Thermal behavior alleviates thermal discomfort during steady-state exercise without affecting whole body heat loss

Abstract

We tested the hypothesis that thermal behavior resulting in reductions in mean skin temperature alleviates thermal discomfort and mitigates the rise in core temperature during light-intensity exercise. In a 27 ± 0°C, 48 ± 6% relative humidity environment, 12 healthy subjects (6 men, 6 women) completed 60 min of recumbent cycling. In both trials, subjects wore a water-perfused suit top continually perfusing 34 ± 0°C water. In the behavior trial, subjects maintained their upper body thermally comfortable by pressing a button to perfuse cool water (2.2 ± 0.5°C) through the top for 2 min per button press. Metabolic heat production (control: 404 ± 52 W, behavior: 397 ± 65 W; P = 0.44) was similar between trials. Mean skin temperature was reduced in the behavior trial (by −2.1 ± 1.8°C, P < 0.01) because of voluntary reductions in water-perfused top temperature ( P < 0.01). Whole body ( P = 0.02) and local sweat rates were lower in the behavior trial ( P ≤ 0.05). Absolute core temperature was similar ( P ≥ 0.30); however, the change in core temperature was greater in the behavior trial after 40 min of exercise ( P ≤ 0.03). Partitional calorimetry did not reveal any differences in cumulative heat storage (control: 554 ± 229, behavior: 544 ± 283 kJ; P = 0.90). Thermal behavior alleviated whole body thermal discomfort during exercise (by −1.17 ± 0.40 arbitrary units, P < 0.01). Despite lower evaporative cooling in the behavior trial, similar heat loss was achieved by voluntarily employing convective cooling. Therefore, thermal behavior resulting in large reductions in skin temperature is effective at alleviating thermal discomfort during exercise without affecting whole body heat loss. NEW & NOTEWORTHY This study aimed to determine the effectiveness of thermal behavior in maintaining thermal comfort during exercise by allowing subjects to voluntarily cool their torso and upper limbs with 2°C water throughout a light-intensity exercise protocol. We show that voluntary cooling of the upper body alleviates thermal discomfort while maintaining heat balance through convective rather than evaporative means of heat loss.

Thermal Behavior Augments Heat Loss Following Low Intensity Exercise

“…Visit one was the consent visit and visits two and three were experimental trials. Data from one of these experimental trials (i.e., the control trial) have been reported previously in a study that tested a different hypothesis [12]. The behavior trial in the current study was a completely separate trial from those presented previously in that subjects were restricted from behaving during exercise and only allowed to behave during recovery.…”

“…The distance of 6 mm was chosen because it ensured that the humidity sensor of the iButton would not become artificially supersaturated due to a droplet of sweat entering the hygrosensor [10,11]. Relative humidity from the iButtons and skin temperature from the adjacent thermocouple placed on each site were used to determine the water vapor pressure of the skin using standard calculations as previously reported [12,21]. Local skin wettedness was calculated according to the methods of Gagge [22] (See Appendix B).…”

“…Using these data, dry and evaporative heat losses, and cumulative heat losses throughout recovery were estimated. Notably, this analysis was added post hoc based on the reported findings, as has previously been reported [12], to help inform decision making regarding conclusions. Details of the partitional calorimetry methods can be found in Appendix C. The same equations were employed as our previous study [12], but we calculated different specific correction factors from those reported previously.…”

“…We recently identified that voluntarily cooling the skin of the upper body during exercise promotes dry heat loss and thermal comfort, despite attenuating sweat rate [12]. Given that sudomotor activity is rapidly withdrawn during exercise recovery, engaging in thermal behavior that promotes dry heat loss at this time, may accelerate the recovery of core temperature to pre-exercise levels.…”

Effects of Acute Exercise on Cutaneous Thermal Sensation

“…For example, as heat production from active musculature increases, various neutrally mediated skin blood-flow and sweating reflexes occur in order to facilitate heat dissipation [3]. Despite these autonomic adjustments in order to serve cardiovascular and thermoregulatory functions, behavioural thermoregulation is still active during exercise and plays an important role in exercise intensity and local microclimate (i.e., seeking cooling and/or removing clothing) selection during exercise, ultimately to help limit thermal discomfort and avoid heat-related illness [4]. Furthermore, behavioural thermoregulation remains engaged after the cessation of exercise, which is particularly important due to the withdrawal of autonomic thermoeffectors [5] and the extended elevation of internal temperature and thermal discomfort post-exercise [6].…”