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 using 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/1 mph wind). We tested this novel protocol (6 mph run for 30 min followed by 3.5 mph walk for 60 min in a 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 ± 16 beats/min) compared with control exercise (P < 0.01; n = 15). In study II, we conducted heat stress tests (60 min at 50% maximal oxygen uptake in 40°C/40%RH/3 mph wind in shorts/t-shirt) before and after five 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 ± 11 beats/min) during exercise in the heat. These adaptations were superior compared with 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.NEW & NOTEWORTHY We developed a novel exercise with overdressing protocol for heat acclimation and validated it using two randomized crossover trials. Five sessions of exercise with overdressing in a gym-like environment significantly lowered resting core temperature and peak core temperature, skin temperature, and heart rate during exercise in the heat. These adaptations were superior compared with five sessions of control exercise. This practical exercise with overdressing protocol induces heat acclimation without requiring hot weather.