Body temperature and thermoregulation during submaximal exercise after 115-day spaceflight

Background: Altered thermoregulation has been reported following spaceflight simulations (bed rest and water immersion) but has never been examined after actual spaceflight. Hypothesis: We tested the null hypothesis that body temperatures and heat loss responses during exercise would be similar before and after spaceflight. Methods: Two male crewmembers of the 115-d Mir 18 mission performed supine submaximal cycle exercise (20 min at 40% and 20 min at 65% of preflight VO 2peak ) once at 145-146 d preflight and once at 5 d postflight (R + 5). Results: After flight neither crewmember could complete the exercise protocol, stopping after 28-29 min. The core temperature (Tin, ingestible telemetry pill) at test termination was similar (37.8°C for both subjects) pre- and postflight despite shorter postflight test duration. The slopes of the skin blood flow (laser Doppler)/Tin relationship (subject 1: 396 vs. 214; subject 2: 704 vs. 143% change Perfusion Unit/°C), and the sweating rate (dew point hygrometry)/Tin relationship (subject 1: 6.3 vs. 2.0; subject 2: 4.6 vs. 0.7 mg.min -1 . cm -2 . °C -1 ), were both reduced postflight without appreciable change in the Tin thresholds for sweating or skin blood flow. Conclusion: In this preliminary report for two crewmembers, the sensitivity of the heat loss responses were reduced after long-duration spaceflight, resulting in a faster rate of rise in core temperature.