Short-term, circuit-based heat acclimation enhances intermittent work capacity, reaction time, and neuromuscular function under heat stress in active males

Exertional heat stress impairs cardiovascular, thermoregulatory, cognitive, and neuromuscular function, compromising health and performance. Although effective, traditional active heat acclimation (HA) relies on prolonged, low- to moderate-intensity exercise heat exposures and may therefore lack context-specificity. We investigated whether short-duration, high-intensity, circuit-based HA elicits favorable performance and thermophysiological adaptations. Eighteen participants completed either five circuit training sessions in hot (HEAT: 35°C; n=9) or thermoneutral conditions (CON: 20°C; n=9) over eight days. Intermittent work capacity and concurrent thermophysiological responses under heat stress were assessed pre- and post-intervention. Additionally, reaction time and neuromuscular function were evaluated to determine whether circuit-based HA attenuates hyperthermia- and fatigue-related impairments following intermittent exercise in the heat. Intermittent work capacity increased to a greater extent following HEAT compared to CON (p=0.001). Furthermore, reaction time improved in HEAT with no meaningful change in CON (p<0.001). Unilateral and bilateral jump height were enhanced (p=0.014−0.044) in HEAT immediately following intermittent work under heat stress. Additionally, increased vastus lateralis excitation during these movements were observed exclusively in HEAT (all p≤0.003). Chronic reductions in thermophysiological strain occurred alongside improvements in intermittent work capacity, with significant condition × time interactions for peak (p=0.022) and Δ (p=0.012) rectal temperature (T rec ), reflecting significant reductions in HEAT and no significant change in CON. Similar interactions were observed for mean, peak, and Δ heart rate (all p≤0.026), representing significant reductions exclusively in HEAT. These findings demonstrate that short-term, circuit-based HA elicits favorable multisystem adaptations relative to thermoneutral training, highlighting its potential as an efficient strategy for time-restricted populations.