Physiological Responses to Overdressing and Exercise-Heat Stress in Trained Runners

AbstractHeat acclimation is the best strategy to improve performance in a hot environment. Many athletes seeking the benefits of heat acclimation lack access to a hot environment for exercise, and thus rely on over-dressing to simulate environmental heat stress. It is currently unknown whether this approach produces the requisite thermoregulatory strain necessary for heat acclimation in trained men and women.PURPOSETo compare physiological and cellular responses to exercise in a hot environment (HOT; 40°C, 30%rh) with minimal clothing (clo= 0.87) and in a temperate environment (CLO; 15°C, 50%rh) with overdressing (clo=1.89) in both men and women.METHODSHeart rate (HR), rectal temperature (Tre), mean skin temperature (Tsk), sweating rate (SR), and extracellular heat shock protein (eHSP)72 were measured in 13 (7M, 6F) well-trained runners (VO2max: 58.7±10.7 ml·kg-1·min-1) in response to ~60 minutes of treadmill running at 50-60% VO2max in HOT and CLO.RESULTSTre increased in both conditions, but the increase was greater in HOT ([INCREMENT]Tre HOT: 2.6 ± 0.1°C; CLO 2.0 ± 0.1°C; p=0.0003). SR was also higher in HOT (1.41± 0.1L·hr-1; CLO: 1.16 ± 0.1L·hr-1; p=0.0001). eHSP72 increased in HOT (% change: 59±11%; p=0.03) but not in CLO (6±2%; p=0.31). Mean Tsk and HR were not different between HOT and CLO in men, but were higher in HOT for women.CONCLUSIONSThese data support the idea that over-dressing during exercise in a temperate environment may produce the high Tre, Tsk, HR, and SR necessary for adaptation, but these responses do not match those in hot, dry environments. It is possible that a greater exercise stimulus, warmer environment, or more clothing may be required to allow for a similar level of acclimation. Heat acclimation is the best strategy to improve performance in a hot environment. Many athletes seeking the benefits of heat acclimation lack access to a hot environment for exercise, and thus rely on over-dressing to simulate environmental heat stress. It is currently unknown whether this approach produces the requisite thermoregulatory strain necessary for heat acclimation in trained men and women. PURPOSE To compare physiological and cellular responses to exercise in a hot environment (HOT; 40°C, 30%rh) with minimal clothing (clo= 0.87) and in a temperate environment (CLO; 15°C, 50%rh) with overdressing (clo=1.89) in both men and women. METHODS Heart rate (HR), rectal temperature (Tre), mean skin temperature (Tsk), sweating rate (SR), and extracellular heat shock protein (eHSP)72 were measured in 13 (7M, 6F) well-trained runners (VO2max: 58.7±10.7 ml·kg-1·min-1) in response to ~60 minutes of treadmill running at 50-60% VO2max in HOT and CLO. RESULTS Tre increased in both conditions, but the increase was greater in HOT ([INCREMENT]Tre HOT: 2.6 ± 0.1°C; CLO 2.0 ± 0.1°C; p=0.0003). SR was also higher in HOT (1.41± 0.1L·hr-1; CLO: 1.16 ± 0.1L·hr-1; p=0.0001). eHSP72 increased in HOT (% change: 59±11%; p=0.03) but not in CLO (6±2%; p=0.31). Mean Tsk and HR were not different between HOT and CLO in men, but were higher in HOT for women. CONCLUSIONS These data support the idea that over-dressing during exercise in a temperate environment may produce the high Tre, Tsk, HR, and SR necessary for adaptation, but these responses do not match those in hot, dry environments. It is possible that a greater exercise stimulus, warmer environment, or more clothing may be required to allow for a similar level of acclimation. Corresponding Author: Christopher T. Minson, PhD, Department of Human Physiology, 1240 University of Oregon, 122C Esslinger Hall, Eugene, OR 97403, Email: minson@uoregon.edu Funding was provided by the Kenneth and Kenda Singer Endowment. We do not have any conflicts of interest, and all results are presented clearly, honestly, and without fabrication, falsification, or inappropriate data manipulation. The results of the present study do not constitute endorsement by the ACSM. Accepted for Publication: 1 December 2017 © 2018 American College of Sports Medicine

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