Influence of relative humidity on prolonged exercise capacity in a warm environment

This study examined the influence of relative humidity on endurance exercise performance in a warm environment. Eight male volunteers performed four cycle exercise trials at 70% maximum oxygen uptake until volitional exhaustion in an environmental chamber maintained at 30.2 +/- A 0.2A degrees 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 were recorded at rest and at regular intervals during exercise. Mean +/- A SD time to exhaustion was 68 +/- A 19, 60 +/- A 17, 54 +/- A 17, and 46 +/- A 14 min at 24, 40, 60, and 80% rh, respectively ( < 0.001); exercise time was significantly less at 60% ( = 0.013) and 80% ( = 0.005) rh than recorded at 24% rh. There were no differences in core temperature ( = 0.480) and heart rate ( = 0.097) between trials. Core temperature at exhaustion was 39.0 +/- A 0.3A degrees C at 24, 40, and 60% rh and 39.1 +/- A 0.3A degrees C at 80% rh ( = 0.159). Mean skin temperature at the point of exhaustion was higher at 80% rh than at 24% rh ( < 0.001). Total sweat loss was similar between trials ( = 0.345), but sweating rate was higher at 60 and 80% rh than at 24% rh ( < 0.001). The results suggest that exercise capacity at moderate intensity in a warm environment is progressively impaired as the relative humidity increases.