Running Title: Exercise and skeletal muscle with spaceflight Address for Correspondence:

The aim of this investigation was to document the exercise program used by crewmembers (n=9; 45±2 y) while aboard the International Space Station (ISS) for 6 months and examine its effectiveness for preserving calf muscle characteristics. Before and after spaceflight, we assessed calf muscle volume (MRI), static and dynamic calf muscle performance, and muscle fiber types (gastrocnemius and soleus). While on the ISS, crewmembers had access to a running treadmill, cycle ergometer and resistance exercise device. The exercise regimen varied among the crewmembers with aerobic exercise performed ~5 hr/wk at a moderate intensity and resistance exercise performed 3-6 d/wk incorporating multiple lower leg exercises. Calf muscle volume decreased (P<0.05) 13±2 % with greater (P<0.05) atrophy of the soleus (-15±2%) compared to the gastrocnemius (-10±2%). Peak power was 32 % lower (P<0.05) after spaceflight. Force-velocity characteristics were reduced (P<0.05)-20 to-29 % across the velocity spectrum. There was a 12-17 % shift in MHC phenotype of the gastrocnemius and soleus with a decrease (P<0.05) in MHC I fibers and a redistribution among the faste