Exercise heat acclimation and post-exercise hot water immersion improve resting and exercise responses to heat stress in the elderly

Objectives: To investigate the efficacy of heat acclimation (HA) in the young (Y _EX) and elderly (E _EX) following exercise-HA, and the elderly utilising post-exercise hot water immersion HA (E _HWI). Design: Cross-sectional study. Method: Twenty-six participants (Y _EX: n = 11 aged 22 ± 2 years, E _EX: n = 8 aged 68 ± 3 years, E _HWI: n = 7 aged 73 ± 3 years) completed two pre-/post-tests, separated by five intervention days. Y _EX and E _EX exercised in hot conditions to raise rectal temperature (T _rec) ≥38.5 °C within 60 min, with this increase maintained for a further 60 min. E _HWI completed 30 min of cycling in temperate conditions, then 30 min of HWI (40 °C), followed by 30 min seated blanket wrap. Pre- and post-testing comprised 30 min rest, followed by 30 min of cycling exercise (3.5 W·kg ⁻¹ Ḣ _prod), and a six-minute walk test (6MWT), all in 35 °C, 50% RH. Results: The HA protocols did not elicit different mean heart rate (HR), T _rec, and duration T _rec ≥ 38.5 °C (p > 0.05) between Y _EX, E _EX, and E _HWI groups. Resting T _rec, peak skin temperature, systolic and mean arterial pressure, perceived exertion and thermal sensation decreased, and 6MWT distance increased pre- to post-HA (p < 0.05), with no difference between groups. Y _EX also demonstrated a reduction in resting HR (p < 0.05). No change was observed in peak T _rec or HR, vascular conductance, sweat rate, or thermal comfort in any group (p > 0.05). Conclusions: Irrespective of age or intervention, HA induced thermoregulatory, perceptual and exercise performance improvements. Both exercise-HA (E _EX), and post-exercise HWI (E _HWI) are considered viable interventions to prepare the elderly for heat stress.