Responses of Muscle Volume, Strength, and Motor Unit Properties to Bed Rest and Retraining in Young Males and Females

Sex differences in motor unit (MU) properties to simulated microgravity are unknown. Young males and females completed a 21-day head-down bed rest followed by a 21-day retraining. Bed rest reduced knee extensor strength and volume without sex differences. MU firing rate decreased in both groups, more in females, alongside increased serum C-terminal agrin fragment. After retraining, these measures returned to baseline, whereas strength only partially recovered, remaining below baseline.