Muscular work as thermal behavior in humans.

Human subjects were placed on a bicycle ergometer and left to pedal ad lib. for 43 min to warm themselves and thereby attain thermal comfort at an ambient temperature of 10 degrees C. Esophageal (Tes), tympanic (Tty), forehead, and hand skin temperatures were recorded. In addition, the work of pedaling was noted for each 2-min period. In some experiments sweating rate was also recorded. Two series of experiments were performed, the first with face fanning and the second with thermal insulation of the head. Face fanning resulted in decreased Tty, but in substantially increased Tes, which was 1.5 degrees C higher than Tty after an intense pedaling effort of 46-50 W. In experiments with head covering, a mild increase of both Tes and Tty was observed toward a position halfway between the extreme values obtained during face fanning. Simultaneously, pedaling intensity decreased to 16.7 W and the rate of sweating increased. In general, pedaling work was inversely proportional to intracranial temperature (Tty) and independent of trunk temperature (Tes). Therefore, motivation for muscular activity in humans exposed to cold appears to be goverened chiefly by the brain temperature.