Does cross-training balance strength asymmetry in healthy subjects? A proof-of-concept trial

Question: We investigated whether a unilateral, maximal isokinetic strength training of the stronger leg may balance strength asymmetry between sides, inducing in the contralateral untrained side a worthy strength-transfer, i.e. a cross-training effect (CT) as well as cortico-spinal adaptations. Methods: Ten healthy volunteers (25.2±4.3yrs) underwent: i) bilateral measurements of the tibialis anterior (TA) strength with isokinetic dynamometry; ii) assessment of both cortical and spinal drives to the untrained TA, through recordings of central motor conduction time, input-output curve, short-interval intracortical inhibition, intracortical facilitation, shortinterval intracortical facilitation, cortical silent period (cSP), short-latency afferent inhibition, H-reflex, V-wave and M-wave. Dynanometric and neurophysiological tests were performed before and after a 4-week maximal isokinetic training (16 sessions) of the stronger leg. Results: The trained TA showed a maximum strength increase of 24.8% (p=0.008;d=0.8) at 90°/s angular velocity and 21.5% (p=0.001;d=0.7) at 40°/s, whilst the untrained TA showed respectively 30.2% (p=0.014;d=0.8) and 23.9% (p=0.001;d=0.6) increases in strength. Notably, after training no significant difference in strength output was anymore detectable between sides at 90°/S (pre: p=0.004; post: p=0.558). A significant increase in corticospinal drive to the untrained TA was revealed by reduction of cSP duration (p=0.01,d=0.5) and by increase of the V-wave amplitude (p=0.01; d>0.8). Conclusion: In our cohort, the CT-effect was remarkable and capable of significantly balancing strength asymmetry. These findings disclose new potential implications for CT as a promising approach to neurological conditions where unilateral muscle weakness does not allow or makes difficult performing a conventional strength training of the weaker limb.