Effectiveness of Exercise Countermeasures for Protecting Muscle on the International Space Station (ISS)

Abstract

These investigators document(ed) the exercise program used by crewmembers aboard the ISS and examine(d) its effectiveness for preserving skeletal muscle size and function. Participants were 10 American and Russian ISS crewmembers with mission durations of 161 to 192 days. A treadmill, cycle ergometers, and a resistance exercise device were available on the ISS and programs were individualized for each subject. Countermeasure performance varied widely among the subjects. The study concentrated on the calf muscles because they atrophy more than other leg or upper body muscles with spaceflight. After missions, soleus muscles had atrophied more than gastrocnemius muscles, with great variation among crewmembers. All measured muscle performance variables were reduced after flight and the muscles also showed changes, and individual variation, in myosin heavy chain fiber types. Analyses of caloric intake and individual exercise indicated the caloric intake was 17 to 23% below the predicted caloric need. The investigators conclude that the exercise program did not completely protect the calf muscle. They identified four factors in addition to the microgravity environment contributed to muscle loss: 1) calf muscle volume before spaceflight; 2) volume of treadmill exercise; 3) treadmill mode (passive mode may protect better); and 4) inadequate caloric intake. In addition to providing evidence the current exercise countermeasure program does not fully protect human skeletal muscle in spaceflight, this research provides valuable information for future monitoring and for the development of an effective countermeasure program.