Similar Physiological Adaptations to Heat Acclimation Observed between a Three-Day, Two-Session a Day and Five-Day, One-Session a Day Protocols

Objective: To determine if a 5-day, 1-session a day heat acclimation (HA) protocol (TRAD; n=40) and a 3-day, 2-session a day protocol (2D; n=10) were effective in eliciting HA using physiological variables of pre- and post-heat tolerance tests (HTTs; 40°C, 40% relative humidity). Hypothesis: Both protocols will be successful in eliciting HA, indicated by improvements in rectal temperature (Trec) and heart rate (HR). Methods: 50 healthy adults (54% females; age 23±3y; VO 2 max 47.8±6.mL·kg -1 ·min -1 ; % body fat 21.6±7.1%) participated in one of two HA protocols (TRAD or 2D). Participants completed the Israeli Defense Force heat tolerance test (HTT1; 5.0km·h -1 , 2% grade, 120min) and the next day completed a modified heat tolerance test (HTT2; 65% vVO2max, 2% grade, until Trec ≥39.5°C) before (pre-HA) and after (post-HA) HA. Peak Trec, average Trec, change in Trec ( Δ; ending Trec – starting Trec), Trec rate of rise (RoR), peak HR, average HR, and time to temperature (T2T; for HTT2 only) were analyzed using a 2-way ANOVA with Bonferroni corrections used for post hoc pairwise comparisons (data are mean ±SE of the difference; significance p≤0.05). Results: All participants, independent of group, acclimated based on significant differences in all metrics (all p<0.007). HTT1 pre-HA peak Trec (-0.42±0.17°C; p=0.016) and average Trec (-0.36±0.14°C; p=0.017) were lower in TRAD vs. 2D. No other sig (p≤0.05) differences were found between groups for other HTT1 and HTT2 variables pre-HA and post-HA. The TRAD group had reductions in peak Trec (-0.32±0.06°C; p<0.001), average Trec (-0.30±0.05°C; p<0.001), Δ Trec (0.20±0.07°C; p=0.004), RoR (-0.002±0.001°C·min -1 ; p=0.002), peak HR (-18±2 BPM; p<0.001), and average HR (-15±2 BPM; p<0.001) from pre-HA to post-HA during HTT1, as well as reductions in RoR (-0.006±0.002°C·min -1 ; p=0.001), peak HR (-6±2BPM; p=0.001), average HR (-4±2BPM; p=0.012), and increased T2T (5.241± 1.463min; p<0.001 from pre-HA to post-HA during HTT2. During HTT1, the 2D group had reductions in peak Trec (-0.52±0.13°C; p<0.001), average Trec (-0.41±0.11°C; p<0.001), peak HR (-20±4BPM; p<0.001), and average HR (-15±2BPM; p<0.001) from pre-HA to post-HA in addition to reduced RoR (-0.007±0.003°C·min -1 ; p=0.040) and peak HR (-10±4BPM; p=0.010) from pre-HA to post-HA during HTT2. Conclusion: Both TRAD and 2D protocols demonstrated heat acclimation through improvements in Trec and HR. Thus, factors such as time allocation, personnel availability, and resource management should be considered for implementation of a HA protocol. DoD BA200299 and DoD BA220010 This abstract was presented at the American Physiology Summit 2025 and is only available in HTML format. There is no downloadable file or PDF version. The Physiology editorial board was not involved in the peer review process.