Effects of Solar Radiation Exposure on Self-regulated Exercise Intensity and Thermoregulation in the Heat Outdoors

Abstract

High radiant heat load reduces endurance exercise performance in the heat indoors, but this remains unconfirmed in outdoor exercise. PURPOSE: The present study examined the effects of variations in solar radiation exposure on self-regulated exercise intensity and thermoregulatory responses in the heat outdoors at a fixed rating of perceived exertion (RPE). METHODS: Ten male participants completed 45-min cycling exercise in hot outdoor environments (about 31°C) at a freely chosen resistance and pedal cadence at an RPE of 13 (somewhat hard). Participants were blinded to resistance, pedal cadence, distance and elapsed time and exercised at three sunlight exposure conditions: clear sky (mean±SD: 1072±91 W·m−2; HIGH); thin cloud (592±32 W·m−2; MID); and thick cloud (306±52 W·m−2; LOW). Rectal and skin (chest, upper arm, thigh and calf) temperatures, heart rate, skin blood flow and blood pressure were recorded at rest and during exercise. RESULTS: Power output (HIGH 96±22 W; MID 103±20 W; LOW 108±20 W) and resistance (HIGH 1.3±0.3 kp; MID 1.4±0.2 kp; LOW 1.5±0.3 kp) were lower in HIGH than MID and LOW (Power output p<0.001; Resistance p<0.01). Pedal cadence was lower as solar radiation increases (HIGH 64.5±5.1 rpm; MID 65.9±4.2 rpm; LOW 67.7±3.9 rpm) and was different between all trials (all p<0.001). The core-to-skin temperature gradient was narrower, body heat gain from the sun (SHG) was greater and thermal sensation was higher with increasing solar radiation and all variables were different between all trials (all p<0.01). Mean skin temperature was higher in HIGH than MID and LOW (p<0.01), but rectal temperature was similar between trials (p=0.485). CONCLUSIONS: We conclude that self-regulated exercise intensity in the heat outdoors at a fixed RPE of somewhat hard is reduced with increasing solar radiation because of greater thermoregulatory strain, perceived thermal stress and SHG. Moreover, solar radiation below about 600 W·m−2 might be of benefit to maintain endurance physical performance in the heat.