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

In young males and females, 21-day head-down bed rest reduces knee extensor strength and muscle volume, while motor unit firing rate drops in both sexes, more in females.

• Why it matters: Sex-specific responses of motor unit properties to simulated microgravity are unclear, limiting how rehabilitation and risk might be tailored after unloading.
• What they did: Young males and females underwent 21 days of head-down bed rest and 21 days of retraining; knee extensor strength and volume were measured, and motor unit firing rate was tracked alongside serum C-terminal agrin fragment.
• The result: Bed rest lowered firing rate in both sexes (greater in females) and increased serum C-terminal agrin fragment; after retraining, firing rate and volume returned to baseline, but strength only partially recovered and stayed below baseline.

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