Abstract
Adaptations from heat acclimation (HA) have been suggested to alter physiological responses during a maximal exercise test in trained individuals. These responses include a decreased oxygen cost for a given workload (improved exercise economy), or an increase in the power output at which the ventilatory threshold (VT) occurs; both factors that predict endurance performance. However, these findings are inconsistent in the literature, particularly in untrained individuals, therefore the effects of HA on maximal exercise responses remain unclear. PURPOSE: To examine any differences in VO2peak, peak power output (PPO), and VT pre- and post-HA in untrained individuals. METHODS: Thirteen healthy men (mean ± SD; age: 21 ± 3 yr; ht: 172 ± 8 cm; wt: 76 ± 13 kg) participated in the study. Subjects completed two peak oxygen consumption tests (VO2peak; on a cycle ergometer) that were separated by an 8-day exercise-HA protocol (treadmill walking: 5 km·hr-1, 2% grade; 40°C, and 40% RH). The VO2peak test workload started at 50-75 watts (W), and increased by 25 W every min until volitional exhaustion. Metabolic data were collected continuously over the course of the test. VT was determined using the ventilatory equivalence method (an increase in VE/VO2 with no change in VE/VCO2). The percent change in plasma volume (%ΔPV) was calculated from blood draws pre- and post-HA. A paired t-test was used to assess differences in physiological responses during the VO2peak test and from the HA protocol. Significance was set at an alpha level of P < 0.05. RESULTS: HA was achieved as indicated by a lower core temperature (Day 1: 38.1 ± 0.3 vs Day 8: 37.8 ± 0.3 °C; P < 0.01) and HR (Day 1: 134 ± 17 vs Day 8: 121 ± 13 bpm; P < 0.01) on Day 8 compared to Day 1. The %ΔPV from pre-to post HA was 22.8 ± 7.6 % over the course of the HA period. VO2peak (3.2 ± 0.4 vs 3.1 ± 0.5 L/min) and PPO (253 ± 37 vs 257 ± 38 W, P > 0.05 for both) were not different after 8 days of HA. There were no differences pre- to post-HA in VO2 (2.2 ± 0.4 vs 2.1 ± 0.4 L/min), power output (169 ± 27 vs 167 ± 31 W), VE (53.4 ± 9.6 vs 53.6 ± 10.8 L/min), or the RER (0.94 ± 0.05 vs 0.95 ± 0.06, P > 0.05 for all) at which VT occurred. CONCLUSION: An 8-day HA protocol did not lead to changes in VO2peak, PPO, or in the VO2 and PO at which the VT occurred during a peak oxygen consumption test. Supported by USARMDC; authors views not official US Army or DoD policy.
Published Version
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