Time-of-day effect on nonthermal control of sweating response to maintained static exercise in humans

To investigate the influence of nonthermal factors in the time-of-day effect on the sweating response to maintained static exercise, eight healthy male subjects performed handgrip exercise at 20%, 35% and 50% maximal voluntary contraction (MVC) for 60 s at 0600 hours (morning) and at 1800 hours (evening). Oesophageal temperature (Toes) before the experiment showed a diurnal rhythm [mean (SEM)] [36.3 (0.1) (morning) compared to 36.8 (0.1) °C (evening), P<0.01]. Experiments were conducted with subjects in a state of mild hyperthermia during which the mean skin temperature (Tsk) was kept constant at 35.5–36.5 °C using a water-perfused suit to activate sudomotor responses. The Toes and mean Tsk remained stable during the pre-exercise, handgrip exercise and recovery periods. The response in sweating rate (ΔSR) on the chest and forearm to handgrip exercise increased significantly with increasing exercise intensity in both the morning and evening tests (P<0.05). The ΔSR on the palm did not change significantly with increasing exercise intensity in the morning test (P>0.1). During handgrip exercise at 50% MVC only, ΔSR on the chest, forearm and palm in the evening was significantly higher than in the morning (P<0.05). On the other hand, mean arterial blood pressure and the rating of perceived exertion during 50% MVC handgrip exercise were not significantly different between the morning and evening (P>0.1). These results indicate the presence of a time-of-day effect on nonthermal control of the sweating response to isometric handgrip exercise, and that this effect is dependent on exercise intensity.