Neuromuscular Sparing Effects of Cross-Education During Immobilization After Prior Strength Training

Abstract

Cross-education (CE) refers to the contralateral effect of unilateral training whereby performance gain is observed in the opposite, untrained, homologous limb. Previous research has demonstrated concurrent training during limb immobilization spares muscle size and strength in the immobilized contralateral limb in healthy, untrained individuals. It is unknown whether CE sparing effects differ in a population of previously trained individuals which warrants investigation. This study primarily aimed to determine if CE sparing effects occur in a population who underwent strength training prior to subsequent immobilization. The study design involved 4-weeks of progressive isokinetic eccentric (ECC) wrist extension and flexion training of both arms (3 days/wk) followed by 4-weeks of left arm immobilization via cast; where a control group ceased training and an intervention group continued right arm training during immobilization. The study involved four testing sessions: Two baseline tests prior to ECC-training, one post-test after ECC-training before immobilization, and one test following the immobilization period. Forearm muscle cross sectional area (MCSA) (peripheral quantitative computed tomography), ECC, concentric (CON) and isometric (ISO) maximal voluntary contraction peak torque (isokinetic dynamometer), grip strength (handgrip dynamometer), normalized electromyography (EMG), wrist flexor contractile properties (evoked resting peak twitch, peak twitch torque, rate of torque development, time to peak torque, total time, & half relaxation time) and wrist flexor voluntary activation (VA) via interpolated twitch technique were assessed during each testing session. During data collection, the Covid-19 world pandemic caused the study to shut down before completion; six participants completed baseline testing (M=1, F=5, 19.8±1.9yrs, 165.9±9.3cm, 69.7±15.9kg), five completed post-eccentric training and three completed immobilization. Five right-handed, untrained participants demonstrated a significant ~5-6% increase in MCSA following ECC training (p<.05). Pooled across all contraction types, peak torque significantly increased in left and right arm flexion and right extension (p<.05) but not left extension. Left arm flexion strength increase was accompanied by increased agonist EMG activity, whereas right arm flexion and extension were associated with decreased agonist EMG. Increased antagonist EMG was evident for all tasks excluding right arm extension. No significant changes were noted in VA (p=.053) and the majority of contractile property measures (p>.05) aside from half relaxation time which significantly increased following training (p=.047). Three participants completed the immobilization phase: continued-training n=1, control n=2. Continued training of the right arm during immobilization partially spared MCSA (continued-training: -5.4% vs. control: -7.5%), grip strength (continued-training: -33.3% vs. control: -62.5%) and extension strength (continued-training: -0.5Nm vs. control: -4.1Nm) in the left immobilized limb. Peak flexion torque was increased in the left arm (+2.3Nm) for the continued training participant and decreased (-11.6Nm) without intervention. Increased agonist EMG activation accompanied increased strength and decreased activation accompanied decreases in strength for both flexion and extension tasks. The continued-training participant’s antagonist EMG activity decreased following immobilization. VA was fully spared for the continued training participant and decreased in the control (-39.7%). Contractile properties did not appear to be spared. Strength and muscle size were partially spared with implementation of a CE training program during immobilization with previous strength training, prior to immobilization. Despite the Covid-19 pandemic preventing the completion of the study, and limiting any conclusions that can be made, these data are useful to inform future work on the sparing effects of cross-education and in designing interventions to offset the decline in limb size and function that follows unilateral limb disuse.