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 21.1plus minus18.2% (p<0.0001) in 22 RCTs (467 subjects) that measured at least one neurophysiological variable in the untrained side, including: EMG (14 studies), MEP (8 studies), SICI, RC and M-wave (6 studies), cSP (5), IHI, ICF and H-reflex (2), V-wave, SICF, SAI and LAI (1). Only EMG, MEP, ICF, cSP and SICI could be included 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, type and intensity of training. The magnitude of CE did not correlate with changes in these 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.