Cardiorespiratory Fitness Level May Affect Adaptations In Response To Heat Acclimation

BACKGROUND: A higher fitness level often comes with adaptations that may improve the response to heat stress relative to untrained individuals. Purpose: To evaluate the influence of fitness level on heat acclimation (HA) among American College of Sports Medicine (ACSM) defined fitness categories. METHODS: Twenty-seven volunteers (Mean + SD; Age 23 + 5 yrs) completed a treadmill VO2max test. Volunteers were categorized into 3 groups based on ACSM guidelines: GOOD: (n = 8, 4 F; VO2max: 46.9 + 7.0 mL/kg/min), FAIR: (n = 13, 8 F; VO2max: 40.8 + 6.3 mL/kg/min), and POOR (n = 6, 4 F; VO2max: 34.9 + 4.7 mL/kg/min). Volunteers completed a 10-day walking HA protocol (120-min, 3.1 mph: 2% grade) in the heat (40 °C, 40% RH). Core temperature (Tc), heart rate (HR), and whole body sweat rate (WBSR) were collected on all days. Paired t-tests were used to compare peak Tc, peak HR, and WBSL between days 1 and 10 (D1, D10) for all volunteers when grouped. Two-way RM ANOVA was used to compare days D1 and D10 of HA across fitness categories for all variables. RESULTS: After HA, peak Tc (D1: 38.8 + 0.5 vs D10: 38.4 + 0.4 °C; p < 0.0001) and peak HR (D1: 57 + 20 vs D10: 43 + 21 bpm; p < 0.001) decreased in all volunteers, while WBSR increased (D1: 0.68 + 0.2 vs D10: 0.80 + 0.2 L/hr; p < 0.001). Peak Tc decreased on D10 compared to D1 in the GOOD (D1: 38.7 + 0.6 vs D10: 38.0 + 0.2 °C; p < 0.05) and FAIR groups (D1: 39.0 + 0.5 vs D10: 38.5 + 0.4 °C; p < 0.01). Peak HR was lower on D10 compared to D1 in both the GOOD (D1: 147 + 22 vs D10: 125 + 12 bpm; p < 0.01) and FAIR groups (D1: 163 + 19 vs D10: 144 + 16 bpm; p < 0.01). WBSR increased on D10 compared to D1 for the GOOD (D1: 0.65 + 0.24 vs D10: 0.82 + 0.26 L/hr; p < 0.05) and FAIR groups (D1: 0.65 + 0.12 vs D10: 0.77 + 0.12 L/hr; p < 0.05). Mean differences in the POOR fitness group were smaller and did not reach statistical significance due to small sample size. CONCLUSION: HA lowered peak Tc and HR, and increased WBSR in all volunteers regardless of fitness category using a controlled work rate protocol. Subgroup analysis suggested possible impact of fitness, with a higher fitness level potentially augmenting thermoregulatory and cardiovascular adaptations. However, more research is needed that is adequately powered for comparison among fitness levels. Funded by USAMRDC; Author views not representative of the Army/DOD. Supported by: Funded by USAMRDC (MO220032)