A Novel Classification Method for Adaptation Responsiveness to a 5-day Heat Acclimation Protocol

Purpose: To present a novel way of classifying individuals based on their adaptive response to a 5-day heat acclimation (HA) protocol. Methods: Physically active individuals (n=42) first completed the Israeli Defense Force heat tolerance test (HTT1; 5.0 km·h -1 , 2% grade, 120min, 40°C, 40% relative humidity) and then a modified heat tolerance test (HTT2; 65% vVO 2 max, 2% grade treadmill run, 40°C, 40% relative humidity, until rectal temperature reached 39.5°C) before (pre-HA) and after (post-HA) a 5-day HA protocol. Absolute changes (Δ) for twenty-five variables across both HTTs were calculated between post-HA and pre-HA. HTT1 variables include pre-exercise (T0) heart rate (HR), peak HR, average HR, baseline rectal temperature (T rec ), T0 T rec , peak T rec , average T rec , T rec rate of rise (RoR), peak mean skin temperature, whole body sweat loss (WBSL), post-exercise rating of perceived exertion (RPE), post-exercise thermal sensation (TS), and post-exercise overall fatigue (FAT). HTT2 variables include total exercise time, T0 HR, peak HR, average HR, baseline T rec , T0 T rec , T rec RoR, peak mean skin temperature, WBSL, RPE, TS, and FAT. SPSS visual binning was utilized to categorize each variable from both HTTs into four quartiles with quartile 1 being the lowest response and quartile 4 being the highest response. Next, quartile category numbers for each variable were summed for the total score, and then relativized by dividing the summed score by the highest possible score for all participants. Lastly, visual binning was used to classify the relativized score for each participant into four quartiles: Low (L), Low-Mid (LM), High-Mid (HM), and High (H) Responders. Comparisons of participant characteristics and Δ of the variables were conducted using one-way ANOVAs and post hoc Tukey HSD (data are mean±SD; significance p≤0.05). Results: H was significantly younger than HM (H: 21±2 years, HM: 25±5 years, p<0.05), and H had a significantly lower VO 2 max v. L (H: 43.9±3.56 mL·kg -1 ·min -1 , L: 51.2±6.8 mL·kg -1 ·min -1 , p<0.05). H had significant reductions in Δbaseline T rec , ΔT0 T rec , Δpeak T rec and Δaverage T rec (all v. L and LM, p<0.05) during HTT1. HTT2 variables were significantly reduced in H for Δbaseline T rec , ΔT0 T rec , ΔT0 HR (all v. L, p<0.05), Δpeak HR (v. LM, p<0.05), while H had greater Δtotal exercise time (v. L, LM & HM, p<0.05) and ΔWBSL (v. LM & HM, p<0.05). Conclusion: The intrinsic variable of aerobic fitness (i.e., VO 2 max) reflects a component of the acute exposome that includes how the heat exposures affect a person given their state. Our method of classification provides a novel, wholistic overview by analytically acknowledging the complex and diverse nature of the heat stress exposome and its corresponding response. Funding: DoD BA200299 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.