Effects of solar radiation on endurance exercise capacity in a hot environment

The present study investigated the effects of variations in solar radiation on endurance exercise capacity and thermoregulatory responses in a hot environment.

Eight male volunteers performed four cycle exercise trials at 70 % maximum oxygen uptake until exhaustion in an environmental chamber maintained at 30 °C and 50 % relative humidity. Volunteers were tested under four solar radiation conditions: 800, 500, 250 and 0 W/m².

Exercise time to exhaustion was less on the 800 W/m² trial (23 ± 4 min) than on all the other trials (500 W/m² 30 ± 7 min; P < 0.05, 250 W/m² 43 ± 10 min; P < 0.001, 0 W/m² 46 ± 10 min; P < 0.001), and on the 500 W/m² trial than the 250 W/m² (P < 0.05) and 0 W/m² (P < 0.01) trials. There were no differences in core (rectal) temperature, total sweat loss, heart rate, skin blood flow, cutaneous vascular conductance and percentage changes in plasma volume between trials (P > 0.05). Mean skin temperature was higher on the 800 W/m² trial than the 250 and 0 W/m² trials (P < 0.05), and on the 500 W/m² trial than the 0 W/m² trial (P < 0.05). The core-to-skin temperature gradient was narrower on the 800 W/m² trial than the 250 and 0 W/m² trials (P < 0.05).

The present study demonstrates that endurance exercise capacity in a hot environment falls progressively as solar radiation increases.