The Relationship of Individual Characteristics and Physiological Responses in Two Acclimation Protocols during a Heat Tolerance Test

Heat acclimation (HA) enhances thermoregulatory efficiency in hot environments, but interindividual variability may influence physiological adaptations in different HA protocols. Purpose: To examine the relationship between individual characteristics and physiological responses between two HA protocols. We hypothesized that individual characteristics and physiological responses would have a statistically significant association (p<0.05) between two different HA protocols. Methods: Physically active males and females completed exercise-heat sessions (40°C,40%rh) through either a traditional HA protocol (N=40; 53% female; VO 2 max=47.8±6.4 ml·kg- 1 ·min -1 ;% body fat [%BF]=21.4±7.2) with 1 session/day for 5 days (TRAD) or Two-A-Day HA protocol (N=10; 60% female; VO 2 max=47.7±5.3 ml·kg- 1 ·min -1 ; %BF=22.6±7.0) with 2 sessions/day for 3 days (TWO). A modified heat tolerance test (HTT; 65% vVO 2 max, 2% grade treadmill run, until rectal temperature reached 39.5°C) was conducted before (preHA) and after (postHA) the respective HA protocol. Pearson’s (r) and Spearman’s (ρ) correlations examined the strength of relationships (<0.2 poor; 0.3-0.5 fair, 0.6-0.7 moderate, >0.8 very strong) between individual characteristics (age, VO 2 max, body surface area [BSA], and %BF) with mean rectal temperature [T REC ], peak heart rate [HR], T REC rate of rise [RoR], absolute change [Δ] T REC , and time to reach 39.5°C both preHA and postHA (p<0.05). Fisher’s Z-Test (Z-score >±1.96, p<0.05) compared correlation coefficients for individual characteristics and physiological responses between TRAD and TWO preHA and postHA. Results: TRAD had negative correlations of VO 2 max and pre and postHA HR (pre: r=-0.362, p=0.022; post: r=-0.411, p=0.008); BSA and postHA mean T REC (r=-0.426, p=0.006); %BF and RoR postHA (r=-0.346, p=0.033); and positive correlations of VO 2 max and preHA ΔT REC (r=0.343, p=0.030); BSA and RoR pre and postHA (pre: r=0.347, p=0.028; post: r=0.455, p=0.003); VO 2 max and RoR postHA (r=0.370, p=0.019).TWO had negative correlations of BSA and HR postHA (r=-0.859, p=0.001). The correlation of BSA and HR postHA in the TRAD (r1=-0.253, n1=40) differed from TWO (r2=-0.859, n2=10), Z=2.50. Conclusion: Between two HA protocols, individual characteristics influenced physiological responses with varying strengths. In TRAD, VO 2 max and BSA had fair negative correlations with HR and post-HA mean T REC , respectively. In TWO, BSA showed a very strong negative correlation with post-HA HR. Our findings highlight the potential relationships of protocol-specific adaptations with body characteristics (e.g. BSA) and fitness (e.g. VO 2 max). Funding Source: 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.