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Abstract
This investigation determined whether the progressive rise in heart rate (HR) and fall in stroke volume (SV) during prolonged, moderate-intensity exercise (cardiovascular drift, CV drift) in hot and cool environments is associated with a reduction in V.O2max. CV drift was measured in hot (H, 35C) and cool (C, 22C) ambient temperatures in ten male endurance athletes (age = 23 3 yr, V.O2max = 64.7 8.7 ml/kg/min) between 15 and 45 min of cycling at a power output that elicited 59.2 1.9 %V.O2max, followed immediately by measurement of V . O2max. V.O2max in both conditions also was measured on separate days after 15 min at the same power output used in the 45 min trials. All four experimental trials included fluid replacement that prevented any significant change in body weight (0.3 0.4%). CV drift and the associated decrease in V.O2max was greater (p<0.05) in H than C. Between 15 and 45 min of cycling in H, an increase in HR of 16 4 bpm (11%, p<0.05) and decrease in SV of 12 3 ml (11%, p<0.05) was associated with a 15% decrease in V.O2max (4.7 0.6 vs 4.0 0.5 l/min, p<0.05). Between 15 and 45 min of cycling in C, HR increased only slightly (3 2 bpm, 2%, p<0.05), SV decreased only slightly (2 1, 2%, p<0.05) and there was no significant decrease in V.O2max (4.7 0.6 vs 4.5 0.5 l/min, 5%, p>0.05). Nevertheless, there was a significant relationship between ?HR andV. O2max in H (r=0.94, SEE=0.15 l/min, p<0.05) and C (r=0.94, SEE=0.09 l/min, p<0.05). Submaximal V.O2 was unchanged over time (p>0.05), but %V.O2max increased from 57 4% at 15 min to 67 8% at 45 min in H (p<0.05), and from 54 3% at 15 min to 57 3% at 45 min in C (p<0.05). It is concluded that CV drift during prolonged exercise (1) is greater in a hot than in a cool environment, (2) is associated with a decrease in V.O2max in the heat and (3) reflects an increase in relative metabolic intensity.