Effects of marked hyperthermia with and without dehydration onV˙o 2 kinetics during intense exercise

Abstract

This study determined whether marked hyperthermia alone or in combination with dehydration reduces the initial rate of rise in O2 consumption (V˙o 2on-kinetics) and the maximal rate of O2 uptake (V˙o 2 max) during intense cycling exercise. Six endurance-trained male cyclists completed four maximal cycle ergometer exercise tests (402 ± 4 W) when euhydrated or dehydrated (4% body wt) with normal (starting esophageal temperature, 37.5 ± 0.2°C; mean skin temperature, ∼31°C) or elevated (+1 and +6°C, respectively) thermal strain. In the euhydrated and normal condition, subjects reachedV˙o 2 max (4.7 ± 0.2 l/min) in 228 ± 34 s, with a mean response time of 42 ± 2 s, and fatigued after 353 ± 39 s. Hyperthermia alone or in combination with dehydration reduced mean response time (17–23%),V˙o 2 max (16%), and performance time (51–53%) (all P < 0.01) but did not alter the absolute response time (i.e., the time to reach 63% response in the control trial, 3.2 ± 0.1 l/min, 42 s). Reduction inV˙o 2 max was accompanied by proportional decline in O2 pulse and significantly elevated maximal heart rate (195 vs. 190 beats/min for hyperthermia vs. normal). Preventing hyperthermia in dehydrated subjects restoredV˙o 2 max and performance time by 65 and 50%, respectively. These results demonstrate that impaired high-intensity exercise performance with marked skin and internal body hyperthermia alone or in combination with dehydration is not associated with a diminished rate of rise in V˙o 2 but decreased V˙o 2 max.