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

Altered thermoregulation has been reported following spaceflight simulations (bed rest and water immersion) but has never been examined after actual spaceflight.

We tested the null hypothesis that body temperatures and heat loss responses during exercise would be similar before and after spaceflight.

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 VO2peak) once at 145-146 d preflight and once at 5 d postflight (R + 5).

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 degrees 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/degree 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.degree C-1), were both reduced postflight without appreciable change in the Tin thresholds for sweating or skin blood flow.

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.