Cross-education of wrist extensor strength is not influenced by non-dominant training in right-handers

Age, Sex, and Training Specific Effects on Cross-Education Training

An extensive number of publications have examined cross-education effects with adults, primarily investigating contralateral homologous (same) muscles. There are far fewer investigations on cross-education effects on contralateral heterologous (different) muscles and age (youth vs adult) and no studies investigating sex differences. Hence, the objective was to compare cross-education in female and male youth and young adults to contralateral homologous (chest press [CP], elbow flexors and extensors, handgrip isometric strength, and shot put) and heterologous (leg press, knee extension isometric strength, and countermovement jump) muscles. Twenty-eight female adults, 28 female youth, 28 male adults, and 28 male youth (total: 112) were examined before and after an 8-week (3 sessions/wk) unilateral, dominant arm, CP training program. Unilateral testing assessed dominant and nondominant leg press and CP 1-repetition maximum, knee extensors, elbow extensors, elbow flexors, and handgrip maximum voluntary isometric contraction (MVIC) strength, as well as shot put distance and countermovement jump height. Unilateral CP training induced training specific (CP 1-repetition maximum) and nonspecific (elbow extensors, elbow flexors, handgrip MVIC force, and shot put distance) improvements (P < .04, η2: .45-.85) but no significant lower body improvements. There was evidence for testing limb specificity as the dominant arm provided significantly (P < .021, η2: .17-.75) greater training gains than the nondominant arm. Youth's training adaptations exceeded with unilateral CP 1-repetition maximum, elbow extensors MVIC force, and shot put distance (P < .049, η2: .14-.49). No sex main effect differences were apparent. In conclusion, cross-education was training specific (greatest gains with upper body and dominant limbs) with greater benefits for youth and generally no sex differences with the exception of elbow extensors MVIC.

Is there evidence for the asymmetrical transfer of strength to an untrained limb?

PURPOSE

The literature predominantly addresses cross-education of strength in the dominant limb rather than the non-dominant limb, guided by the hypothesis of an asymmetrical transfer of strength from unilateral training protocols. The purpose of the study was to review the literature and determine how much evidence was available to support this claim. A meta-analysis was performed to estimate the magnitude of this hypothesized asymmetrical transfer of strength.

METHODS

A literature search of all possible records was implemented using Cochrane Library, PubMed, and Scopus from February 2022 to May 2022. Comparison of randomized controlled trials was computed. The change scores and standard deviations of those change scores were extracted for each group. Only three studies met the criteria, from which a total of five effect sizes were extracted and further analyzed.

RESULTS

The overall effect of resistance training of the dominant limb on strength transfer to the non-dominant limb relative to the effects of resistance training the non-dominant limb on strength transfer to the dominant (non-training) limb was 0.46 (SE 0.42). The analysis from this study resulted in minimal support for the asymmetry hypothesis. Given the small number of studies available, we provide the effect but note that the estimate is unlikely to be stable.

CONCLUSION

Although it is repeatedly stated that there is an asymmetrical transfer of strength, our results find little support for that claim. This is not to say that it does not exist, but additional research implementing a control group and a direct comparison between limbs is needed to better understand this question.

Effects of Tempo-Controlled Resistance Training on Corticospinal Tract Plasticity in Healthy Controls: A Systematic Review

After musculoskeletal injuries, there is often a loss of corticospinal control. Current tendon rehabilitation may not adequately address the corticospinal control of the muscle which may contribute to the recalcitrance of symptom recurrence. This review provides a summary of the current literature regarding the effectiveness of tempo-controlled resistance training (TCRT) in (1) promoting corticospinal plasticity, (2) improving physical performance, and (3) improving strength outcomes in healthy adults. A comprehensive literature search was conducted using electronic databases (PubMed, CINAHL, Embase, and Google Scholar) to identify relevant studies published between 2010 and 2023. Randomized control (RCT) studies that included recreationally trained and untrained healthy adults between 18 and 60 years of age and that compared a TCRT intervention to a control condition were included. Twelve of the 1255 studies identified in the initial search were included in the final analysis. Throughout all included studies, TCRT was shown to elicit greater neural adaptations compared to traditional resistance training methods (i.e., self-paced strength training). These results indicate that TCRT holds promise as an effective method for modulating corticospinal plasticity in healthy adults and may enhance neuromuscular adaptations, including improvements in CSE, decreased SICI, enhanced motor unit synchronization, and voluntary muscle activation.

Cross-Education of Muscular Endurance: A Scoping Review

BACKGROUND

It is well established that performing unilateral resistance training can increase muscle strength not only in the trained limb but also in the contralateral untrained limb, which is widely known as the cross-education of strength. However, less attention has been paid to the question of whether performing unilateral resistance training can induce cross-education of muscular endurance, despite its significant role in both athletic performance and activities of daily living.

OBJECTIVES

The objectives of this scoping review were to provide an overview of the existing literature on cross-education of muscular endurance, as well as discuss its potential underlying mechanisms and offer considerations for future research.

METHODS

A scoping review was conducted on the effects of unilateral resistance training on changes in muscular endurance in the contralateral untrained limb. This scoping review was conducted in PubMed, SPORTDiscus, and Scopus.

RESULTS

A total of 2000 articles were screened and 21 articles met the inclusion criteria. Among the 21 included studies, eight studies examined the cross-education of endurance via absolute (n = 6) or relative (n = 2) muscular endurance test, while five studies did not clearly indicate whether they examined absolute or relative muscular endurance. The remaining eight studies examined different types of muscular endurance measurements (e.g., time to task failure, total work, and fatigue index).

CONCLUSION

The current body of the literature does not provide sufficient evidence to draw clear conclusions on whether the cross-education of muscular endurance is present. The cross-education of muscular endurance (if it exists) may be potentially driven by neural adaptations (via bilateral access and/or cross-activation models that lead to cross-education of strength) and increased tolerance to exercise-induced discomfort. However, the limited number of available randomized controlled trials and the lack of understanding of underlying mechanisms provide a rationale for future research.

Unilateral high-load resistance training induced a similar cross-education of strength between the dominant and non-dominant arm

It was previously hypothesized that the cross-education of strength is asymmetrical, where a greater transfer of strength is observed from the dominant to the non-dominant limb. The purpose of this study was to examine if the magnitude of cross-education of strength differed between dominant and non-dominant limbs following unilateral high-load resistance training. One hundred and twenty-two participants were randomized to one of the three groups: 1) training on the dominant arm (D-Only), 2) training on the non-dominant arm (ND-Only) and 3) a time-matched non-exercise control (Control). The training groups completed 6 weeks (18 sessions) of unilateral elbow flexion exercise. Each training session started with one-repetition maximum (1RM) training (≤ five attempts), followed by four sets of high-load exercise (i.e. 8-12RM). Strength changes of the untrained arm were compared between groups. Changes in the strength of the untrained arm were greater in D-Only (1.5 kg) and ND-Only (1.3 kg) compared to Control (-0.2 kg), without differences between D-Only and ND-Only. Unilateral resistance training increased strength in the opposite untrained arm, and the magnitude of this effect was similar regardless of which arm was trained. However, there is still considerable uncertainty on this topic and additional research is warranted to confirm the current findings.

Cross-Education of Skill: Assessment of Overhand Throwing Using Product- and Process-Oriented Assessment

Purpose: The aims of the present study were to: (1) investigate the magnitude and direction of the cross-education effect in a unilateral sport skill (overhand throw) and (2) to establish which practice condition (dominant hand only or alternating hands) would yield the best results. Methods: The study involved three experimental groups of 11-year-old children (n = 59). The first group (n = 20) used only the dominant hand to throw the ball. The second group (n = 19) used the nondominant hand only, while the third (n = 20) alternated hands for each throw. A pre- and post-testing of both hands preceded and followed the intervention period. Results: The results of our study revealed no asymmetry in cross-education effect between the limbs for children's overhand ball throwing. It was also shown that training both hands is superior to training the dominant hand alone. Conclusion: Our findings would be of particular interest to physical education teachers and coaches of unilateral sports who are advised to review their unilateral skill teaching methods as bilateral training offers a superior approach to augmenting the process of motor learning and performance.

Can the cross-education of strength attenuate the impact of detraining after a period of strength training? A quasi-randomized trial

PURPOSE

Unilateral strength training may attenuate the decline in muscle strength and size in homologous, contralateral muscles. This study aimed to determine whether the cross-education of strength could specifically attenuate the effects of detraining immediately after a short (prehabilitation-type) period of strength training.

METHODS

Twenty-six strength-trained participants were assigned to either four weeks of unilateral strength training of the stronger arm (UNI) or detraining (Detrain). Motor evoked potential (MEP) and cortical silent period (cSP) responses, muscle cross-sectional area (CSAFlexor; peripheral quantitative computed tomography) and maximal strength, rate of force development (RFD) and muscle activation (EMG) were examined in both elbow flexors before and after the intervention period.

RESULTS

In UNI, one-repetition maximum (1-RM) strength improved in both the trained (∆ = 2.0 ± 0.9 kg) and non-trained (∆ = 0.8 ± 0.9 kg) arms despite cessation of training of the weaker arm, whereas 1-RM strength was unchanged in Detrain. Maximal voluntary isometric contraction, isokinetic peak torque, and RFD did not change in either group. No neural changes were detected in UNI, but cSP increased in Detrain (∆ = 0.010 ± 0.015 s). CSAFlexor increased in the trained arm (∆ = 51 ± 43 mm2) but decreased in the non-trained arm (∆ = -53 ± 50 mm2) in UNI. CSAFlexor decreased in both arms in Detrain and at a similar rate to the non-trained arm in UNI.

CONCLUSION

UNI attenuated the effects of detraining in the weaker arm as shown by the improvement in 1-RM strength. However, the cross-education of strength did not attenuate the decline in muscle size in the contralateral arm.

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.

A randomized controlled trial on the effectiveness of mirror therapy in improving strength, range of movement and muscle activity, in people with carpal tunnel syndrome

BACKGROUND

There is little information on the potential effects of mirror therapy (MT) on motor recovery in individuals with Carpal Tunnel Syndrome (CTS).

PURPOSE

To compare the effectiveness of a MT protocol versus a therapeutic exercise (TE) protocol, in improving strength, range of motion (ROM), muscle activity, pain, and functionality in patients with CTS.

STUDY DESIGN

Randomized clinical trial.

METHODS

Thirty-nine participants with unilateral CTS were divided into two groups: (i) MT group (n = 20) that followed an exercise protocol applied to the unaffected hand reflected in a mirror, and (ii) TE group (n = 19) that followed the same exercise protocol using the unaffected hand but without a mirror. Strength, wrist ROM, muscle activity, pain and functionality, were assessed at baseline (T0), after treatment (T1) and one month after treatment (T2).

RESULTS

At T1, the MT group showed significantly higher wrist flexion-extension ROM compared to TE (p = 0.04, d = 0.8), maintained at T2 (p = 0.02, d = 0.8). No significant changes were observed in ulnar-radius deviation, pronosupination, or fatigue following either MT or TE (p > 0.05). MT exhibited enhanced handgrip strength at T1 (p = 0.001, d = 0.7), as well as an increase in the extensor carpi radialis (ECR) and flexor carpi radialis (FCR) maximum muscle activity (p = 0.04, d = 1.0; p = 0.03, d = 0.4). At T1, both groups decreased pain (p = 0.002, d = 1.1; p = 0.02, d = 0.7), and improved functionality (p < 0.001, d = 0.8; p = 0.01, d = 0.5) (MT and TE respectively).

DISCUSSION

MT led to enhancements in wrist flexion-extension movement, handgrip strength and functionality unlike TE. MT notably increased muscle activity, particularly in the ECR and FCR muscles.

CONCLUSIONS

MT is a favorable strategy to improve wrist flexion-extension ROM, handgrip strength, ECR and FCR muscle activity, and functionality in people with unilateral CTS.

Corticospinal adaptations following resistance training and its relationship with strength: A systematic review and multivariate meta-analysis

Neural adaptations to resistance training (RT) and their correlation with muscle strength remain partially understood. We conducted a systematic review and multivariate meta-analysis to examine the effects of metronome-paced (MP), self-paced (SP), and isometric (IM) training on M1 and corticospinal pathway activity. Following MP RT, a significant increase in corticospinal excitability was observed, correlating with increased strength. Conversely, no significant relationship was found after SP or IM training. RT also reduced the duration of the cortical silent period, but this change did not predict strength changes and was not specific to any training modality. No significant effects were found for short-interval intracortical inhibition. Our findings suggest that changes in corticospinal excitability may contribute to strength gains after RT. Furthermore, the relationship between these adaptations and strength appears dependent on the type of training performed.

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.

Adjustments in the motor unit discharge behavior following neuromuscular electrical stimulation compared to voluntary contractions

Introduction: The application of neuromuscular electrical stimulation superimposed on voluntary muscle contractions (NMES+) has demonstrated a considerable potential to enhance or restore muscle function in both healthy and individuals with neurological or orthopedic disorders. Improvements in muscle strength and power have been commonly associated with specific neural adaptations. In this study, we investigated changes in the discharge characteristics of the tibialis anterior motor units, following three acute exercises consisting of NMES+, passive NMES and voluntary isometric contractions alone. Methods: Seventeen young participants participated in the study. High-density surface electromyography was used to record myoelectric activity in the tibialis anterior muscle during trapezoidal force trajectories involving isometric contractions of ankle dorsi flexors with target forces set at 35, 50% and 70% of maximal voluntary isometric contraction (MVIC). From decomposition of the electromyographic signal, motor unit discharge rate, recruitment and derecruitment thresholds were extracted and the input-output gain of the motoneuron pool was estimated. Results: Global discharge rate increased following the isometric condition compared to baseline at 35% MVIC while it increased after all experimental conditions at 50% MVIC target force. Interestingly, at 70% MVIC target force, only NMES + led to greater discharge rate compared to baseline. Recruitment threshold decreased after the isometric condition, although only at 50% MVIC. Input-output gain of the motoneurons of the tibialis anterior muscle was unaltered after the experimental conditions. Discussion: These results indicated that acute exercise involving NMES + induces an increase in motor unit discharge rate, particularly when higher forces are required. This reflects an enhanced neural drive to the muscle and might be strongly related to the distinctive motor fiber recruitment characterizing NMES+.

A scoping review of the contralateral effects of unilateral peripheral stimulation on neuromuscular function

It is known that resistance exercise using one limb can affect motor function of both the exercised limb and the unexercised contralateral limb, a phenomenon termed cross-education. It has been suggested that cross-education has clinical implications, e.g. in rehabilitation for orthopaedic conditions or post-stroke paresis. Much of the research on the contralateral effect of unilateral intervention on motor output is based on voluntary exercise. This scoping review aimed to map the characteristics of current literature on the cross-education caused by three most frequently utilised peripheral neuromuscular stimulation modalities in this context: electrical stimulation, mechanical vibration and percutaneous needling, that may direct future research and translate to clinical practice. A systematic search of relevant databases (Ebsco, ProQuest, PubMed, Scopus, Web of Science) through to the end of 2020 was conducted following the PRISMA Extension for Scoping Review. Empirical studies on human participants that applied a unilateral peripheral neuromuscular stimulation and assessed neuromuscular function of the stimulated and/or the unstimulated side were selected. By reading the full text, the demographic characteristics, context, design, methods and major findings of the studies were synthesised. The results found that 83 studies were eligible for the review, with the majority (53) utilised electrical stimulation whilst those applied vibration (18) or needling (12) were emerging. Although the contralateral effects appeared to be robust, only 31 studies claimed to be in the context of cross-education, and 25 investigated on clinical patients. The underlying mechanism for the contralateral effects induced by unilateral peripheral stimulation remains unclear. The findings suggest a need to enhance the awareness of cross-education caused by peripheral stimulation, to help improve the translation of theoretical concepts to clinical practice, and aid in developing well-designed clinical trials to determine the efficacy of cross-education therapies.

Neural mechanisms mediating cross education: With additional considerations for the ageing brain

CALVERT, G.H.M., and CARSON, R.G. Neural mechanisms mediating cross education: With additional considerations for the ageing brain. NEUROSCI BIOBEHAV REV 21(1) XXX-XXX, 2021. - Cross education (CE) is the process whereby a regimen of unilateral limb training engenders bilateral improvements in motor function. The contralateral gains thus derived may impart therapeutic benefits for patients with unilateral deficits arising from orthopaedic injury or stroke. Despite this prospective therapeutic utility, there is little consensus concerning its mechanistic basis. The precise means through which the neuroanatomical structures and cellular processes that mediate CE may be influenced by age-related neurodegeneration are also almost entirely unknown. Notwithstanding the increased incidence of unilateral impairment in later life, age-related variations in the expression of CE have been examined only infrequently. In this narrative review, we consider several mechanisms which may mediate the expression of CE with specific reference to the ageing CNS. We focus on the adaptive potential of cellular processes that are subserved by a specific set of neuroanatomical pathways including: the corticospinal tract, corticoreticulospinal projections, transcallosal fibres, and thalamocortical radiations. This analysis may inform the development of interventions that exploit the therapeutic utility of CE training in older persons.

Muscle Activity and Activation in Previously Strain-Injured Lower Limbs: A Systematic Review

BACKGROUND

Lower limb muscle strain injury is highly prevalent in running-based sports and is considered a risk factor for recurrent injury. It is possible that differences in muscle activity and activation in previously strain-injured limbs may contribute to the elevated risk of reinjury.

OBJECTIVES

To systematically review available literature investigating whether muscle activity and/or activation is different in previously strain-injured muscles compared to contralateral uninjured muscles or uninjured controls.

METHODS

A systematic review of literature in SPORTDiscus, MEDLINE Complete, CINAHL and Web of Science was conducted. Full-text English articles which compared indicators of neuromuscular function between injured and uninjured contralateral limbs or control groups in those with a history of muscle strain injury were included.

RESULTS

Twelve studies were included in the review after eligibility criteria were applied. A best evidence synthesis revealed moderate to limited evidence suggesting differences in surface electromyography (sEMG) amplitude, integrated sEMG amplitude, inter-muscle sEMG ratios and voluntary activation in injured limbs, most often during eccentric contractions. Studies utilising sprinting assessments demonstrated conflicting evidence when comparing late swing phase biceps femoris sEMG amplitude between limbs with a history of hamstring strain injury and uninjured contralateral limbs.

CONCLUSIONS

Differences in muscle activity and activation were observed between injured and uninjured limbs across a variety of strength assessments. The evidence supporting these differences was most often moderate or limited and was generally observed during eccentric contractions. Mostly conflicting or limited evidence was found to suggest that participants with previous hamstring strain injury demonstrate no differences in muscle activity during running tasks when compared with their uninjured counterparts or contralateral limbs.

TRIAL REGISTRY

PROSPERO (ID, CRD42019135681).

Contralateral Effects of Unilateral Strength and Skill Training: Modified Delphi Consensus to Establish Key Aspects of Cross-Education

Background

Cross-education refers to increased motor output (i.e., force generation, skill) of the opposite, untrained limb following a period of unilateral exercise training. Despite extensive research, several aspects of the transfer phenomenon remain controversial.

Methods

A modified two-round Delphi online survey was conducted among international experts to reach consensus on terminology, methodology, mechanisms of action, and translational potential of cross-education, and to provide a framework for future research.

Results

Through purposive sampling of the literature, we identified 56 noted experts in the field, of whom 32 completed the survey, and reached consensus (75% threshold) on 17 out of 27 items.

Conclusion

Our consensus-based recommendations for future studies are that (1) the term ‘cross-education’ should be adopted to refer to the transfer phenomenon, also specifying if transfer of strength or skill is meant; (2) functional magnetic resonance imaging, short-interval intracortical inhibition and interhemispheric inhibition appear to be promising tools to study the mechanisms of transfer; (3) strategies which maximize cross-education, such as high-intensity training, eccentric contractions, and mirror illusion, seem worth being included in the intervention plan; (4) study protocols should be designed to include at least 13–18 sessions or 4–6 weeks to produce functionally meaningful transfer of strength, and (5) cross-education could be considered as an adjuvant treatment particularly for unilateral orthopedic conditions and sports injuries. Additionally, a clear gap in views emerged between the research field and the purely clinical field.

The present consensus statement clarifies relevant aspects of cross-education including neurophysiological, neuroanatomical, and methodological characteristics of the transfer phenomenon, and provides guidance on how to improve the quality and usability of future cross-education studies.

Electronic supplementary material

The online version of this article (10.1007/s40279-020-01377-7) contains supplementary material, which is available to authorized users.

Effects of resistance training at different intensities of load on cross-education of muscle strength

The objectives of this study were 1) to compare the extent of cross-transfer of high- versus low-load unilateral resistance training performed with external pacing of the movement (URTEP) and 2) to compare the time course of the two approaches. Fifty subjects were randomized to one of the following three groups: G80 [two sets at 80% and two sets at 40% of one maximum repetition (1RM), 1 concentric second and 3 eccentric seconds controlled by a metronome]; G40 (four sets at 40% of 1 RM, 1s and 3s controlled by a metronome); or CG (control group). At week 1, the G80 increased the elbow flexion 1RM (P<0.05) in contralateral arm. At week 4, both G80 and G40 increased the elbow flexion 1RM (P<0.05) in contralateral arm. However, a greater 1RM gain was observed in the G80 than in the G40 (P< .05). Thus, although higher-load URTEP seems to enhance the cross-education effect when compared to lower-load URTEP, the cross-education of dynamic strength can be achieved in the two approaches after four weeks. Many patients would benefit from cross-education of muscle strength through URPEP, even who are unable to exercise with high loads and in short periods of immobilization. Novelty bullets: (1) Unilateral resistance training promotes cross-education of dynamic muscle strength. (2) However, higher-load resistance training enhances the effects of cross-education of muscle strength.

Corticospinal excitability and motor representation after long-term resistance training

It is poorly understood how the central nervous system adapts to resistance training, especially after years of exposure. We compared corticospinal excitability and motor representation assessed with transcranial magnetic stimulation (TMS) between long-term resistance trained (LRT, ≥3 years) versus untrained (UNT) males (n = 15/group). Motor-evoked potentials (MEPs) were obtained from the biceps brachii during isometric elbow flexion. Stimulus-response curves were created at the hotspot during 10% maximum voluntary torque (MVT) contractions. Maximum peak-to-peak MEP amplitude (MEPmax) was acquired with 100% stimulator output intensity, whilst 25%-100% MVT was produced. Maps were created during 10% MVT contractions, with an individualised TMS intensity eliciting 20% MEPmax at the hotspot. LRT had a 48% lower stimulus-response curve slope than UNT (p < .05). LRT also had a 66% larger absolute map size, although TMS intensity used for mapping was greater in LRT versus UNT (48% vs. 26% above active motor threshold) to achieve a target 20% MEPmax at the hotspot, due to the lower slope of LRT. Map size was strongly correlated with the TMS intensity used for mapping (r = 0.776, p < .001). Once map size was normalised to TMS intensity, there was no difference between the groups (p = .683). We conclude that LRT had a lower stimulus-response curve slope/excitability, suggesting higher neural efficiency. TMS map size was overwhelmingly determined by TMS intensity, even when the MEP response at the hotspot was matched among individuals, likely due to larger current spread with higher intensities. Motor representation appears similar between LRT and UNT given no difference in the normalised map size.

Lack of Evidence for Non-Local Muscle Fatigue and Performance Enhancement in Young Adults

Post-activation performance enhancement (PAPE) is an improvement to voluntary muscle performance following a conditioning activity. There is evidence of fatigue resistance deficits in non-exercised muscles following unilateral fatiguing exercise of a contralateral muscle. The purpose of this study was to determine if a unilateral conditioning exercise protocol could induce PAPE in a contralateral, non-exercised muscle in young healthy adults. Thirty-two recreationally trained (n = 16) and athletically trained (n = 16) participants (16 males; age: 22.9 ± 2.03 years; height: 1.81 ± 0.06 m; weight: 82.8 ± 9.43 kg, and 16 females; age: 23.1 ± 2.80 years; height: 1.67 ± 0.07 m; weight: 66.4 ± 11.09 kg) were randomly allocated into two groups (dominant or non-dominant limb intervention). The experimental intervention, involved a conditioning exercise (4-repetitions of 5-seconds knee extension maximal voluntary isometric contractions: MVIC) with either the dominant (DOM) (n = 16) or non-dominant (ND) (n = 16) knee extensors with testing of the same (exercised) or contralateral (non-exercised) leg as well as a control (no conditioning exercise: n = 32) condition. Testing was performed before, 1-minute and 10-minutes after a high intensity, low volume, conditioning protocol (2 sets of 2x5-s MVIC). Pre- and post-testing included MVIC force and F100 (force developed in the first 100 ms: a proxy measure of rate of force development) and unilateral drop jump (DJ) height and contact time. There were no significant MVIC peak force or EMG nor DJ height or contact time interactions (intervention x limb dominance x time). The pre-test (0.50 ± 0.13) dominant leg MVIC F100 forces exceeded (p = 0.02) both post-test and post-10 min by a small magnitude 8.7% (d = 0.31). There was also a significant (p = 0.02) time x intervention leg x testing leg intervention, although it was observed that the control condition was as likely to demonstrate small to large magnitude changes as were the dominant and non-dominant legs. Following the conditioning activity, there was no significant evidence for non-local improvements (PAPE), or performance decreases.

Nondominant hand computer mouse training and the bilateral transfer effect to the dominant hand

This study explored the effects of training computer mouse use in the nondominant hand on clicking performance of the dominant and nondominant hands. Computer mouse use is a daily operation in the workplace and requires minute hand and wrist movements developed and refined through practice and training for many years. Our study had eleven right-handed computer mouse users train their nondominant hand for 15 min a day, five days per week, for six weeks. This study found improved performance with the computer mouse in the dominant hand following nondominant hand training because of the bilateral transfer effect of training. Additionally, our study showed that the nondominant hand is capable of learning the complex movements that our dominant hand has trained for many years. Last, our research showed that nondominant hand performance decreases when the skill is not trained for over a year, but the performance is significantly higher than that prior to the original training and can be rapidly relearned. Overall, training the nondominant hand on the computer mouse will allow for improved performance in industry while allowing safer, sustainable, and more achievable work in a multitude of economies.

Unilateral Strength Training of the Less Affected Hand Improves Cortical Excitability and Clinical Outcomes in Patients With Subacute Stroke: A Randomized Controlled Trial

OBJECTIVES

To investigate whether unilateral strength training helps improve cortical excitability and clinical outcomes after stroke.

DESIGN

Randomized controlled trial.

SETTING

Rehabilitation sciences research center.

PARTICIPANTS

Patients with subacute stroke (N=26) were randomly assigned to a control group (n=13) or the experimental group (n=13).

INTERVENTIONS

Participants in both groups received conventional physiotherapy. The experimental group also received unilateral strength training of the less affected wrist extensors. Interventions were applied for 4 weeks (12 sessions, 3 d/wk).

MAIN OUTCOME MEASURES

Cortical excitability in both the ipsilesional hemisphere (ipsiH) and contralesional hemisphere (contraH) was assessed by measuring resting motor threshold (RMT), active motor threshold (AMT), motor evoked potential (MEP), and cortical silent period (CSP) at baseline and after the 4-week intervention period. Clinical outcomes were obtained by evaluating wrist extension strength in both the more affected and less affected hands, upper extremity motor function, activities of daily living (ADL), and spasticity.

RESULTS

The experimental group showed greater MEP amplitude (P=.001) in the ipsiH and shorter CSP duration in both the ipsiH (P=.042) and contraH (P=.038) compared with the control group. However, the reductions in RMT and AMT in both hemispheres were not significantly different between groups. Improvements in wrist extension strength in the more affected (P=.029) and less affected (P=.001) hand, upper extremity motor function (P=.04), and spasticity (P=.014) were greater in the experimental group. No significant difference in ADLs was detected between groups.

CONCLUSIONS

A combination of unilateral strength training and conventional physiotherapy appears to be a beneficial therapeutic modality for improving cortical excitability and some clinical outcomes in patients with stroke.

The knowns and unknowns of neural adaptations to resistance training

The initial increases in force production with resistance training are thought to be primarily underpinned by neural adaptations. This notion is firmly supported by evidence displaying motor unit adaptations following resistance training; however, the precise locus of neural adaptation remains elusive. The purpose of this review is to clarify and critically discuss the literature concerning the site(s) of putative neural adaptations to short-term resistance training. The proliferation of studies employing non-invasive stimulation techniques to investigate evoked responses have yielded variable results, but generally support the notion that resistance training alters intracortical inhibition. Nevertheless, methodological inconsistencies and the limitations of techniques, e.g. limited relation to behavioural outcomes and the inability to measure volitional muscle activity, preclude firm conclusions. Much of the literature has focused on the corticospinal tract; however, preliminary research in non-human primates suggests reticulospinal tract is a potential substrate for neural adaptations to resistance training, though human data is lacking due to methodological constraints. Recent advances in technology have provided substantial evidence of adaptations within a large motor unit population following resistance training. However, their activity represents the transformation of afferent and efferent inputs, making it challenging to establish the source of adaptation. Whilst much has been learned about the nature of neural adaptations to resistance training, the puzzle remains to be solved. Additional analyses of motoneuron firing during different training regimes or coupling with other methodologies (e.g., electroencephalography) may facilitate the estimation of the site(s) of neural adaptations to resistance training in the future.

Training load but not fatigue affects cross-education of maximal voluntary force

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 ± 4.3 years), low-load RT to failure (LLF, n = 11, 21 ± 1.3 years, three sets to failure at 25% of 1RM), high-load RT to failure (HLF, n = 11, 21 ± 1.4 years, three sets to failure at 75% of 1RM), and high-load RT without failure (HLNF, n = 11, 22 ± 1.5 years, six sets of five repetitions at 75% of 1RM) groups. Before and after the four weeks of training, 1RM, maximal voluntary isometric force, 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.

Priming the Motor Cortex With Anodal Transcranial Direct Current Stimulation Affects the Acute Inhibitory Corticospinal Responses to Strength Training

Frazer, AK, Howatson, G, Ahtiainen, JP, Avela, J, Rantalainen, T, and Kidgell, DJ. Priming the motor cortex with anodal transcranial direct current stimulation affects the acute inhibitory corticospinal responses to strength training. J Strength Cond Res 33(2): 307-317, 2019-Synaptic plasticity in the motor cortex (M1) is associated with strength training (ST) and can be modified by transcranial direct current stimulation (tDCS). The M1 responses to ST increase when anodal tDCS is applied during training due to gating. An additional approach to improve the M1 responses to ST, which has not been explored, is to use anodal tDCS to prime the M1 before a bout of ST. We examined the priming effects of anodal tDCS of M1 on the acute corticospinal responses to ST. In a randomized double-blinded cross-over design, changes in isometric strength, corticospinal excitability, and inhibition (assessed as area under the recruitment curve [AURC] using transcranial magnetic stimulation) were analyzed in 13 adults exposed to 20 minutes of anodal tDCS and sham tDCS followed by a ST session of the right elbow flexors. We observed a significant decrease in isometric elbow-flexor strength immediately after training (11-12%; p < 0.05), which was not different between anodal tDCS and sham tDCS. Transcranial magnetic stimulation revealed a 24% increase in AURC for corticospinal excitability after anodal tDCS and ST; this increase was not different between conditions. However, there was a 14% reduction in AURC for corticospinal inhibition when anodal tDCS was applied before ST when compared with sham tDCS and ST (all p < 0.05). Priming anodal tDCS had a limited effect in facilitating corticospinal excitability after an acute bout of ST. Interestingly, the interaction of anodal tDCS and ST seems to affect the excitability of intracortical inhibitory circuits of the M1 through nonhomeostatic mechanisms.

Unilateral Strength Training and Mirror Therapy in Patients With Chronic Stroke: A Pilot Randomized Trial

OBJECTIVE

The aim of the study was to investigate the feasibility and potential effectiveness of mirror-aided cross-education compared with cross-education alone in poststroke upper limb recovery.

DESIGN

A pilot randomized controlled parallel group study was carried out. Thirty-two patients with chronic stroke followed a 4-wk isometric strength training program performed with the less-affected upper limb three times per week. Participants in the mirror and strength training group observed the reflection of the exercising arm in the mirror. Participants in the strength training only group exercised without a mirror entirely. Participant compliance, adverse effects, and suitability of outcome measures assessed feasibility. Effectiveness outcomes included maximal isometric strength measured with the Biodex Dynamometer, the Modified Ashworth Scale, and the Chedoke Arm and Hand Activity Inventory.

RESULTS

Compliance was high with no adverse effects. The use of the Biodex Dynamometer must be reviewed. Mirror therapy did not augment the cross-education effect (P > 0.05) in patients with chronic stroke when training isometrically.

CONCLUSIONS

This pilot trial established the feasibility of a randomized controlled trial comparing mirror-aided cross-education with cross-education alone for poststroke upper limb recovery. Mirror therapy did not augment cross-education when training isometrically. However, results indicate that the combination of interventions should be investigated further applying an altered training protocol.

Determining the early corticospinal-motoneuronal responses to strength training: a systematic review and meta-analysis

Several studies have used transcranial magnetic stimulation to probe the corticospinal-motoneuronal responses to a single session of strength training; however, the findings are inconsistent. This systematic review and meta-analysis examined whether a single bout of strength training affects the excitability and inhibition of intracortical circuits of the primary motor cortex (M1) and the corticospinal-motoneuronal pathway. A systematic review was completed, tracking studies between January 1990 and May 2018. The methodological quality of studies was determined using the Downs and Black quality index. Data were synthesised and interpreted from meta-analysis. Nine studies (n=107) investigating the acute corticospinal-motoneuronal responses to strength training met the inclusion criteria. Meta-analyses detected that after strength training compared to control, corticospinal excitability [standardised mean difference (SMD), 1.26; 95% confidence interval (CI), 0.88, 1.63; p<0.0001] and intracortical facilitation (ICF) (SMD, 1.60; 95% CI, 0.18, 3.02; p=0.003) were increased. The duration of the corticospinal silent period was reduced (SMD, −17.57; 95% CI, −21.12, −14.01; p=0.00001), but strength training had no effect on the excitability of the intracortical inhibitory circuits [short-interval intracortical inhibition (SICI) SMD, 1.01; 95% CI, −1.67, 3.69; p=0.46; long-interval intracortical inhibition (LICI) SMD, 0.50; 95% CI, −1.13, 2.13; p=0.55]. Strength training increased the excitability of corticospinal axons (SMD, 4.47; 95% CI, 3.45, 5.49; p<0.0001). This systematic review and meta-analyses revealed that the acute neural changes to strength training involve subtle changes along the entire neuroaxis from the M1 to the spinal cord. These findings suggest that strength training is a clinically useful tool to modulate intracortical circuits involved in motor control.

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.

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.

Neurophysiological adaptations in the untrained side in conjunction with cross-education of muscle strength: a systematic review and meta-analysis

We reviewed the evidence from randomized controlled trials (RCTs) focusing on the neurophysiological adaptations in the untrained side associated with cross-education of strength (CE) and pooled data into definite effect estimates for neurophysiological variables assessed in chronic CE studies. Furthermore, scoping directions for future research were provided to enhance the homogeneity and comparability of studies investigating the neural responses to CE. The magnitude of CE was 21.1 ± 18.2% (mean ± SD; P < 0.0001) in 22 RCTs ( n = 467 subjects) that measured at least 1 neurophysiological variable in the untrained side, including the following: electromyography (EMG; 14 studies); motor evoked potential (MEP; 8 studies); short-interval intracortical inhibition (SICI), recruitment curve, and M wave (6 studies); cortical silent period (cSP; 5 studies); interhemispheric inhibition, intracortical facilitation (ICF), and H reflex (2 studies); and V wave, short-interval ICF, short-latency afferent inhibition, and long-latency afferent inhibition (1 study). Only EMG, MEP, ICF, cSP, and SICI could be included in the meta-analysis (18 studies, n = 387). EMG ( P = 0.26, n = 235) and MEP amplitude ( P = 0.11, n = 145) did not change in the untrained limb after CE. cSP duration ( P = 0.02, n = 114) and SICI ( P = 0.001, n = 95) decreased in the untrained hemisphere according to body region and type and intensity of training. The magnitude of CE did not correlate with changes in these transcranial magnetic stimulation (TMS) measures. The design of this meta-analytical study and the lack of correlations prevented the ability to link mechanistically the observed neurophysiological changes to CE. Notwithstanding the limited amount of data available for pooling, the use of TMS to assess the ipsilateral neurophysiological responses to unilateral training still confirms the central neural origin hypothesis of chronic CE induced by strength training. However, how these neural adaptations contribute to CE remains unclear.

The ipsilateral corticospinal responses to cross-education are dependent upon the motor-training intervention

This study aimed to identify the ipsilateral corticospinal responses of the contralateral limb following different types of unilateral motor-training. Three groups performing unilateral slow-paced strength training (SPST), non-paced strength training (NPST) or visuomotor skill training (VT) were compared to a control group. It was hypothesised that 4 weeks of unilateral SPST and VT, but not NPST, would increase ipsilateral corticospinal excitability (CSE) and reduce short-interval cortical inhibition (SICI), resulting in greater performance gains of the untrained limb. Tracking error of the untrained limb reduced by 29 and 41% following 2 and 4 weeks of VT. Strength of the untrained limb increased by 8 and 16% following 2 and 4 weeks of SPST and by 6 and 13% following NPST. There was no difference in cross-education of strength or tracking error. For the trained limb, SPST and NPST increased strength (28 and 26%), and VT improved by 47 and 58%. SPST and VT increased ipsilateral CSE by 89 and 71% at 2 weeks. Ipsilateral CSE increased 105 and 81% at 4 weeks following SPST and VT. The NPST group and control group showed no changes at 2 and 4 weeks. SPST and VT reduced ipsilateral SICI by 45 and 47% at 2 weeks; at 4 weeks, SPST and VT reduced SICI by 48 and 38%. The ipsilateral corticospinal responses are determined by the type of motor-training. There were no differences in motor performance between SPST, NPST and VT. The data suggests that the corticospinal responses to cross-education are different and determined by the type of motor-training.

Does ipsilateral corticospinal excitability play a decisive role in the cross-education effect caused by unilateral resistance training? A systematic review

INTRODUCTION

Unilateral resistance training has been shown to improve muscle strength in both the trained and the untrained limb. One of the most widely accepted theories is that this improved performance is due to nervous system adaptations, specifically in the primary motor cortex. According to this hypothesis, increased corticospinal excitability (CSE), measured with transcranial magnetic stimulation, is one of the main adaptations observed following prolonged periods of training. The principal aim of this review is to determine the degree of adaptation of CSE and its possible functional association with increased strength in the untrained limb.

DEVELOPMENT

We performed a systematic literature review of studies published between January 1970 and December 2016, extracted from Medline (via PubMed), Ovid, Web of Science, and Science Direct online databases. The search terms were as follows: (transcranial magnetic stimulation OR excitability) AND (strength training OR resistance training OR force) AND (cross transfer OR contralateral limb OR cross education). A total of 10 articles were found.

CONCLUSION

Results regarding increased CSE were inconsistent. Although the possibility that the methodology had a role in this inconsistency cannot be ruled out, the results appear to suggest that there may not be a functional association between increases in muscle strength and in CSE.

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.