The contralateral repeated bout effect is not caused by adaptations in skeletal muscle

The repeated bout effect (RBE) refers to the phenomenon whereby the recovery of maximal voluntary contraction, a parameter considered to reflect muscle damage, is enhanced in a subsequent bout of exercise following an initial damaging bout. To investigate whether the ipsilateral RBE (IL-RBE) and contralateral RBE (CL-RBE) involve peripheral skeletal muscle adaptations, we assessed strength recovery following damaging eccentric contractions (ECCs) using supramaximal electrical stimulation to recruit all muscle fibers. Male Wistar rats were randomly assigned to one of the four groups: nondamaging control (CNT), damage (DMG), IL-RBE, and CL-RBE. The plantar flexors were exposed to 100 repeated ECCs with supramaximal electrical stimulation: once in the DMG group and twice at 2-wk intervals in the IL-RBE and CL-RBE groups. In the DMG group, the maximum isometric torque (MIT) at a stimulation frequency of 100 Hz remained 30% lower than the initial value even 4 days after ECCs. This was accompanied by an increased number of Evans Blue Dye-positive fibers, activation of calpain 1, and decreased expression of excitation-contraction coupling proteins. In the IL-RBE group, membrane damage and protein degradation were almost completely prevented and MIT returned to baseline by 1 day after ECCs. Conversely, the CL-RBE group did not show these beneficial effects observed in the IL-RBE group. These findings suggest that protective peripheral muscle adaptations contribute to the IL-RBE, but similar adaptations are unlikely to play a role in the CL-RBE.<b>NEW & NOTEWORTHY</b> When damaging contractions are repeatedly applied to a muscle, voluntary strength recovery is promoted not only in the exercised muscle but also in the contralateral muscle. However, the mechanism underlying this contralateral repeated bout effect (CL-RBE) remains unclear. Our findings show that strength recovery, assessed by electrical stimulation, was promoted only in the exercised muscle, not the contralateral side, suggesting the absence of adaptations within peripheral skeletal muscle in CL-RBE.