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Contralateral strength training attenuates muscle performance loss following anterior cruciate ligament (ACL) reconstruction: a randomised-controlled trial

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Claire Minshull at Leeds Beckett University
Claire Minshull
Peter Gallacher
Peter Gallacher
Peter Gallacher
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Simon Roberts
Simon Roberts
Simon Roberts
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Andrew Barnett at Royal National Orthopaedic Hospital NHS Trust
Andrew Barnett
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PurposeTo investigate the effects of cross-education (CE) exercise on strength and performance at 10 and 24 weeks post anterior cruciate ligament (ACL) surgery.Methods Design: randomised controlled trial. N = 44 ACL-reconstruction patients, randomly-allocated into: CE: strength training of the non-operative limb, or CON: sham exercise of upper limb stretching. Each patient underwent standardised ACL rehabilitation, plus 8 weeks of thrice weekly CE or CON, commencing at 2 weeks post surgery. The primary outcome was quadriceps peak force (QPF) of the ACL-reconstructed limb at 10 weeks post surgery. Secondary measures were hamstrings peak force (HPF), rate of force development (RFD) and International Knee Documentation Committee score (IKDC) at 10 and 24 weeks; QPF and hop for distance (HOP) at 24 weeks post surgery.ResultsCE significantly attenuated the decline in QPF of the ACL-reconstructed limb at 10 weeks compared to CON (16.6% decrease vs. 32.0%, respectively); that advantage was not retained at 24 weeks. A training effect was observed in the trained limb for HPF and QPF, which was retained at 24 weeks. No significant differences were observed for IKDC, HOP, RFD, or HPF of the reconstructed limb. Inter-limb symmetry (ILS) ranged from 0.78 to 0.89 and was not significantly different between groups.Conclusion High-intensity CE strength training attenuated the post-operative decline in QPF and should be considered in early-phase ACL rehabilitation. ILS data showed good symmetry, but it masked significantly inferior performance between groups and should be used with caution.Trial registration numberNCT02722876.
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Vol.:(0123456789)
1 3
European Journal of Applied Physiology (2021) 121:3551–3559
https://doi.org/10.1007/s00421-021-04812-3
ORIGINAL ARTICLE
Contralateral strength training attenuates muscle performance
loss followinganterior cruciate ligament (ACL) reconstruction:
arandomised‑controlled trial
ClaireMinshull1,2 · PeterGallacher1· SimonRoberts1· AndrewBarnett1· JanHermanKuiper1,3· AndreaBailey1
Received: 26 April 2021 / Accepted: 11 September 2021 / Published online: 20 September 2021
© The Author(s), under exclusive licence to Springer-Verlag GmbH Germany, part of Springer Nature 2021
Abstract
Purpose To investigate the effects of cross-education (CE) exercise on strength and performance at 10 and 24weeks post
anterior cruciate ligament (ACL) surgery.
Methods Design: randomised controlled trial. N = 44 ACL-reconstruction patients, randomly-allocated into: CE: strength
training of the non-operative limb, or CON: sham exercise of upper limb stretching. Each patient underwent standardised
ACL rehabilitation, plus 8weeks of thrice weekly CE or CON, commencing at 2weeks post surgery. The primary outcome
was quadriceps peak force (QPF) of the ACL-reconstructed limb at 10weeks post surgery. Secondary measures were ham-
strings peak force (HPF), rate of force development (RFD) and International Knee Documentation Committee score (IKDC)
at 10 and 24weeks; QPF and hop for distance (HOP) at 24weeks post surgery.
Results CE significantly attenuated the decline in QPF of the ACL-reconstructed limb at 10weeks compared to CON (16.6%
decrease vs. 32.0%, respectively); that advantage was not retained at 24weeks. A training effect was observed in the trained
limb for HPF and QPF, which was retained at 24weeks. No significant differences were observed for IKDC, HOP, RFD,
or HPF of the reconstructed limb. Inter-limb symmetry (ILS) ranged from 0.78 to 0.89 and was not significantly different
between groups.
Conclusion High-intensity CE strength training attenuated the post-operative decline in QPF and should be considered in
early-phase ACL rehabilitation. ILS data showed good symmetry, but it masked significantly inferior performance between
groups and should be used with caution.
Trial registration number NCT02722876.
Keywords Cross-education· Cross-transfer· Rehabilitation· Strength training
Abbreviations
ACL Anterior cruciate ligament
CE Cross-education
CON Control
QPF Quadriceps peak force
HPF Hamstrings peak force
RFD Rate of force development
ILS Inter-limb symmetry
IKDC International knee documentation
Introduction
Anterior cruciate ligament (ACL) injury is a common and
debilitating injury (Ardern etal. 2011; Sanders etal. 2016)
and without intervention often prevents return to play. Surgi-
cal reconstruction to restore mechanical rotational instability
(Krause etal. 2018) is the preferred treatment option, where
patients have ongoing symptoms of instability despite con-
servative treatment (Schmitt etal. 2012).
ACL reconstruction (ACLR) often results in significant
and prolonged functional impairments, in particular asym-
metry of quadriceps strength (Gokeler etal. 2014; Kuenze
Communicated by Toshio Moritani.
* Claire Minshull
minshullc@hotmail.com
1 Research Department, RJAH Orthopaedic Hospital NHS
Foundation Trust, OswestrySY107AG, UK
2 Get Back To Sport Ltd, Nottingham, UK
3 School ofPharmacy andBioengineering, Keele University,
Keele, StaffordshireST55BG, UK
Content courtesy of Springer Nature, terms of use apply. Rights reserved.
... This adaptation can occur with only a few weeks of training (Barss et al. 2018;Carr et al. 2019) but has little relevance for individuals with two well-functioning limbs. However, in a scenario where one limb is debilitated, unilateral training is a viable intervention to preserve neuromuscular function following orthopedic injury (Magnus et al. 2013;Harput et al. 2019;Minshull et al. 2021) or neurotrauma (Dragert and Zehr 2013;Manca et al. 2020). Despite evidence showing the therapeutic efficacy of cross-education, unilateral training is still poorly prescribed (Collins et al. 2017). ...
... In these scenarios, cross-education training provides a low-risk strategy that, at minimum, helps to prevent global deconditioning during periods of inactivity. The emerging evidence showing cross-education interventions preserve muscle function during orthopedic limb immobilization (Farthing et al. 2009;Magnus et al. 2010;Pearce et al. 2013;Andrushko et al. 2018b;Valdes et al. 2021), following orthopedic injury (Papandreou et al. 2007(Papandreou et al. , 2013Magnus et al. 2013;Harput et al. 2019;Minshull et al. 2021), and enhance the strength of the more affected limb for stroke survivors (Dragert and Zehr 2013;Ehrensberger et al. 2019;Simpson et al. 2019), and individuals living with multiple sclerosis (Manca et al. 2015(Manca et al. , 2017a provide confidence that unilateral training has broad clinical applications. The intriguing data showing that crosseducation attenuates muscle atrophy following nonindicated orthopedic immobilization have been discussed in recent reviews, with candidate mechanisms being the physiological mirror activity, preserved muscle protein synthesis or inhibition of protein degradation pathways, and localized trophic responses that may confer preservation effects with chronic unilateral training (Hendy and Lamon 2017;Andrushko et al. 2018a). ...
... Moreover, there are little data from clinical trials that have employed a cross-education intervention in patients. Of those that have, there is evidence (Papandreou et al. 2013;Magnus et al. 2013;Minshull et al. 2021) showing that crosseducation provides superior adaptations compared to standard care, whereas others (Zult et al. 2018(Zult et al. , 2019 show similar outcomes with or without unilateral training. Differences in exercise design and prescription likely account for some of the disparity in these clinical trials (Papandreou et al. 2013;Zult et al. 2018Zult et al. , 2019Minshull et al. 2021). ...
Exercise prescription and strategies to promote the cross-education of strength: a scoping review
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  • May 2023
This review examines the experimental evidence regarding unilateral resistance training frequency, intensity, the type of training, training volume, and adjuvant therapies on the cross-education of strength. CINAHL, MEDLINE, APA PsycInfo, SPORTDiscus, and Web of Science were systematically searched with gray literature searches and pearling of references thereafter. Experiments were included in the review if they performed a unilateral resistance training intervention that directly compared the dose of a training variable on the cross-education response in healthy or clinical populations following a minimum of two weeks of training. Experiments must have reported maximal strength outcomes for the untrained limb. For each experiment, the study population, intervention methods, the dosage of the training variable being studied, and the outcomes for the untrained, contralateral limb were identified and collectively synthesized. The search returned a total of 912 articles, 57 of which qualified for inclusion. The results show that experimental trials have been conducted on resistance training frequency (n = 4), intensity (n = 7), the type of training (n = 26), training volume (n = 3), and adjuvant therapies (n = 17) on the cross-education of strength. This review maps the available evidence regarding exercise design and prescription strategies to promote the cross-education of strength. It appears that traditional resistance training frequencies (i.e., 2–3×/week) at high intensities are effective at promoting cross-education with eccentric muscle actions showing additive benefits. There is experimental evidence that neuromodulatory techniques can augment cross-education when layered with unilateral resistance training versus training alone. Registration identifier (osf.io/9sh5b). Novelty The cross-education of strength is moderated by exercise design and prescription in clinical and non-clinical populations. This review synthesizes the available evidence regarding exercise design strategies for unilateral resistance training and provides evidence-based recommendations for the prescription of unilateral training to maximize the cross-education of strength. Greater insights regarding the timing and effectiveness of cross-education interventions in clinical scenarios will strengthen the use of unilateral resistance training for individuals who may benefit from its use.
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... Positive psychological intervention can help patients and their families build confi-dence in rehabilitation, eliminate negative emotions, and improve patients' compliance with treatment and rehabilitation training (23). Muscle strength training and joint range of motion training can improve the circulation and limb function of patients, prevent the aggravation of the disease, and improve the QoL of patients (24). It was found that the satisfaction score of patients with nursing quality after home care intervention was significantly higher than that of routine care. ...
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... It is necessary to investigate whether eccentric resistance training of nonimmobilized muscles is effective for attenuating or maintaining the negative effects of immobilization in real injuries such as ligament sprains or tears, bone fracture, and postsurgery (e.g., joint replacement, anterior cruciate ligament) that accompany inflammation. Previous studies (45)(46)(47)(48)(49)(50) have shown that the cross-education effects are still observed in musculoskeletal injuries when a nonaffected limb receives a resistance training, but ET was not used in these studies. It is also interesting to apply the contralateral eccentric resistance training to a less impaired limb for patients with stroke, as two studies showed that resistance exercise training of a less impaired limb provides positive effects on an impaired limb (48,49). ...
Effects of Unilateral Eccentric versus Concentric Training of Non-Immobilized Arm During Immobilization
Article
Full-text available
  • Feb 2023
Introduction: The present study tested the hypothesis that eccentric training (ET) of non-immobilized arm would attenuate negative effects of immobilization and provide greater protective effects against muscle damage induced by eccentric exercise after immobilization, when compared with concentric training (CT). Methods: Sedentary young men were placed to ET, CT or control group (n = 12/group), and their non-dominant arms were immobilized for 3 weeks. During the immobilization period, ET and CT groups performed 5 sets of 6 dumbbell curl eccentric-only and concentric-only contractions, respectively at 20-80% of maximal voluntary isometric contraction (MVCiso) strength over six sessions. MVCiso torque, root-mean square (RMS) of electromyographic activity during MVCiso, and bicep brachii muscle cross-sectional area (CSA) were measured before and after immobilization for both arms. All participants performed 30 eccentric contractions of the elbow flexors (30EC) by the immobilized arm after the cast was removed. Several indirect muscle damage markers were measured before, immediately after, and for 5 days following 30EC. Results: ET increased MVCiso (17 ± 7%), RMS (24 ± 8%), and CSA (9 ± 2%) greater (P < 0.05) than CT (6 ± 4%, 9 ± 4%, 3 ± 2%) for the trained arm. The control group showed decreases in MVCiso (-17 ± 2%), RMS (-26 ± 6%), and CSA (-12 ± 3%) for the immobilized arm, but these changes were attenuated greater (P < 0.05) by ET (3 ± 3%, -0.1 ± 2%, 0.1 ± 0.3%) than CT (-4 ± 2%, -4 ± 2%, -1.3 ± 0.4%). Changes in all muscle damage markers after 30EC were smaller (P < 0.05) for the ET and CT than control, and ET than CT group (e.g., peak plasma creatine kinase activity, ET: 860 ± 688, CT: 2,390 ± 1104, control: 7,819 ± 4,011 IU/L). Conclusions: These results showed that ET of the non-immobilized arm was effective for eliminating the negative effects of immobilization and attenuating eccentric exercise-induced muscle damage after immobilization.
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... One possible application of this phenomenon can be found in the field of exercise and rehabilitation, where one may train the intact side if the opposite homologous (contralateral) limb cannot be trained due to injuries or diseases (e.g., surgery-induced immobilization, stroke). However, as a relatively novel tool, mixed findings have been reported in recent literature [4][5][6][7]. One big challenge to moving this research forward is identifying the specific exercise mode that can induce the largest magnitude of the contralateral cross-education effect. ...
Effects of Unilateral Neuromuscular Electrical Stimulation with Illusionary Mirror Visual Feedback on the Contralateral Muscle: A Pilot Study
Article
Full-text available
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... Seven studies [160][161][162][163][164][165][166] investigated the effect of contralateral limb strength training on the injured limb outcomes after ACL surgery. Summary ► There is conflicting evidence for an effect of cross-training on quadriceps strength at the early and intermediate phase. ...
Aspetar clinical practice guideline on rehabilitation after anterior cruciate ligament reconstruction
Article
Full-text available
  • Feb 2023
  • BRIT J SPORT MED
This guideline was developed to inform clinical practice on rehabilitation after anterior cruciate ligament reconstruction (ACLR) and was performed in accordance with the Appraisal of Guidelines for REsearch & Evaluation (AGREE II) instrument and used the Grading of Recommendations, Assessment, Development and Evaluation (GRADE) approach. A Guideline Development Group systematically searched and reviewed evidence using randomised clinical trials and systematic reviews to evaluate the effectiveness of rehabilitation interventions and guide clinicians and patients on the content of the optimal rehabilitation protocol after ACLR. The guideline targets patients during rehabilitation after ACLR and investigates the effectiveness of the available interventions to the physiotherapist, alone or in combination (e.g., exercise, modalities, objective progression criteria). Exercise interventions should be considered the mainstay of ACLR rehabilitation. However, there is little evidence on the dose-response relationship between volume and/or intensity of exercise and outcomes. Physical therapy modalities can be helpful as an adjunct in the early phase of rehabilitation when pain, swelling, and limitations in range of motion are present. Adding modalities in the early phase may allow earlier pain-free commencement of exercise rehabilitation. Return to running and return to training/activity are key milestones for rehabilitation after ACLR. However, there is no evidence on which progression or discharge criteria should be used. While there is a very low level of certainty for most components of rehabilitation, most of the recommendations provided in this guideline were agreed to by expert clinicians. This guideline also highlights several new elements of ACLR management not reported previously.
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Optimising the Early-Stage Rehabilitation Process Post-ACL Reconstruction
Article
Full-text available
  • Oct 2023
  • SPORTS MED
Outcomes following anterior cruciate ligament reconstruction (ACLR) need improving, with poor return-to-sport rates and a high risk of secondary re-injury. There is a need to improve rehabilitation strategies post-ACLR, if we can support enhanced patient outcomes. This paper discusses how to optimise the early-stage rehabilitation process post-ACLR. Early-stage rehabilitation is the vital foundation on which successful rehabilitation post-ACLR can occur. Without high-quality early-stage (and pre-operative) rehabilitation, patients often do not overcome major aspects of dysfunction, which limits knee function and the ability to transition through subsequent stages of rehabilitation optimally. We highlight six main dimensions during the early stage: (1) pain and swelling; (2) knee joint range of motion; (3) arthrogenic muscle inhibition and muscle strength; (4) movement quality/neuromuscular control during activities of daily living (5) psycho-social-cultural and environmental factors and (6) physical fitness preservation. The six do not share equal importance and the extent of time commitment devoted to each will depend on the individual patient. The paper provides recommendations on how to implement these into practice, discussing training planning and programming, and suggests specific screening to monitor work and when the athlete can progress to the next stage (e.g. mid-stage rehabilitation entry criteria).
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Contractile rate of force development after ACL reconstruction – a comprehensive review and meta‐analysis
Article
Full-text available
  • Jun 2020
  • SCAND J MED SCI SPOR
Study design Comprehensive review and meta‐analysis. Background The recovery in rapid force production measured as the rate of force development (RFD) is not clear after anterior cruciate ligament reconstruction (ACLR). Objectives To evaluate (1) time‐course change of between‐limb asymmetries in isometric knee extension/flexion RFD in individuals post‐ACLR and (2) differences in RFD between individuals post‐ACLR and healthy controls. Methods A literature search of Web of Sciences, SPORTDiscus, PubMed‐Medline and ScienceDirect identified 10 eligible studies (n= 246) assessing RFD after ACLR. Results Standard mean difference (SMD) for early phase (<100ms) knee extensor RFD was ‐1.07 (95% CI: ‐1.46, ‐0.68) when comparing ACLR vs. uninjured limb, while SMD for late phase (≥100ms) RFD was ‐0.85 (95CI%: ‐1.27, ‐0.42). SMD for early and late phase knee flexor RFD were ‐0.74 (95% CI: ‐1.19, ‐0.29) and ‐0.79 (95% CI: ‐1.19, ‐0.39), respectively. Comparing ACLR limbs to uninjured controls knee extensor SMD for early and late phase RFD were ‐1.42 (95% CI: ‐2.10, ‐0.73) and 1.09 (95% CI: ‐1.81, ‐0.38). For the knee flexors SMD for early and late phase RFD were ‐0.78 (95% CI: ‐1.96, ‐0.39) and ‐1.14 (95% CI: ‐1.60, ‐0.67). Conclusions ACLR limbs demonstrated sustained post‐surgical suppresion in RFD capacity for the knee extensors/flexors compared to the contralateral limb as well as to healthy controls. Monitoring of RFD should be considered throughout rehabilitation and return to sport (RTS) after ACLR to assess the effectiveness of post‐operative rehabilitation. Post‐surgical ACLR rehabilitation should include training interventions to enhance RFD.
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The time course of cross-education during short-term isometric strength training
Article
Full-text available
  • Jun 2019
  • EUR J APPL PHYSIOL
PurposeThis study examined the time course of contralateral adaptations in maximal isometric strength (MVC), rate of force development (RFD), and rate of electromyographic (EMG) rise (RER) during 4 weeks of unilateral isometric strength training with the non-dominant elbow flexors.Methods Twenty participants were allocated to strength training (n = 10, three female, two left hand dominant) or control (n = 10, three female, two left hand dominant) groups. Both groups completed testing at baseline and following each week of training to evaluate MVC strength, EMG amplitude, RFD and RER at early (RFD50, RER50) and late (RFD200, RER200) contraction phases for the dominant ‘untrained’ elbow flexors. The training group completed 11 unilateral isometric training sessions across 4 weeks.ResultsThe contralateral improvements for MVC strength (P < 0.01) and RFD200 (P = 0.017) were evidenced after 2 weeks, whereas RFD50 (P < 0.01) and RER50 (P = 0.02) showed significant improvements after 3 weeks. Each of the dependent variables was significantly (P < 0.05) greater than baseline values at the end of the training intervention for the trained arm. No changes in any of the variables were observed for the control group (P > 0.10).Conclusions Unilateral isometric strength training for 2–3 weeks can produce substantial increases in isometric muscle strength and RFD for both the trained and untrained arms. These data have implications for rehabilitative exercise design and prescription.
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Operative Versus Conservative Treatment of Anterior Cruciate Ligament Rupture
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Full-text available
  • Dec 2018
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Cross-education improves quadriceps strength recovery after ACL reconstruction: a randomized controlled trial
Article
Full-text available
  • Jun 2018
Purpose The aim of this study was to investigate the effects of concentric and eccentric cross-education (CE) on quadriceps strength and knee function recoveries after anterior cruciate ligament (ACL) reconstruction. Methods Forty-eight patients (age: 29.5±6.8 years, body mass index: 26.1±3.2 kg/m2 ) who had undergone ACL reconstruction with hamstring tendon autograft were included in the study. The patients were randomly divided into three groups when they reached four weeks post surgery: (1) concentric CE (n=16); (2) eccentric CE (n=16); and (3) control (n=16). All groups followed the same post-surgical rehabilitation program for their reconstructed limb. Additionally, the two experimental groups followed eight weeks of isokinetic training for the uninjured knee at 60°/s for 3 days per week. Quadriceps maximum voluntary isometric strength (MVIC) was measured during the 4th week (pre-training), 12th week (post training), and 24th week post surgery. The single-leg hop distance and International Knee Documentary Committee (IKDC) scores were also evaluated during the 24th week post surgery. Analysis of variance was used for statistical analysis. Results Group-by-time interaction was significant for quadriceps MVICs for reconstructed and healthy limbs (p=0.02). Quadriceps strength of both knees was greater in concentric and eccentric CE groups compared to control group during the 12th- and 24th weeks post surgery (p<0.05). Strength gain was 28% and 31% in concentric and eccentric CE groups, respectively, when compared with the control group. Concentric and eccentric CE had similar effects on quadriceps strength recovery (n.s.). IKDC score, and single-leg hop distances were not significantly different among groups (n.s.). Conclusions Concentric and eccentric quadricep strengthening of healthy limbs in early phases of ACL rehabilitation improved post-surgical quadriceps strength recovery of the reconstructed limb. CE should be integrated into ACL reconstruction rehabilitation, especially in the early rehabilitative phases to restore quadriceps strength. Level of evidence Randomized controlled trial, Level I
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Cross-education does not accelerate the rehabilitation of neuromuscular functions after ACL reconstruction: a randomized controlled clinical trial
Article
Full-text available
  • Aug 2018
  • EUR J APPL PHYSIOL
PurposeCross-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. ResultsThe 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.
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The time has come to incorporate a greater focus on rate of force development training in the sports injury rehabilitation process
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Full-text available
  • Jan 2017
This narrative and literature review discusses the relevance of Rate of Force Development (RFD) (the slope of the force time curve) for Return To Sport (RTS), its determinants and the influence of training practices on it expression, with the purpose to enhance clinicians' awareness of how RFD training may enhance RTS success. RFD is considered functionally more relevant than maximal muscle strength during certain very fast actions including rapid joint stabilisation following mechanical perturbation. Deficits in RFD are reported following conventional rehabilitation programmes despite full restoration of maximal strength, which may contribute to the less than satisfactory RTS outcomes reported in the literature. RFD determinants vary as a function of time from force onset with a diminishing role of maximal strength as the time available for force development decreases. Factors such as neural activation, fibre type composition and muscle contractile properties influence RFD also and to a much greater extent during the early periods of rapid force development. Conventional resistance training using moderate loads typical of most rehabilitation programmes is insufficient at restoring or enhancing RFD, thus incorporating periodised resistance training programmes and explosive training techniques in the final stages of rehabilitation prior to RTS is recommended. Level of evidence: V.
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Cross-education of muscular strength following unilateral resistance training: a meta-analysis
Article
Full-text available
  • Nov 2017
  • EUR J APPL PHYSIOL
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.
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The Cross-Education Phenomenon: Brain and Beyond
Article
Full-text available
  • May 2017
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.
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Hamstring Muscle Activity After Primary Anterior Cruciate Ligament Reconstruction—A Protective Mechanism in Those Who Do Not Sustain a Secondary Injury? A Preliminary Study
Article
  • Jun 2019
Background: Individuals who experience a subsequent ipsilateral anterior cruci (cruciate)ate ligament (ACL) reinjury may use hazardous muscle activation strategies after primary ACL reconstruction (ACLR). The purpose of this study was to compare electromyograms (EMGs) of the quadriceps, hamstrings, and gastrocnemius muscles during a dynamic hopping task among individuals with a single ACL injury (ACLx1), individuals who went on to have secondary ipsilateral ACL injury (ACLx2), and individuals who have never sustained an ACL injury (ACLx0). Hypothesis: We expected that individuals who went on to experience a secondary ACL injury would use less quadriceps muscle activity as compared with individuals who experienced a single ACL injury. Study design: Cross-sectional study. Level of evidence: Level 3. Methods: Fourteen individuals that were returned to play post-ACLR and 7 non-ACL-injured individuals participated. Individuals who had undergone an ACLR were placed into groups depending on whether they had experienced a secondary ipsilateral ACL reinjury postprimary ACLR. EMG data of the vastus lateralis, biceps femoris, and lateral gastrocnemius were measured during 2 phases of a single-leg dynamic hopping task: preactivity (100 ms prior to ground contact) and reactivity (250 ms post-ground contact). Processed EMG data were compared across groups using 1-way analyses of variance, with post hoc independent t tests where appropriate (P ≤ 0.05). Results: At preactivity, ACLx1 (0.48% ± 0.2%max) was found to use significantly more hamstring activity than ACLx2 (0.20% ± 0.1%max, P = 0.018), but not than ACLx0 (0.38% ± 0.1%max, P > 0.05). At reactivity, both ACL groups were found to use less quadriceps activity than ACLx0 (ACLx1: 0.38% ± 0.1%max, P = 0.016; ACLx2: 0.40% ± 0.1%max, P = 0.033; ACLx0: 0.58% ± 0.1%max), but not than each other (P > 0.05). Conclusion: Quadriceps muscle activity during landing was diminished in all ACL participants as compared with participants who had never sustained an ACL injury. Individuals who did not experience a secondary ipsilateral ACL reinjury (ACLx1) used greater levels of hamstring activity prior to landing. Clinical relevance: The higher hamstring activity in patients who did not experience a secondary injury may be interpreted as a protective mechanism that is used to dynamically stabilize the reconstructed limb.
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