Abstract:
Introduction/Purpose: This study investigated whether the progressive rise in heart rate (HR) and fall in stroke volume (SV) during prolonged, constant-rate, moderate-intensity exercise (cardiovascular drift, CVdrift) in a hot environment is associated with a reduction in VO2max. Methods: CVdrift was measured in nine male cyclists between 15 and 45 min of cycling at 60% VO2max in 35degreesC that was immediately followed by measurement of VO2max. VO2max also was measured after 15 min of cycling on a separate day, so that any change in VO2max between 15 and 45 min could be associated with the CVdrift that occurred during that time interval. This protocol was performed under one condition in which fluid was ingested and there was no significant body weight change (0.3 +/- 0.4%), and under another in which no fluid was ingested and dehydration occurred (2.5 +/- 1%, P < 0.05). Results: Fluid ingestion did not affect CVdrift or change in VO2max. A 12% increase in HR (151 +/- 9 vs 169 +/- 10 bpm, P < 0.05) and 16% decrease in SV (120 +/- 12 vs 101 +/- 10 mL(.)beat(-1), P < 0.05) between 15 and 45 min was accompanied by a 19% decrease in VO2max (4.4 +/- 0.6 vs 3.6 +/- 0.4 L(.)min(-1), P < 0.05) despite attainment of a higher maximal HR (P < 0.05) at 45 min (194 +/- 5 bpm) vs 15 min (191 +/- 5 bpm). Submaximal VO2 increased only slightly over time, but %VO2max increased from 63 +/- 5% at 15 min to 78 +/- 8% at 45 min (P < 0.05). Conclusion: We conclude CVdrift during 45 min of exercise in the heat is associated with decreased VO2max and increased relative metabolic intensity. The results support the validity of using changes in HR to reflect changes in relative metabolic intensity during prolonged exercise in a hot environment in which CVdrift occurs.
