Human thermoregulatory function during exercise and immersion after 35 days of horizontal bed-rest and recovery

Abstract

The present study evaluated the effect of 35 days of experimental horizontal bed-rest on exercise and immersion thermoregulatory function. Fifteen healthy male volunteers were assigned to either a Control (n=5) or Bed-rest (n=10) group. Thermoregulatory function was evaluated during a 30-min bout of submaximal exercise on a cycle ergometer, followed immediately by a 100-min immersion in 28°C water. For the Bed-rest group, exercise and immersion thermoregulatory responses observed post-bed-rest were compared with those after a 5 week supervised active recovery period. In both trials, the absolute work load during the exercise portion of the test was identical. During the exercise and immersion, we recorded skin temperature, rectal temperature, the difference in temperature between the forearm and third digit of the right hand (ΔT_{forearm-fingertip})— an index of skin blood flow, sweating rate from the forehead, oxygen uptake and heart rate at minute intervals. Subjects provided ratings of temperature perception and thermal comfort at 5-min intervals. Exercise thermoregulatory responses after bed-rest and recovery were similar. Subjective ratings of temperature perception and thermal comfort during immersion indicated that subjects perceived similar combinations of Tsk and Tre to be warmer and thermally less uncomfortable after bed-rest. The average (SD) exercise-induced increase in Tre relative to resting values was not significantly different between the Post-bed-rest (0.4 (0.2)°C) and Recovery (0.5 (0.2)°C) trials. During the post-exercise immersion, the decrease in Tre, relative to resting values, was significantly (P<0.05) greater in the Post-bed-rest trial (0.9 (0.5)°C) than after recovery (0.4 (0.3)°C). ΔT_{forearm-fingertip} was 5.2 (0.9)°C and 5.8 (1.0)°C at the end of the post-bed-rest and recovery immersions, respectively. The gain of the shivering response (increase in V̇O₂ relative to the decrease in Tre; V̇O₂/Tre) was 1.19 l min⁻¹°C⁻¹ in the Recovery trial, and was significantly attenuated to 0.51 l min⁻¹°C⁻¹ in the Post-bed-rest trial. The greater cooling rate observed in the post-bed-rest trial is attributed to the greater heat loss and reduced heat production. The former is the result of attenuated cold-induced vasoconstriction and enhanced sweating rate, and the latter a result of a lower shivering V̇O₂ response.