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 degrees 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 (DeltaT(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) degrees C) and Recovery (0.5 (0.2) degrees 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) degrees C) than after recovery (0.4 (0.3) degrees C). DeltaT(forearm-fingertip) was 5.2 (0.9) degrees C and 5.8 (1.0) degrees C at the end of the post-bed-rest and recovery immersions, respectively. The gain of the shivering response (increase in VO(2) relative to the decrease in Tre; VO(2)/Tre) was 1.19 l min(-1) degrees C(-1) in the Recovery trial, and was significantly attenuated to 0.51 l min(-1) degrees 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 VO(2) response.

Effect of 21 days of horizontal bed rest on behavioural thermoregulation

“…Inappropriate behavioural regulation of body temperature may contribute, at least in part, to the observed thermal imbalance during prolonged bed rest or microgravity. Changes in thermal comfort and thermal sensation have been reported in previous studies during hypokinesia (Panferova 1976(Panferova , 1989) and experimental bed rest (Fortney et al 1996;Greenleaf 1989;Mekjavic et al 2005). However, the eVects of bed rest on behavioural thermoregulatory responses have not yet been investigated.…”

“…In addition, since activation of behavioural thermoregulatory responses relies on similar thermal aVerent neural information as autonomic thermoregulatory responses, it was hypothesised that behavioural responses might also be aVected in such conditions. Mekjavic et al (2005) reported that subjects perceived similar combinations of skin and the core temperatures as warmer and thermally less uncomfortable after 35 days of horizontal bed rest. Reports from earlier studies indicate that some changes in thermal sensitivity occur (Fortney et al 1996;Panferova 1976) and that subjects in bed rest studies often complain of cold feet (Fortney et al 1996;Mekjavic et al 2005;Taylor et al 1949).…”

“…Exercise-induced heat loss (sweating and vasodilatation) and heat production responses (shivering) were shown to be modiWed after bed rest (Fortney et al 1996(Fortney et al , 1998Greenleaf 1989;Mekjavic et al 2005;Pavy-Le Traon et al 2007). Changes in the eYciency and/or sensitivity of these autonomic responses were suggested to contribute to the elevation in core temperature observed during spaceXights (Rimmer et al 1999) and in bed rest experiments (Ertl et al 2000;Fortney et al 1998;Greenleaf 1997).…”

Increased distensibility in dependent veins following prolonged bedrest

“…Video surveillance and results from criterion tests and other experiments indicated that all subjects complied with the requirements of the bedrest protocol (Mekjavic et al 2005;Berg et al 2007;Eiken et al 2008). Prior to bedrest, all ten subjects could endure the 10-min stand test with normal cardiovascular responses and without experiencing any unwarranted symptoms.…”

“…Results from these criterion tests have, for the most part, been reported in detail elsewhere (cf. Mekjavic et al 2005;Berg et al 2007;Eiken et al 2008). …”