Exercise with overdressing for heat acclimation: a multilayered approach using biophysical modeling and two randomized crossover trials.

Individuals who work in the heat, such as military personnel and athletes, are often required to rapidly transition from temperate or cooler climates to hot environments. Thus, acclimation strategies are needed for individuals lacking access to hot weather. We sought to develop and validate a practical exercise with overdressing protocol for heat acclimation. We began by utilizing biophysical modeling to identify a combination of clothing and treadmill exercise (speed/duration) predicted to facilitate appropriate increases in core temperature in a gym-like environment (20°C/50% RH/1mph wind). We tested this novel protocol (6mph run for 30min followed by 3.5mph walk for 60min in standardized overdressing ensemble) against control exercise (shorts/t-shirt) using two randomized crossover trials in fit males and females. In study I, we showed that a single session of exercise with overdressing elicited significantly higher peak core temperature (38.9±0.4 vs. 38.5±0.3°C), skin temperature (35.3±0.6 vs. 32.5±0.7°C), and heart rate (166±20 vs. 147±16bpm) compared to control exercise (p<0.01; n=15). In study II, we conducted heat stress tests (60min at 50% maximal oxygen uptake in 40°C/40%RH/3mph wind in shorts/t-shirt) before and after 5 sessions of exercise with overdressing or control exercise (n=12). Five days of exercise with overdressing in a gym-like environment significantly (p<0.01) lowered resting core temperature (- 0.3±0.2°C), peak core temperature (-0.4±0.2°C), skin temperature (-0.5±0.6°C), and heart rate (-11±11bpm) during exercise in the heat. These adaptations were superior compared to control exercise (interactions p<0.05). This practical exercise with overdressing approach effectively induces heat acclimation in fit males and females without requiring hot weather.