Aerobic and anaerobic exercise tolerance in the heat : effect of acclimatization
The effect of heat acclimatization on aerobic exercise tolerance in the heat, and on subsequent anaerobic exercise performance was investigated. Ten male subjects were acclimatized by exercising 90 minutes per day for eight days in the heat (39.7 C db, 31.0% RH) on a bicycle ergometer at a workload corresponding to 54% of V02 max. Prior to, and following heat acclimatization, the subjects performed six hours of intermittent, submaximal (50% V02 max) exercise in the heat (HET). A 45 sec maximal bicycle ride was performed before (sprint 1) and after (sprint 2) each HET. Mean ( ± SE) resting plasma volume was higher (+9.2 + 1.7%) in acclimatized subjects (ACC) compared with unacclimatized (UN) subjects (P<0.001). Acclimatization resulted in reductions in the mean exercise heart rate (160 + 3 vs. 144 ± 3, P<0.001) and mean rectal temperature (38.13 + 0.08 vs 37.91 + 0.05, P<0.01). Muscle glycogen utilization during the HET was greater in the UN trial (57.4 + 5.1 mmol/kg) than the ACC trial (28.6 + 6.4 mmol/kg, P<0.05). No differences were noted between the UN and ACC trials with respect to blood glucose, lactate (LA), or respiratory exchange ratio during the HET. Total work output during sprint 2 was reduced compared with sprint 1 in the UN trial (24.01 ± 0.80 vs. 21.56 + 1.18 kJ, P<0.05). Post exercise muscle pH was significantly (P<0.05) higher following sprint 2 (6.86) than sprint 1 (6.67) in the UN trial. Associated with this reduction in anaerobic exercise performance, an attenuated accumulation of LA and fall in pH was noted in the blood. No differences between sprints 1 and 2 were observed for the ACC trial in total work, musce pH, or blood LA. As a result of sprint exercise in both UN and ACC trials, a decrease in muscle potassium (K+) was noted (P<0.05), which was accompanied by a 0.8 to 2.2 mmol/l increase in antecubital venous blood K+. Following sprint exercise, serum K+ was below pre sprint values for up to 15 minutes, suggesting a rapid and accelerated uptake of K+.These data indicate that eight days of heat acclimatization produced a shift in fuel selection during submaximal exercise in the heat through an enhanced delivery of blood borne substrate. The observed muscle glycogen sparing during submaximal exercise may be associated with the enhanced ability to maintain high levels of anaerobic energy release and performance of highly intense exercise following prolonged exertion in the heat.