The effect of diurnal variation on the regional differences in sweating and skin blood flow during exercise

Abstract

The aim of the present study was to examine changes in the control of heat-dissipation responses to exercise associated with the diurnal variation in core temperature from the viewpoint of the regional response patterns. We studied seven men during exercise on a cycle ergometer at 100 W for 40 min at 25°C at 0630 (morning) 1630 (evening) hours on 2 separate days. Oesophageal temperature (T _oes), local skin temperature, local sweating rate ($\dot m_{sw}$) on the forehead, back, forearm and thigh, and skin blood flow by laser Doppler flowmeter (LDF) on the back and forearm were measured continuously. TheT _oes at rest was significantly higher in the evening than in the morning, the difference averaging approximately 0.4°C (P < 0.05). TheT _oes thresholds for each site in$\dot m_{sw}$ and that for back in LDF were significantly different between the two times of day (P < 0.05). The change inT _oes thresholds for sweating and vasodilatation for morning and evening were similar toT _oes at rest. Although$\dot m_{sw}$ on the forehead was significantly higher in the morning than in the evening,$\dot m_{sw}$ on the back was significantly higher in the evening than in the morning (P < 0.05). Total local sweating rate ($\dot m_{sw} ,_{tot}$) for each site during exercise was significantly higher on the forehead than on the forearm in the morning, and on the back than on the forearm in the evening, respectively (P < 0.05). The results would suggest that the diurnal variation of heat-dissipation responses to exercise is influenced not only by a central controlling mechanism but also by changes in the regional differences.