs. Subjects’ daily steps were reduced by 80 ± 2% during SR (P < 0.001) compared with baseline. Leg fat-free mass decreased with SR (−124 ± 61 g) and increased in the SR + RT (+126 ± 68 g; P = 0.003). Myofibrillar FSR was lower (P < 0.0001) in the SR as compared with the SR + RT leg in the postabsorptive (0.026 ± 0.001%/h vs. 0.045 ± 0.001%/h) and postprandial states (0.055 ± 0.002%/h vs. 0.115 ± 0.003%/h). We conclude that low-load RT, but not supplementation with CIT, can attenuate the deleterious effects of SR in aging muscle.

Increased cross-education of muscle strength and reduced corticospinal inhibition following eccentric strength training

AIM

Strength training of one limb results in a substantial increase in the strength of the untrained limb, however, it remains unknown what the corticospinal responses are following either eccentric or concentric strength training and how this relates to the cross-education of strength. The aim of this study was to determine if eccentric or concentric unilateral strength training differentially modulates corticospinal excitability, inhibition and the cross-transfer of strength.

METHODS

Changes in contralateral (left limb) concentric strength, eccentric strength, motor-evoked potentials, short-interval intracortical inhibition and silent period durations were analyzed in groups of young adults who exercised the right wrist flexors with either eccentric (N=9) or concentric (N=9) contractions for 12 sessions over 4weeks. Control subjects (N=9) did not train.

RESULTS

Following training, both groups exhibited a significant strength gain in the trained limb (concentric group increased concentric strength by 64% and eccentric group increased eccentric strength by 62%) and the extent of the cross-transfer of strength was 28% and 47% for the concentric and eccentric group, respectively, which was different between groups (P=0.031). Transcranial magnetic stimulation revealed that eccentric training reduced intracortical inhibition (37%), silent period duration (15-27%) and increased corticospinal excitability (51%) compared to concentric training for the untrained limb (P=0.033). There was no change in the control group.

CONCLUSION

The results show that eccentric training uniquely modulates corticospinal excitability and inhibition of the untrained limb to a greater extent than concentric training. These findings suggest that unilateral eccentric contractions provide a greater stimulus in cross-education paradigms and should be an integral part of the rehabilitative process following unilateral injury to maximize the response.

Unilateral muscle overuse causes bilateral changes in muscle fiber composition and vascular supply

Unilateral strength training can cause cross-transfer strength effects to the homologous contralateral muscles. However, the impact of the cross-over effects on the muscle tissue is unclear. To test the hypothesis that unilateral muscle overuse causes bilateral alterations in muscle fiber composition and vascular supply, we have used an experimental rabbit model with unilateral unloaded overstrain exercise via electrical muscle stimulation (E/EMS). The soleus (SOL) and gastrocnemius (GA) muscles of both exercised (E) and contralateral non-exercised (NE) legs (n = 24) were morphologically analyzed after 1 w, 3 w and 6 w of EMS. Non-exercised rabbits served as controls (n = 6). After unilateral intervention the muscles of both E and NE legs showed myositis and structural and molecular tissue changes that to various degrees mirrored each other. The fiber area was bilaterally smaller than in controls after 3 w of E/EMS in both SOL (E 4420 and NE 4333 µm2 vs. 5183 µm2, p<0.05) and GA (E 3572 and NE 2983 µm2 vs. 4697 µm2, p<0.02) muscles. After 6 w of E/EMS, the percentage of slow MyHCI fibers was lower than in controls in the NE legs of SOL (88.1% vs. 98.1%, p<0.009), while the percentage of fast MyHCIIa fibers was higher in the NE legs of GA (25.7% vs. 15.8%, p = 0.02). The number of capillaries around fibers in the E and NE legs was lower (SOL 13% and 15%, respectively, GA 25% and 23%, respectively, p<0.05) than in controls. The overall alterations were more marked in the fast GA muscle than in the slow SOL muscle, which on the other hand showed more histopathological muscle changes. We conclude that unilateral repetitive unloaded overuse exercise via EMS causes myositis and muscle changes in fiber type proportions, fiber area and fiber capillarization not only in the exercised leg, but also in the homologous muscles in the non-exercised leg.

Role of the mirror-neuron system in cross-education

The present review proposes the untested hypothesis that cross-education performed with a mirror increases the transfer of motor function to the resting limb compared with standard cross-education interventions without a mirror. The hypothesis is based on neuroanatomical evidence suggesting an overlap in activated brain areas when a unilateral motor task is performed with and without a mirror in the context of cross-education of the upper extremities. The review shows that the mirror-neuron system (MNS), connecting sensory neurons responding to visual properties of an observed action and motor neurons that discharge action potentials during the execution of a similar action, has the potential to enhance cross-education.

Restoring symmetry: clinical applications of cross-education

The "restoring symmetry" hypothesis poses that cross-education of strength--a crossed-limb adaptation after unilateral training--is best applied to clinical conditions presenting with asymmetries. Cross-education mechanisms should be viewed as evolutionarily conserved circuits that have a small impact on daily life but a meaningful impact for rehabilitation. Two recently published examples are hemiparesis after stroke and unilateral orthopedic injury.

Neural pathways mediating cross education of motor function

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.

Cross-education for improving strength and mobility after distal radius fractures: a randomized controlled trial

OBJECTIVE

To evaluate the effects of cross-education (contralateral effect of unilateral strength training) during recovery from unilateral distal radius fractures on muscle strength, range of motion (ROM), and function.

DESIGN

Randomized controlled trial (26-wk follow-up).

SETTING

Hospital, orthopedic fracture clinic.

PARTICIPANTS

Women older than 50 years with a unilateral distal radius fracture. Fifty-one participants were randomized and 39 participants were included in the final data analysis.

INTERVENTIONS

Participants were randomized to standard rehabilitation (Control) or standard rehabilitation plus strength training (Train). Standard rehabilitation included forearm casting for 40.4±6.2 days and hand exercises for the fractured extremity. Nonfractured hand strength training for the training group began immediately postfracture and was conducted at home 3 times/week for 26 weeks.

MAIN OUTCOME MEASURES

The primary outcome measure was peak force (handgrip dynamometer). Secondary outcomes were ROM (flexion/extension; supination/pronation) via goniometer and the Patient Rated Wrist Evaluation questionnaire score for the fractured arm.

RESULTS

For the fractured hand, the training group (17.3±7.4kg) was significantly stronger than the control group (11.8±5.8kg) at 12 weeks postfracture (P<.017). There were no significant strength differences between the training and control groups at 9 (12.5±8.2kg; 11.3±6.9kg) or 26 weeks (23.0±7.6kg; 19.6±5.5kg) postfracture, respectively. Fractured hand ROM showed that the training group had significantly improved wrist flexion/extension (100.5°±19.2°) than the control group (80.2°±18.7°) at 12 weeks postfracture (P<.017). There were no significant differences between the training and control groups for flexion/extension ROM at 9 (78.0°±20.7°; 81.7°±25.7°) or 26 weeks (104.4°±15.5°; 106.0°±26.5°) or supination/pronation ROM at 9 (153.9°±23.9°; 151.8°±33.0°), 12 (170.9°±9.3°; 156.7°±20.8°) or 26 weeks (169.4°±11.9°; 162.8°±18.1°), respectively. There were no significant differences in Patient Rated Wrist Evaluation questionnaire scores between the training and control groups at 9 (54.2±39.0; 65.2±28.9), 12 (36.4±37.2; 46.2±35.3), or 26 weeks (23.6±25.6; 19.4±16.5), respectively.

CONCLUSIONS

Strength training for the nonfractured limb after a distal radius fracture was associated with improved strength and ROM in the fractured limb at 12 weeks postfracture. These results have important implications for rehabilitation strategies after unilateral injuries.

Nutritional strategies to attenuate muscle disuse atrophy

Situations such as recovery from injury or illness require otherwise healthy humans to undergo periods of disuse, which lead to considerable losses of skeletal muscle mass and, subsequently, numerous negative health consequences. It has been established that prolonged disuse (>10 days) leads to a decline in basal and postprandial rates of muscle protein synthesis, without an apparent change in muscle protein breakdown. It also seems, however, that an early and transient (1-5 days) increase in basal muscle protein breakdown may also contribute to disuse atrophy. A period of disuse reduces energy requirements and appetite. Consequently, food intake generally declines, resulting in an inadequate dietary protein consumption to allow proper muscle mass maintenance. Evidence suggests that maintaining protein intake during a period of disuse attenuates disuse atrophy. Furthermore, supplementation with dietary protein and/or essential amino acids can be applied to further aid in muscle mass preservation during disuse. Such strategies are of particular relevance to the older patient at risk of developing sarcopenia. More work is required to elucidate the impact of disuse on basal and postprandial rates of muscle protein synthesis and breakdown. Such information will provide novel targets for nutritional interventions to further attenuate muscle disuse atrophy and, as such, support healthy aging.

At-home resistance tubing strength training increases shoulder strength in the trained and untrained limb

The purpose was to determine if an at-home resistance tubing strength training program on one shoulder (that is commonly used in rehabilitation settings) would produce increases in strength in the trained and untrained shoulders via cross-education. Twenty-three participants were randomized to TRAIN (strength-trained one shoulder; n = 13) or CONTROL (no intervention; n = 10). Strength training was completed at home using resistance tubing and consisted of maximal shoulder external rotation, internal rotation, scaption, retraction, and flexion 3 days/week for 4 weeks. Strength was measured via handheld dynamometry and muscle size measured via ultrasound. For external rotation strength, the trained (10.9 ± 10.9%) and untrained (12.7 ± 9.6%) arm of TRAIN was significantly different than CONTROL (1.6 ± 13.2%; -2.7 ± 12.3%; pooled across arm; P < 0.05). For internal rotation strength, the trained (14.8 ± 11.3%) and untrained (14.6 ± 10.1%) arm of TRAIN was significantly different than CONTROL (6.4 ± 11.2%; 5.1 ± 8.8%; pooled across arm; P < 0.05). There were no significant differences for scaption strength (P = 0.056). TRAIN significantly increased muscle size in the training arm of the supraspinatus (1.90 ± 0.32 to 1.99 ± 0.31 cm), and the anterior deltoid (1.08 ± 0.37 to 1.21 ± 0.39 cm; P < 0.05). This study suggests that an at-home resistance tubing training program on one limb can produce increases in strength in both limbs, and has implications for rehabilitation after unilateral shoulder injuries.

Pronounced effects of acute endurance exercise on gene expression in resting and exercising human skeletal muscle

Regular physical activity positively influences whole body energy metabolism and substrate handling in exercising muscle. While it is recognized that the effects of exercise extend beyond exercising muscle, it is unclear to what extent exercise impacts non-exercising muscles. Here we investigated the effects of an acute endurance exercise bouts on gene expression in exercising and non-exercising human muscle. To that end, 12 male subjects aged 44–56 performed one hour of one-legged cycling at 50% W_max. Muscle biopsies were taken from the exercising and non-exercising leg before and immediately after exercise and analyzed by microarray. One-legged cycling raised plasma lactate, free fatty acids, cortisol, noradrenalin, and adrenalin levels. Surprisingly, acute endurance exercise not only caused pronounced gene expression changes in exercising muscle but also in non-exercising muscle. In the exercising leg the three most highly induced genes were all part of the NR4A family. Remarkably, many genes induced in non-exercising muscle were PPAR targets or related to PPAR signalling, including PDK4, ANGPTL4 and SLC22A5. Pathway analysis confirmed this finding. In conclusion, our data indicate that acute endurance exercise elicits pronounced changes in gene expression in non-exercising muscle, which are likely mediated by changes in circulating factors such as free fatty acids. The study points to a major influence of exercise beyond the contracting muscle.