Thermoregulation during prolonged actual and laboratory-simulated bicycling

Thermoregulatory and cardiorespiratory responses to bicycling 55 km (mean speed 9.7 m·s⁻¹) outdoors (15‡ C DB) were compared to equivalent cycle ergometry (90 min at 65% \(\dot V_{O_{2\max } } \) ) in the laboratory (20–23‡ C DB, 50% RH) in 7 trained cyclists. Outdoor environmental conditions were simulated with fans and lamps, and were contrasted with standard no-wind, no-sun laboratory conditions. Sweating rate was similar during outdoor and laboratory simulated outdoor cycling (0.90 and 0.87 to 0.94 l·h⁻¹ respectively). During outdoor bicycling, mean heart rate (161 bt·min⁻¹) was 7–13% higher (p<.05) than under laboratory conditions, suggesting a greater strain for a similar external workrate. The increase in rectal temperature (0.8‡ C) was 33–50% less (p<0.05) at the cooler outdoor ambient temperature than in the laboratory. Thermoregulatory stress was greater under the no-fan, no-lamp laboratory condition than during simulated outdoor conditions (36–38% greater (p<0.05) sweating rate, 15–18% greater (p<0.01) mean skin temperature, 6.4 to 7.8 fold greater (p<0.01) amount of clothing-retrained sweat). The cooling wind encountered in actual road bicycling apparently reduces thermoregulatory and circulatory demands compared with stationary cycle ergometry indoors. Failure to account for this enhanced cooling may result in overestimation of the physiological stress of actual road cycling. Additionally, core temperature and heart rate rose throughout the period of laboratory cycling at a constant velocity, indicating the need for caution in inferring relative metabolic rate ( \(\dot V_{O_2 } \) / \(\dot V_{O_{2\max } } \) ) from heart rate during prolonged exercise without adequate rehydration.