Neurophysiological adaptations in the untrained side and cross-education of muscle strength: a systematic review and meta-analysis
Introduction
The phenomenon of cross-education (CE), or cross-transfer, refers to the ability of unilateral training (of only one side of the body) to induce increases in strength in the opposite untrained side. It is a finding of great interest both for the clinical-rehabilitation field and for the sports and training field. However, despite the phenomenon being well documented, the underlying neurophysiological mechanisms remain unclear.
With this study, Andrea Manca and colleagues have attempted to fill part of this gap, proposing a systematic review and a meta-analysis on the neurophysiological adaptations associated with CE, with particular attention to the changes measured in the untrained side.
Study objectives
- To identify and synthesize evidence from RCTs (randomized clinical trials) that have evaluated neurophysiological changes in the untrained side following a unilateral training program.
- Estimate, through meta-analysis, the effect size of these adaptations.
- Propose directions to standardize future methodologies and clarify the central neural mechanisms underlying CE.
Methods
Inclusion criteria:
- Randomized controlled trials
- Healthy adult participants
- Unilateral strength training
- Pre- and post-intervention measurements on the untrained side
- Neurophysiological assessments (EMG, TMS, H-reflex, etc.)
Neurophysiological adaptations analyzed:
- EMG (muscle activity)
- MEP (motor evoked potential)
- SICI (short interval intracortical inhibition)
- cSP (cortical silent period)
- Recruitment curve
- M-wave and H-reflex
- Interhemispheric inhibition (IHI)
In total, 22 studies were included, with 467 healthy participants.
Main results
Strength cross-education:
- The mean increase in strength on the untrained side was 21.1%, confirming the effectiveness of unilateral training in generating contralateral gains as well.
EMG activity:
- No significant changes were observed in surface EMG activity on the untrained side. This suggests that the strength gain is not attributable to increased peripheral muscle activation, but probably to central mechanisms.
Motor evoked potential (MEP):
- MEPs also did not show significant changes, indicating that corticospinal excitability may not be substantially modified on the untrained side.
SICI and cSP (intracortical inhibition):
- Significant reduction in SICI and cSP duration in the hemisphere contralateral to the trained side.
o This indicates a decrease in intracortical inhibition, an adaptation consistent with increased availability of motor output.
Other neurophysiological measures:
- There was no consistent evidence for recruitment curve, interhemispheric inhibition or spinal reflexes (M-wave, H-reflex).
Interpretation and implications
The results suggest that the central mechanisms underlying CE involve intracortical rather than peripheral changes. In particular, the decrease in intracortical inhibition is the most consistent finding in the literature.
However, despite the presence of these adaptations, the correlation between neurophysiological changes and increased strength on the untrained side was not significant. This leaves two interpretations open:
- The observed adaptations are necessary but not sufficient to explain CE.
- Or there are other mechanisms (e.g. interhemispheric reorganization, subcortical plasticity) not adequately measured in the current studies.
Methodological criticalities emerged:
- Variability in training protocols (duration, intensity, frequency)
- Differences in instrumentation used for neurophysiological measurements
- Small sample size
- Lack of long-term follow-up
Conclusions and future perspectives
The study by Manca et al. highlights that cross-transfer of force is accompanied by cortical neurophysiological modifications, mainly in terms of reduced intracortical inhibition. However, the complexity of the phenomenon requires more robust studies, with standardized experimental designs and greater attention to the integration between cortical, subcortical and spinal levels.
For clinical and sports practice, these results support the use of unilateral training as an effective strategy even in contexts of immobilization, injuries or rehabilitation.
CITATION
Manca A, Hortobágyi T, Rothwell J, Deriu F. Neurophysiological adaptations in the untrained side in conjunction with cross-education of muscle strength: a systematic review and meta-analysis. J Appl Physiol (1985). 2018 Jun 1;124(6):1502-1518. doi: 10.1152/japplphysiol.01016.2017. Epub 2018 Feb 15. PMID: 29446711.
REFERENCE – ORIGINAL ARTICLE
POSTSCRIPT:
The implications of the phenomenon of cross-education for sports training are multiple and potentially very interesting, both in the performance and preventive or rehabilitative fields. Here are some evidences that emerged from the meta-analysis of Ammann et al. (2018) and related studies:
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Strength maintenance in case of unilateral injury
One of the most immediate uses is the prevention of the loss of strength during periods of immobilization. If an athlete has an injured limb (e.g. arm in a cast), training the healthy limb can:
- Reduce muscle atrophy of the immobilized limb
- Limit neural deconditioning
- Promote a faster return to performance
This is supported by the fact that cross-education has a central basis: we train the brain, not just the muscles.
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Symmetrical Strength Development
In sports that require functional bilateralism (e.g. athletics, martial arts, gymnastics), targeted unilateral training can have a positive impact on the other side, helping to balance any strength asymmetries. This is especially useful in young athletes or in recovery phases from functional imbalances.
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Improved Neuromuscular Efficiency
Training one side of the body improves the efficiency of the central nervous system for the other side as well. This can optimize motor recruitment, intermuscular coordination, and activation speed, all of which are crucial for neuromuscularly intensive sports (e.g. sprinting, weightlifting, tennis, fencing).
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Integration into training programming
This knowledge can be integrated into:
- Compensatory training: when one side is fatigued, you can train only the other without losing the effect on the entire body
- Rehabilitation periodization: designing programs that exploit cross-education in the initial stages of return to activity
- Training in constraint conditions (e.g. swimming pool, suspensions, unilateral isometric exercises)
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Practical applications:
- Use of high-intensity unilateral isometric exercises
- Maximal voluntary activation
- Training with submaximal loads but with neural focus (high concentration, visualization, explosive contraction)
