Heat Acclimation Protocol Optimization for VO2max Preservation and Thermoregulatory Adaptation in Elite Endurance Athletes: A Crossover Trial

This study investigates comparison of three heat acclimation protocols (passive, exercise-induced, and controlled hyperthermia) on VO2max preservation, sweat rate adaptation, and 5 km time trial performance in elite endurance athletes within the context of exercise physiology and sports science, an area of growing scientific importance given its implications for pre-competition heat preparation for athletes competing in hot climates and military/occupational heat acclimation protocol design. Using randomized crossover trial with 4-week washout comparing 10-day passive, exercise-induced, and controlled hyperthermia acclimation at 38 C/50% RH with pre/post thermoregulatory and performance testing, we examine repeated heat exposure triggering plasma volume expansion, earlier onset and increased sweat rate, heart rate adaptation, and heat shock protein upregulation that collectively reduce physiological strain and preserve aerobic capacity in heat in 24 well-trained endurance athletes (VO2max 64.2 ± 4.8 mL/kg/min, 18 male/6 female) completing all 3 protocols in counterbalanced crossover with 4-week washout drawn from climate-controlled environmental chamber at 38 C, 50% RH with metabolic cart VO2max testing at 18 C thermoneutral condition pre and post each acclimation block. Results indicate that controlled hyperthermia protocol produces greatest sweat rate adaptation (+18.4%) and heat VO2max preservation (+6.8%) with 5 km TT improvement of 84 seconds versus passive acclimation (+28 seconds) and exercise-induced (+52 seconds) (p < 0.001), with controlled hyperthermia: +18.4% sweat rate, +6.8% heat VO2max, -84s TT as the primary quantitative benchmark. Concordance between primary and confirmatory measurement approaches exceeded 93%, validating the analytical framework. These findings contribute empirically to exercise physiology and sports science and carry actionable implications for the design of programs and policies targeting pre-competition heat preparation for athletes competing in hot climates and military/occupational heat acclimation protocol design.