The Thermoregulatory Response of Pre-. Mid- and Late-Pubertal Boys Exercising in the Heat

Abstract

<p>Th~ thermoregulatory response to exercise in the heat, especia!ly sweating rate, differs between children and adults. This project investigated the effect of physical maturation on the thermoregulatory response to exercise (50% V02mc.x) in the heat (42°C 20%RH) among circum-pubertal boys, using a mixed cross-sectional, longitudinal design. Subjects were initially divided into three groups, based on Tanner (pubic hair) staging: 16 pre-pubenal (PP, Tanner 1), 15 mid-pubertal (MP, Tanner 2,3,4) and 5 Iatepubertal (LP, Tanner 5). The thermoregulatory response was observed every 6 months for a period of 18 months (4 sessions). Thirty of the 36 boys completed the four sessions. During each session, the exercise task :onsisted of three 20-min bouts of cycling and 10 min rest periods. Measurements included rectal and skin temperatures and hean rate continuously, VO:! at the midpoint of the second bout, forearm blood flow (FBF) upon entry to the climatic chamber and immediately following each exercise bout, sweat collection during each bout, photography of sweat drops after bouts 1 and 2, and whole body sweating rate. Blood samples before and after exercise in the heat were analyzed for hormonal, lactate and electrolyte concentrations. Body temperatures tended to be higher among LP and FBF was higher among PP. However, the rate of heat exchange and heat storage per kg body weight were similar among groups. Sweating rate per body surface area and per gland were higher among LP compared to PP. This was accompanied by lower sweat lactate concentrations during the initial stages of exercise, hi~her lactate excretion rate per gland. ~l')wer activated sw~at gland population density, and higher mean sweat drop area. Aldosterone and prolactin concentrations increased during exercise in the heat in all groups. The increase in prolactin was greater among LP compared to PP. Longitudinal observations tended to support cross-sectional findings. It is concluded that, in boys a) heat exchange and heat storage are similar among preto late-pubescents exercising in dry heat; b) physical maturation is characterized by enhanced sweating rate per body surface area and per gland, and this may be associated with increased sweat gland anaerobic metabolism; c) physical maturation may potentiate the increase in prolactin but not the increase in aldosterone during exercise in the heat.</p>