Foot forces during exercise on the International Space Station

Long-duration exposure to microgravity has been shown to have detrimental effects on the human musculoskeletal system. To date, exercise countermeasures have been the primary approach to maintain bone and muscle mass and they have not been successful. Up until 2008, the three exercise countermeasure devices available on the International Space Station (ISS) were the treadmill with vibration isolation and stabilization (TVIS), the cycle ergometer with vibration isolation and stabilization (CEVIS), and the interim resistance exercise device (iRED). This article examines the available envelope of mechanical loads to the lower extremity that these exercise devices can generate based on direct in-shoe force measurements performed on the ISS. Four male crewmembers who flew on long-duration ISS missions participated in this study. In-shoe forces were recorded during activities designed to elicit maximum loads from the various exercise devices. Data from typical exercise sessions on Earth and on-orbit were also available for comparison. Maximum on-orbit single-leg loads from TVIS were 1.77 body weight (BW) while running at 8mph. The largest single-leg forces during resistance exercise were 0.72 BW during single-leg heel raises and 0.68 BW during double-leg squats. Forces during CEVIS exercise were small, approaching only 0.19 BW at 210W and 95RPM. We conclude that the three exercise devices studied were not able to elicit loads comparable to exercise on Earth, with the exception of CEVIS at its maximal setting. The decrements were, on average, 77% for walking, 75% for running, and 65% for squats when each device was at its maximum setting. Future developments must include an improved harness to apply higher gravity replacement loads during locomotor exercise and the provision of greater resistance exercise capability. The present data set provides a benchmark that will enable future researchers to judge whether or not the new generation of exercise countermeasures recently added to the ISS will address the need for greater loading.