Influence Of Immediate Versus Delayed Exercise On Cold-wet Body Temperatures

Water saturated clothing worn in a cold air environment can significantly increase risk of hypothermia. While exercise heat production generally reduces cold injury risk, cold-wet threat to body temperatures is of greatest relevance in operational scenarios that require remaining static, when exercise heat production is low. PURPOSE: Evaluate body temperature responses to cold air during a static-to-exercise and exercise-to-static sequence wearing water saturated clothing. METHODS: Eight volunteers (7 M, 1F; 26 ± 9 yrs; 25.5 ± 8.6% BF) performed two randomized trials (static-exercise; exercise-static) while wearing an Army Combat Uniform. For both trials, volunteers immersed to the neck in thermoneutral water (33.9 ± 0.3 °C) for two minutes before entering a cold chamber (5 °C, 45-55%RH). Once in the cold, the static-exercise trial sequence consisted of 60 min of static standing followed by 60 min of ruck-loaded treadmill walking (0.89 m/s, 2.6 ± 1.1% grade) for a total of 120 min. The exercise-static trial reversed this sequence. RESULTS: Area under the curve (AUC) analysis indicated that across the first 60 min of cold air exposure (Period 1), rectal temperature (Trec) was 1.25 [1.12-1.37] times greater (p < 0.001) with the exercise-static sequence. Trec remained elevated across the static 60 min (Period 2) in cold air (p = 0.011). The static-exercise sequence also increased Trec, albeit to a lesser degree (Period 1 ΔTrec, exercise-static: 0.75 ± 0.26 vs. static-exercise: 0.38 ± 0.25 °C; Period 2, exercise-static: -0.63 ± 0.18 vs. static-exercise: -0.02 ± 0.31 °C). Mean, foot, and finger skin temperature (Tsk) AUC did not differ between the two trials across either period (p > 0.05). Direct comparison of static periods, irrespective of time, indicate mean Tsk AUC was 2.17 [1.25-3.79] times higher when volunteers were static first (p = 0.014) despite lower Trec and greater uniform water mass. CONCLUSION: Hypothermia risk over two hours in 5 °C air wearing a water saturated Army Combat Uniform is minimal, regardless of whether physical activity is immediately initiated or delayed. Remaining static for 60 min following clothing saturation may better defend skin temperatures due to an unperturbed skin-clothing microenvironment. Supported by Defense Health Program JPC5; author views not official US Army or DoD policy