Author(s): DUNN, R., LUK, H., APPELL, C.R., JIWAN, N.C., KEEFE, M.S., SEKIGUCHI, Y., Institution: TEXAS TECH UNIVERSITY, Country: UNITED STATES, Abstract-ID: 755

INTRODUCTION:
Bouts of eccentric muscle-damaging exercise or heat exposure have the potential to reduce physiological and cellular stress during future exertional-heat exposure. Given potential cross adaptations, this study investigated the effect of muscle-damaging exercise in the heat on reducing physiological and cellular stress during future exertional-heat exposure.
METHODS:
Ten healthy, physically active males (mean ± SD; age, 23 ± 3 years; body mass, 78.7 ± 11.5 kg; height, 176.9 ± 4.7 cm) completed this study. In a randomised, counterbalanced order, participants were assigned into two groups; a) downhill running (DHR) in the heat (ambient temperature [Tamb], 35°C; relative humidity [RH], 40%), and b) DHR in thermoneutral (Tamb, 20°C; RH, 20%) to evoke muscle damage. Seven days following DHR, participants performed a 45-minute flat run in the heat (FlatHEAT [Tamb, 35°C; RH, 40%]). During exercise trials, heart rate (HR) and rectal temperature (Trec) were recorded at baseline and every 5-minutes. Peripheral blood mononuclear cells were isolated and homogenised to assess heat shock protein 72 (Hsp72) concentration between conditions at baseline, immediately post-DHR, and immediately pre- and post-FlatHEAT.
RESULTS:
Mean Trec during FlatHEAT between hot (38.23 ± 0.38 oC) and thermoneutral (38.26 ± 0.38 oC) were not significantly different (p = 0.68), with no mean HR differences during FlatHEAT between hot (172 ± 15 beats.min-1) and thermoneutral (174 ± 8 beats.min-1; p = 0.58). Hsp72 concentration change from baseline to immediately pre-FlatHEAT was significantly lower in hot (-51.4%) compared to thermoneutral (+24.2%; p = 0.025), with Hsp72 change from baseline to immediately post-FlatHEAT also lower in hot (-52.6%) compared to thermoneutral (+26.3%; p = 0.047).
CONCLUSION:
A singular bout of muscle-damaging exercise in the heat reduces cellular stress levels prior to and immediately following future exertional-heat exposure. Individuals regularly exposed to exertional-heat stress (e.g., athletes, military personnel, and firefighters) would likely benefit from implementing this preconditioning modality during heat preparation, particularly when the window to perform more robust heat adaptation methods is insufficient (e.g., heat acclimation/acclimatisation).