Development Of A Standardized Overdressing + Exercise Protocol To Induce Targeted Heat Stress In A Military Setting

BACKGROUND: Heat acclimatization (HA) is the gold standard countermeasure to reduce heat illness risk and optimize physical performance in hot environments. Importantly, Soldiers stationed in cooler climates lack the ambient heat required for classical HA. Restricting heat loss and enhancing heat storage via overdressing (OD) could facilitate targeted hyperthermia to promote heat acclimation during exercise in temperate conditions. However, a validated OD protocol for military populations is lacking. PURPOSE: To evaluate the thermal responses to a novel, standardized OD + exercise protocol developed through biophysical modeling. We hypothesized that TC would reach, and remain, between 38.3 °C and 39.5 °C for at least 45 minutes with OD and would be substantially higher than similar exercise in standard physical training (PT) clothing. METHODS: In temperate (gym-like) conditions (20 °C, 50% RH, 1 mph air velocity), 7 healthy males (28 ± 7 yrs.; 75.1 ± 16.5 kg; 17.7 ± 2.7 %BF) completed a 30 min run (6.0 mph, 0% grade), followed immediately by a 60 min walk (3.5 mph; 0% grade) on two separate occasions in a randomized, crossover manner. The OD clothing ensemble consisted of an Army cold-wet weather jacket and trousers, combat uniform, PT shirt and shorts, gloves, and a fleece cap. The PT ensemble consisted of PT shirt and shorts only. TC, skin temperature (TSk), and heart rate (HR) were monitored continuously. Thermal sensation (TS) and thermal comfort (TC) were recorded at 15 min intervals. RESULTS: Time spent with Tc between 38.3 °C and 39.5 °C was greater with OD (53.2 ± 27.0 min) vs. PT (19.5 ± 18.6 min; P = 0.007). Peak TC (39.1 °C ± 0.5 vs 38.4 °C ± 0.4) and ∆TC (2.0 °C ± 0.5 vs 1.4 °C ± 0.2) were greater in OD relative to PT (P = 0.005 and P = 0.006, respectively). Peak TC remained <39.5 °C for all participants. TSk, HR, and TS were all significantly higher in OD compared to PT exercise whereas TC was lower in OD (P < 0.01 for all comparisons). CONCLUSIONS: Exercise in OD clothing elevated TC, TSk, and thermal perceptual strain to a greater degree when compared to similar exercise performed in PT clothing. Based on the appropriate increase in TC with this protocol, future work will determine the safety and efficacy of chronic exercise with OD to induce heat acclimation in military personnel. Funded by USAMRDC; author views not representative of the Army/DoD.