Examining The Contralateral Repeated Bout Effect in Females While Accounting for Menstrual Cycle-Related Estrogen Fluctuations.

The Contralateral Repeated Bout Effect (CL-RBE) occurs when an initial exercise protects the opposite, unexercised muscle from muscle damage during subsequent exercises. CL-RBE has been widely demonstrated in male populations, but research involving females remains limited. Recent research, including exclusively female subjects, reported an absence of CL-RBE, suggesting a possible sex-specific difference in the phenomenon. This null result may have been due to estrogen changes during the menstrual cycle, since estrogen has been suggested to protect from muscle damage. Therefore, this study controlled for estrogen fluctuations in fourteen females while testing for CL-RBE. Each participant completed the same eccentric exercise consisting of 60 repetitions on opposite arms, two weeks apart, with muscle damage markers assessed pre-exercise, immediately (muscle strength only), 24 hours, and 48 hours post-exercise. Significant main effects of time were observed in strength (p = .006) and soreness (p < .001), confirming muscle damage after the eccentric exercises. A significant bout effect was observed for strength (p < .028). However, no bout × time interaction emerged for any measured marker (p > .05). These findings suggest that estrogen fluctuation alone may not explain the absence of CL-RBE in females, and support emerging evidence of sex-specific differences in CL-RBE. While this study incorporated increased exercise volume and controlled for estrogen fluctuation, limitations include the indirect estimation of estrogen levels, which may have allowed subtle CL-RBE to go undetected. This highlights the need for future investigation into female-specific mechanisms governing muscle adaptation to develop evidence-based rehabilitation and training protocols that accurately reflect female physiology.