Physiological and biochemical responses of well-trained military personnel in a multi-day high-intensity training course

Consecutive days of high-intensity military training impose significant physiological and biochemical demands on personnel, heightening risks for exertional heat-related illnesses (EHI), exercise-associated hyponatremia (EAH) and exertional rhabdomyolysis (ER). We characterised continuous body core temperature (T c ), biochemical responses and illness risk profiles during a 5-day training course in a warm-humid environment. Twenty-three well-trained male military personnel were recruited. T c and wet-bulb globe temperature (WBGT) were continuously monitored. Venous blood was collected Pre, Mid, Post and three weeks post-course (Post2). Serum concentrations of sodium, calcium, potassium, chloride, bicarbonate and urea were analysed at Pre, Post and Post2. Total creatine kinase (CK) and creatine kinase-myocardial band (CKMB) concentrations were determined at all four time-points. The relationship between CK and CKMB was analysed using correlation, linear regression and Bland-Altman analyses. The training course was predominantly conducted under WBGT ≤ 30.9 °C. Mean overall peak T c was 39.5 ± 0.5 °C (38.8–40.9 °C). 26% of participants had overall peak 39.5 < T c ≤ 40 °C and 17% >40 °C, but clinical symptoms of EHI were not observed. 48% of participants developed asymptomatic EAH (sodium concentration < 135 mmol/L, 131 ± 3 (126–134) mmol/L). Although CK increased by up to 45-times relative to Pre, ER was not observed. CK and CKMB were strongly correlated ( r = 0.759, p < 0.001), but large individual variability limits the use of CKMB as a proxy for CK (mean bias = 0.45 U/L, 95% limits of agreement = -11426 to 11425 U/L). 30% of participants met risk thresholds for EHI, EAH and ER across the 5-day period, forming a ‘triple-risk’ subgroup. Well-trained military personnel are resilient to extreme physiological and biochemical changes. Subgroups of participants meeting multiple risk thresholds highlight individual susceptibility despite overall cohort resilience and suggest shared underlying mechanisms. These findings support refining safety thresholds to distinguish between expected training responses and clinical risk to optimise safety and performance.