Effect of short-duration spaceflight on thigh and leg muscle volume

Purpose: Human skeletal muscle probably atrophies as a result of spacefligh t, but few studies have examined this issue. Thus, little is known about th e influence of microgravity upon human skeletal muscle, nor is it possible to assess the validity of ground based models of spaceflight. This study te sted the hypothesis that the magnitude of spaceflight induced muscle atroph y would be a function of flight duration and greater than that of bed rest. Methods: Three astronauts flew 9, 15, and 16 d in space. Volume of the kne e extensor (quadriceps femoris), knee flexor (hamstrings, sartorius, and gr acilis), and plantar flexor (triceps surae) muscle groups was measured usin g magnetic resonance imaging before and after spaceflight and during recove ry. The Volume of each muscle group in each image was determined by multipl ying cross-sectional area by slice thickness. These values were subsequentl y summed to calculate muscle volume. Results: Volume changes in the knee ex tensor. knee flexor, and plantar flexor muscle groups ranged from -15.4 to -5.5, -14.1 to -5.6, and -8.8 to -15.9%, respectively. Muscle volume decrea ses normalized by flight duration ranged from 0.62 to 1.04%.d(-1). These re lative changes appeared to be greater than those that we have reported prev iously for bed rest (Akima et al., J. Gravitat. Physiol. 4:15-22, 1997). Co nclusions: These results suggest that atrophy as a result of at least 2 wk of spaceflight varied among individuals and muscle groups and that the degr ee of atrophy appeared to be greater than that induced by 20 d of bed rest.