Aerobic Performance is Degraded, Despite Modest Hypothermia, In Hot Environments
ARMY RESEARCH INST OF ENVIRONMENTAL MEDICINE NATICK MA THERMAL AND MOUNTAIN MEDICINE DIVISION
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Environmental heat stress degrades aerobic performance however, little research has focused on performance when the selected task elicits modest elevations in core body temperature 38.5 deg C. Purpose To determine the effect of environmental heat stress, with modest hyperthermia, on aerobic performance and pacing strategies. Methods After a 30-min cycling preload at 50 VO sub 2peak, eight euhydrated men performed a 15-min time trial on a cycle ergometer in temperate TEMP 21 deg C, 50 RH and hot HOT 40 deg C, 25 RH environments. Core and skin temperature Tc and Tsk, respectively and HR were continuously monitored. Performance was assessed by the total work kJ completed in 15 min. Pacing was quantified by comparing the percent difference in actual work performed in each of five 3-min blocks normalized to the mean work performed per 3-min block. Pace over the final 2 min was compared with the average pace from minutes 0 to 13 for end spurt analysis. Results Tc and HR rose continually throughout both time trials. Peak Tc remained modestly elevated in both environments mean range HOT 38.20 deg C 37.97-38.42 deg C TEMP 38.11 deg C 38.07-38.24 deg C, whereas Tsk was higher in HOT 36.19 or - 0.40 deg C vs 31.14 or - 1.14 deg C, and final HR reached 95 of age-predicted maximum in both environments. Total work performed in HOT 147.7 or - 23.9 kJ was 17 less P 0.05 than TEMP 177.0 or - 25.0 kJ. Pace was evenly maintained in TEMP, but in HOT, volunteers were unable to maintain initial pace, slowing progressively over time. A significant end spurt was produced in both environments. Conclusions During a brief aerobic exercise time trial where excessive hyperthermia is avoided, total work is significantly reduced by heat stress because of a gradual slowing of pace over time. These findings demonstrate how aerobic exercise performance degrades in hot environments without marked hyperthermia.
- Stress Physiology