Major sports championships are often scheduled in warm/dry and warm/humid environments, so it is important to be able to quantify the influence of these environmental extremes on work capacity. The aim of the present study was to examine the influence of relative humidity on endurance exercise performance in a warm environment. Eight male volunteers (mean ± SD age 26 ± 4 years; height 1.80 ± 0.03 m; body mass 72.0 ± 7.0 kg; VO2max 4.38 ± 0.65 l/min) performed four cycle exercise trials at 70% maximum oxygen uptake until volitional exhaustion in an environmental chamber maintained at 30.2 ± 0.2°C. Volunteers were tested under four relative humidity (rh) conditions: 24%, 40%, 60% and 80%. Core and weighted mean skin temperature, heart rate, skin blood flow, and cutaneous vascular conductance (CVC) were recorded at rest and at regular intervals during exercise. Mean (SD) time to exhaustion was 68 ± 19, 60 ± 17, 54 ± 17, and 46 ± 14 min at 24, 40, 60, and 80% rh, respectively (P < 0.001); exercise time was significantly less at 60 (P = 0.013) and 80% (P = 0.005) rh than recorded at 24% rh. There were no differences in core temperature (P = 0.480) and heart rate (P = 0.097) between trials. Core temperature at exhaustion was 39.0 ± 0.3°C at 24, 40, and 60% rh and 39.1 ± 0.3°C at 80% rh (P = 0.159). Mean skin temperature at the point of exhaustion was higher at 80% rh than at 24% rh (P < 0.001). Total sweat loss was similar between trials (P = 0.345), but sweating rate was higher at 60 and 80% rh than at 24% rh (P < 0.001). The results suggest that exercise capacity at moderate intensity in a warm environment is progressively impaired as the relative humidity increases. Early fatigue in the higher humidity trials was accompanied by a faster rate of rise in core temperature and a greater weighted mean skin temperature, with no differences in heart rate, skin blood flow or the metabolic response to exercise.
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