Muscle-specific atrophy of the lower limb musculature in response to simulated microgravity exposure in women.

Despite the increased involvement of women in spaceflight missions, the efforts to understand changes in skeletal muscle health in women with microgravity exposure have been limited. The goal of the current investigation was to expand the limited knowledge on skeletal muscle atrophy responses to microgravity in women. Eight women (34 ± 4 yr) underwent 60 days of simulated microgravity (6° head-down tilt bedrest), and changes in MRI-determined skeletal muscle volume of 17 lower limb muscles were determined after 1 and 2 mo. Muscle volume decreased ( < 0.05) in all 17 muscles of the women after 1 and 2 mo of simulated microgravity. There was nearly a threefold difference between the least (rectus femoris: -10%) and most (soleus: -27%, lateral gastrocnemius: -28%, and medial gastrocnemius: -29%) affected muscles. Muscle-specific atrophy was also evident within muscle groups of the knee extensors (rectus femoris: -10%; vastii: -22%, < 0.05) and knee flexors (semimembranosus and biceps femoris long head: -20%; gracilis, biceps femoris short head, semitendinosus: -12%, < 0.05). These findings in women were also compared with data from men (33 ± 7 yr) who previously underwent similar MRI volume determinations of the same 17 muscles after 1 and 2 mo of simulated microgravity exposure. Compared with men, women experienced more pronounced atrophy in 16 of the 17 muscles ( < 0.05), and at least double the atrophy in seven of these muscles. The current findings extend our understanding of the magnitude of muscle- and sex-specific skeletal muscle responses to long-duration microgravity, which should be considered for spaceflight exercise countermeasures program evolution. This study expands the limited amount of data on skeletal muscle changes with long-duration microgravity exposure in women. Microgravity-induced skeletal muscle atrophy of the lower limbs occurs in a muscle- and sex-specific fashion. This new information should be considered in exercise countermeasures development for astronauts on long-duration spaceflight missions.