Oxygen Consumption and Heart Rate Responses in Graded Exercise during Long-Duration Space Flight

Abstract

Astronauts routinely perform submaximal graded exercise testing using cycle ergometry before and during International Space Station (ISS) missions. Aerobic capacity has been estimated from linear extrapolation of heart rate (HR) response and submaximal oxygen consumption (VO2). The elevated HR response to standardized exercise work rates during ISS flight typically has been interpreted as aerobic deconditioning. However, until recently VO2 has not been measured during these tests. PURPOSE: To determine if submaximal VO2 and HR measured during exercise tests on the ISS differ from preflight values as a function of time on board and of exercise intensity. METHODS: Astronauts (n=7) performed exercise tests before flight on a standard cycle ergometer and during flight on the ISS cycle ergometer. In-flight exercise tests were conducted at intervals between days 21 and 149 of the missions. The protocol for submaximal exercise tests consisted of three 5-minute stages at work rates prescribed to elicit 25, 50, and 75% of preflight peak VO2. HR was obtained from ECG and VO2 from metabolic gas analysis. Mixed modeling was used to examine the mean differences between preflight and in-flight measurements of VO2 and of HR over time and at each work rate. RESULTS: During flight, VO2 was significantly lower during the 25% and higher during the 75% exercise stages but did not differ from preflight VO2 at the 50% stage. These differences were consistent across in-flight time points. HR was unchanged from preflight HR at the 25% stage of exercise, although the HR at this stage tended to lower as flight duration increased. At the 50% and 75% stages, HR was elevated early during the missions but did not differ from preflight values later during flight. CONCLUSIONS: The preflight relationship between VO2 and work rate is not maintained during flight, perhaps because of microgravity and/or differences between the ergometers. The changing HR responses across mission duration, some of which do not parallel the VO2 findings, may indicate an acute adaptation early during flight that normalizes later during the missions, and/or an exercise training effect during flight. Thus, estimating aerobic capacity during flight by using only HR responses to submaximal work rates may result in erroneous conclusions. Supported by NASA Contract NAS 9-02078