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Exercise Metabolism Unit, School of Life Sciences and Technology, Victoria University of Technology, Footscray 3001; and Exercise Physiology and Metabolism Laboratory, Department of Physiology, University of Melbourne, Parkville 3052, Australia
To examine
the effect of ambient temperature on metabolism during fatiguing
submaximal exercise, eight men cycled to exhaustion at a workload
requiring 70% peak pulmonary oxygen uptake on three separate
occasions, at least 1 wk apart. These trials were conducted in ambient
temperatures of 3°C (CT), 20°C (NT), and 40°C
(HT). Although no differences in muscle or rectal
temperature were observed before exercise, both muscle and rectal
temperature were higher (P < 0.05)
at fatigue in HT compared with CT and NT. Exercise time
was longer in CT compared with NT, which, in turn, was longer compared
with HT (85 ± 8 vs. 60 ± 11 vs. 30 ± 3 min, respectively; P < 0.05). Plasma epinephrine
concentration was not different at rest or at the point of fatigue when
the three trials were compared, but concentrations of this hormone were
higher (P < 0.05) when HT
was compared with NT, which in turn was higher
(P < 0.05) compared with CT after 20 min of exercise. Muscle glycogen concentration was not different at
rest when the three trials were compared but was higher at fatigue in
HT compared with NT and CT, which were not different (299 ± 33 vs.
153 ± 27 and 116 ± 28 mmol/kg dry wt, respectively;
P < 0.01). Intramuscular lactate concentration was not different at rest when the three trials were
compared but was higher (P < 0.05)
at fatigue in HT compared with CT. No differences in the concentration
of the total intramuscular adenine nucleotide pool (ATP + ADP + AMP),
phosphocreatine, or creatine were observed before or after exercise
when the trials were compared. Although intramuscular IMP
concentrations were not statistically different before or after
exercise when the three trials were compared, there was an
exercise-induced increase (P < 0.01)
in IMP. These results demonstrate that fatigue during prolonged
exercise in hot conditions is not related to carbohydrate availability.
Furthermore, the increased endurance in CT compared with NT is probably
due to a reduced glycogenolytic rate.
glycogen; inosine 5'-monophosphate; heat stress; total adenine nucleotides
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