LAUSR

PURPOSE To determine the vastus lateralis muscle temperature kinetics during and after passive heating, to exam the effect of sex on thermoregulatory responses, and the thermal safety and tolerance of the 42 °C hot-water immersion protocol. METHODS Thirty participants (15 males, 15 females) underwent a 2 h 42 ºC hot-water immersion to the waist level. Vastus lateralis, rectal and skin temperature, thermal sensation, heart rate and blood pressure (BP) were measured during the passive heating and recovery period. Participant recovery was monitored until muscle temperature returned to baseline. RESULTS Vastus lateralis temperature increased to a maximal value of 39.0 ± 0.11 °C (P < 0.001), reaching a plateau after ~ 83.5 min of hot-water immersion and returning to baseline after ~ 115.8 min of recovery. Despite the anthropometric differences between males and females (e.g., height, body mass, body fat %, and fat thickness; P < 0.05), thermoregulatory responses showed no differences between sexes (P > 0.05). No change was found in systolic BP (~ 117 mmHg; P = 0.061). Peak rectal temperature (38.8 ± 0.14 °C; P < 0.001), heart rate (~ 100 bpm; P < 0.001), and diastolic BP (↓ ~ 13 mmHg; P < 0.001) during the hot-water immersion indicated the safety of the protocol. While skin temperature (~ 35.4 °C; P < 0.001) and thermal sensation (~ 5.95 AU; P < 0.001) confirmed protocol tolerance. CONCLUSION These data demonstrate lower-body 42 °C hot-water immersion to increase vastus lateralis temperature and plateau ~ 2.8 °C above baseline. This amplitude of muscle temperature change aligns with reported cellular adaptation and muscle growth. Thermal strain incurred from this protocol appears safe and tolerable, positioning it well for health-related prescription.