Voluntary Cooling‐Seeking Behavior during Heat Exposure is Decreased When Physical Effort is Required

Abstract

Motivational deterrents increase the level of cold discomfort that an individual will tolerate prior to engaging in warm-seeking behavior. It is unknown if this is true for warm discomfort, due to differences in behavioral sensitivities during heat and cold exposure. Purpose: Test the hypothesis that the requirement to engage in physical effort (i.e., the motivational deterrent) to receive cooling during heat exposure attenuates voluntary cool-seeking behavior compared to when no physical effort was required. Methods: Twelve healthy adults (27±4 y, 4 females) underwent 2 h exposures to 41±1°C, 20±0% RH with 15 min work:rest intervals at ~4 METS on 3 occasions. The trial order was quasi-randomized. Subjects wore a tube-lined suit top able to perfuse cold water (2.1±0.2ºC). In the first trial, participants received no cooling (CON). The subsequent trials were randomized and subjects were given the option to freely engage in cool-seeking behavior by either 1) pressing a button to receive 2 min of cooling (BUTTON) or 2) receive cooling while producing 15% maximal isometric handgrip force for as long as desired (HG) or up to 2 min. A 1 min washout period followed each behavior. Mean skin (Tsk, 10 site) and core (Tcore, gastrointestinal pill) temperature, skin wettedness (Wsk, 10 site), forearm skin blood flow (SkBF, laser Doppler), and axilla and thigh local sweat rate (LSR, ventilated capsule) were recorded continuously. The total number of behaviors and cumulative cooling time indicated the amount of cool-seeking behavior between each trial. Results: Tcore, and Wsk increased over time (P<0.01) with no differences between trials (P≥0.28) or at the end of heat exposure (CON: +0.69±0.32ºC; BUTTON: +0.75±0.33ºC; HG: +0.62±0.26ºC, P=0.52). Tsk at the end of heat exposure was lowest in BUTTON (34.2±1.5°C) compared to HG (35.6±0.8°C, P=0.03) and CON (36.9±0.7°C, P<0.01). Axilla and thigh LSR, and SkBF increased over time (P<0.01) with no differences between trials (P≥0.11). The total number of behaviors (8±3 vs. 10±5, P=0.04) and cumulative cooling time (850±323 vs. 1230±616 seconds, P=0.02) were lower in HG compared to BUTTON. Conclusion : Motivational deterents can modulate behavioral thermoregulation during heat exposure such that, when physical effort is required to obtain cooling, the frequency and duration of cool-seeking behavior is reduced.