Intensity-dependent thermoregulatory responses at the onset of dynamic exercise in mildly heated humans

To investigate quantitatively how sweating and cutaneous blood flow responses at the onset of dynamic exercise are affected by increasing exercise intensity in mildly heated humans, 18 healthy male subjects performed cycle exercise at 30, 50, and 70% of maximal O2 uptake (VO2 max) for 60 s in a warm environment. The study was conducted in a climatic chamber with a regulated ambient temperature of 35 degrees C and relative humidity of 50%. The subjects rested in the semisupine position in the chamber for 60 min, and then sweating rate (SR) and skin blood flow were measured during cycle exercise at three different intensities. Changes in the heart rate, rating of perceived exertion, and mean arterial blood pressure were proportional to increasing exercise intensity, whereas esophageal and mean skin temperatures were essentially constant throughout the experiment. The SR on the chest, forearm, and thigh, but not on the palm, increased significantly with increasing exercise intensity (P < 0.05). The mean SR of the chest, forearm, and thigh increased 0.05 mg.cm-2.min-1 with an increase in exercise intensity equivalent to 10% VO2 max. On the other hand, the cutaneous vascular conductance (CVC) on the chest, forearm, and palm decreased significantly with increasing exercise intensity (P < 0.05). The mean CVC of the chest and forearm decreased 5.5% and the CVC on the palm decreased 8.0% with an increase in exercise intensity equivalent to 10% VO2 max. In addition, the reduction in CVC was greater on the palm than on the chest and forearm at all exercise intensities (P < 0.01). We conclude that nonthermal sweating and cutaneous blood flow responses are exercise intensity dependent but directionally opposite at the onset of dynamic exercise in mildly heated humans. Furthermore, cutaneous blood flow responses to increased exercise intensity are greater in glabrous (palm) than in nonglabrous (chest and forearm) skin.