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F-69 Free Communication/Poster - Thermoregulation/Hyperthermia Friday, May 29, 2020, 1: 30 PM - 4: 00 PM Room: CC-Exhibit Hall

Endurance Exercise Capacity & Heat-loss Responses Are Greater In The Late Evening Than Morning

3503 Board #324 May 29 2:30 PM - 4:00 PM

Otani, Hidenori1; Kaya, Mitsuharu2; Goto, Heita3; Tamaki, Akira2

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Medicine & Science in Sports & Exercise: July 2020 - Volume 52 - Issue 7S - p 972
doi: 10.1249/01.mss.0000686136.48893.8c
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Various major sporting events in the heat of summer start from not only in the morning but also in the late evening. However, to date rationales for differences in endurance exercise capacity and thermoregulatory responses to exercise in the heat between morning and late evening have not been reported.

PURPOSE: The current study investigated the diurnal effects of exercise in the late morning and evening on endurance exercise capacity and thermoregulatory responses during a time-to-exhaustion test in the heat.

METHODS: Ten male participants cycled at 70% peak oxygen uptake until exhaustion in the heat (30°C, 50% relative humidity). Participants commenced exercise in the late morning at 10:00 h (AM) or evening at 21:00 h (PM). Upon cessation of exercise, participants completed a 30 min post-exercise recovery.

RESULTS: Time to exhaustion was 28±13% (mean±SD) longer in PM (49.1±16.3 min) than AM (38.7±14.6 min; P<0.001). Rectal temperature before and during exercise were higher in PM than AM (both P<0.01) in accordance with the diurnal variation of core temperature (Tc). The rates of rise in rectal temperature (AM 0.030±0.012°C/min; PM 0.021±0.008°C/min), mean skin temperature (AM 0.095±0.042°C/min; PM 0.068±0.028°C/min), thermal sensation and rating of perceived exertion during exercise were slower in PM than AM (all P<0.05). Rectal temperature at the point of exhaustion was not different by time-of-day (AM 38.3±0.4°C; PM 38.6±0.5°C). Dry and evaporative heat losses and an increase in skin blood flow during exercise were greater in PM than AM (all P<0.05). During 30-min post-exercise recovery, the rates of fall in rectal temperature (AM 0.013±0.004°C/min; PM 0.019±0.010°C/min) and skin blood flow were faster and thermal sensation was lower in PM than AM (all P<0.05). Heart rate during exercise and recovery were not different between trials.

CONCLUSION: This study indicates that endurance exercise capacity is greater and heat-loss responses to control Tc during and following exercise in the heat are more effective in the late evening than morning. Moreover, perceived fatigue during exercise and thermal perception during and following exercise are lower in the late evening than morning.

Copyright © 2020 by the American College of Sports Medicine