Rate of heat storage does not influence exercise intensity at a fixed rating of perceived exertion

Abstract

This study investigated whether different rates of decline in self‐selected work rate (W) were mediated by early differences in heat storage. Five males cycled at a W to maintain a perceived exertion of 16 (Borg scale) at 35°C and 25°C. Heat production (Hprod) was estimated via indirect calorimetry. Esophageal (Tes) and skin (Tsk) temperatures, as well as local sweat rates on the lower back (LSRback) and thigh (LSRthigh), were measured throughout. The level of muscle activation in vastus lateralis (VL) was assessed via integrated surface electromyography (iEMG). While dry heat loss was lower at 35°C (P<0.01), sweating responses were elevated, indicated by greater LSRthigh at 2 min (P<0.05), a trend toward greater LSRback (P=0.07), and earlier onsets at both sites (P=0.02). Similar Hprod and rates of change in Tes were found in both conditions (P>0.05), yet at 35°C, exercise time was shorter (35°C: 22.6±4.9 min; 25°C: 32.1±10.4 min; P=0.03), and W (% of initial W) declined more rapidly by 15 min (35°C: 89.2±8.2%; 25°C: 82.2±8.0%; P<0.01). Mean Tsk was lower throughout exercise at 25°C (P<0.01). Similar changes in VL iEMG activity were found at 25°C and 35°C (P=0.28). In conclusion, reductions in W occur without early differences in heat storage between air temperatures, implicating factors other than the rate of heat storage, such as Tsk, in the regulation of exercise intensity.Supported by a NSERC Discovery Grant (O. Jay)