The effect of passive heating and face cooling on perceived exertion during exercise in the heat

Increased body temperature is thought to be an important component of the higher perception of exertion that is a feature of fatigue during exercise in the heat but a causal relationship has yet to be demonstrated. We have investigated the effect of passive heating on the perception of exertion during a standard bout of exercise and also assessed the effect of cooling the head on compensating for the increased body temperature on the feelings of exertion. Ten male subjects performed a 14-min cycling exercise [average power ~63% of maximum power output (Ẇ _max)] at an ambient temperature of 35°C at resting rectal temperature [mean (SD): 37.49 (0.27)°C; control (CON) trial] on one occasion, and after sitting in a sauna to raise rectal temperature [mean (SD): 38.95(0.13)°C; sauna (SAU) trial]. During the exercise, subjects reported their ratings of overall perceived exertion (RPE), perceived exertion of the legs (RPE_legs) and thermal comfort (TC). A blood sample was collected by the end of the exercise for determination of plasma glucose, lactate and prolactin and haematocrit. RPE values were significantly elevated after passive heating [mean (SE): 14.5 (0.7) units in CON and 17.2 (0.5) units in SAU, at the end of exercise; P<0.001] as were the RPE_legs (P<0.01), while ratings of TC were similar in CON and SAU trials. Passive heating increased blood glucose (P<0.05) but had no effect on lactate at the end of the exercise. Plasma prolactin was markedly elevated as a result of the sauna exposure [mean (SE): 1598 (152) versus 225 (31) mU l⁻¹ in SAU and CON trials, respectively; P<0.001]. Six of the subjects repeated the two trials but with the face cooled during exercise (trials CON_FAN and SAU_FAN) that was achieved by combining face fanning and spraying the face with a mist of cooled water. Face cooling decreased RPE values after sauna to a point that no differences between the two conditions existed. RPE_legs scores and heart rate, however, remained higher in SAU_FAN compared with CON_FAN (P<0.05). We conclude that hyperthermia is a causative element of the increased perception of exertion during submaximal exercise in the heat and that the effect of increased core temperature on the feelings of exertion is modulated by face cooling.