Exercise tolerance and thermoregulatory responses during cycling in boys and men

Purpose: Physiological responses to exercise in the heat differ between prepubertal children and young adults. Whether these maturity-related variations imply lower exercise tolerance, inferior thermoregulation, and greater risk for heat injury in the child is uncertain. This study directly compared thermoregulatory and cardiovascular responses as well as endurance performance between prepubertal boys and adult males during steady-load cycling in moderately hot and cool ambient conditions with moderate humidity. Methods: Eight prepubertal boys (age 11.7 +/- 0.4 yr) and eight adult men (age 31.8 +/- 2.0 yr) performed steady-load cycling to exhaustion at an intensity equivalent to approximately 65% peak VO2 in both hot (similar to 31 degrees C) and cool (similar to 19 degrees C) environments, with fluid intake ad libitum. Results: Exercise duration in the heat was shorter for both groups (hot: men 30.46 +/- 8.84 min, boys 29.30 +/- 6.19 min; cold: men 42.88 +/- 11.79 min, boys 41.38 +/- 6.30 min), with no significant difference between men and boys (P > 0.05). Increases in rectal temperature, heart rate, and cardiac index were similar between groups and conditions. Stroke index, mean arterial pressure, and arterial venous oxygen difference were stable and similar in both conditions, without group differences. No significant dehydration was observed in men or boys. Conclusions: This study failed to reveal differences in exercise tolerance, thermoregulatory adaptation, or cardiovascular response to exercise in the heat between euhydrated prepubertal boys and adult men.