Objectives: To compare heat acclimation adaptations after three and six days of either post-exercise hot water immersion (HWI) or exercise-heat-acclimation (EHA) in recreationally active individuals.

Design: Randomised, mixed model, repeated measures.

Methods: Post-exercise HWI involved a daily 40-min treadmill-run at 65% V̇O_2peak in temperate conditions (19 °C, 45% RH) followed by HWI (≤40 min, 40 °C water; n = 9). Daily EHA involved a ≤60-min treadmill-run in the heat (65% V̇O_2peak; 33 °C, 40% RH; n = 9), chosen to elicit a similar endogenous thermal stimulus to HWI. A thermoneutral exercise intervention (TNE, 19 °C, 45% RH; n = 9), work-matched to EHA, was also included to determine thermoregulatory adaptations to daily exercise in temperate conditions. An exercise-heat-stress-test was performed before and after three and six intervention days and involved a 40-min treadmill-run and time-to-exhaustion (TTE) at 65% V̇O_2peak in the heat (33 °C, 40% RH).

Results: ANCOVA, using baseline values as the covariate, revealed no interaction effects but significant group effects demonstrated that compared to EHA, HWI elicited larger reductions in resting rectal temperature (T_re; p = 0.021), T_re at sweating onset (p = 0.011), and end-exercise T_re during exercise-heat-stress (-0.47 °C; p = 0.042). Despite a similar endogenous thermal stimulus to HWI, EHA elicited a modest reduction in end-exercise T_re (-0.26 °C), which was not different from TNE (-0.25 °C, p = 1.000). There were no main effects or interaction effects for end-exercise T_sk, heart rate, physiological strain index, RPE, thermal sensation, plasma volume, or TTE (all p ≥ 0.154).

Conclusions: Compared with conventional short-term exercise heat acclimation, short-term post-exercise hot water immersion elicited larger thermal adaptations.