Cardiovascular exercise in the U.S. space program: Past, present and future

Exercise deconditioning during space flight may impact a crewmember's ability to perform strenuous or prolonged tasks during and after a spaceflight mission. In this paper, we review the cardiovascular exercise data from U.S. spaceflights from the Mercury Project through International Space Station (ISS) expeditions and potential implications upon current and future missions. During shorter spaceflights (<16 days), the heart rate (HR) response to exercise testing and maximum oxygen consumption (VO2 max) are not changed. The submaximal exercise HR responses during longer duration flights are less consistent, and VO2 max has not been measured. Skylab data demonstrated no change in the exercise HR response during flight which would be consistent with no change in VO2 max; however, during ISS flight exercise HR is elevated early in the mission, but approaches preflight levels later during the missions, perhaps due to performance of exercise countermeasures. An elevated exercise HR is consistently observed after both short and long duration spaceflight, and crewmembers appear to recover at rates which are affected by the length of the mission.