Human Skeletal Muscle Responses to Prolonged Space Flight: Whole Muscle Size and Strength

Abstract

Magnetic resonance imaging (MRI) and isokinetic dynamometry was used to assess lower leg skeletal muscle characteristics from crewmembers (n=9; 45±2 y) that averaged 181-d aboard the International Space Station (ISS). While on the ISS, crewmembers had access to a running treadmill, cycle ergometer and resistance exercise device. The exercise regimen varied among the individuals with groups identified as high treadmill users (HT; n=4) and low treadmill users (LT; n=5). On average, all crewmembers lost 13±2% (p<0.05) of their lower leg muscle mass. HT lost less (p<0.05) muscle mass (8±2%) compared to LT (15±2%). Maximal isometric strength was reduced similarly for all crewmembers (14±2%; p<0.05). Isokinetic strength at six speeds was reduced 20–29% (p<0.05) with no differences between HT and LT. These data show that lower leg muscle volume and strength are significantly reduced after 6 months in space. These data also indicate that the level of treadmill use in space may aid in preserving lower limb muscle mass. Future long duration space missions should consider modifications to the current ISS exercise prescription and/or hardware that would preserve human skeletal muscle mass and function. Supported by NASA grant NCC9-116.