HUMAN THERMOREGULATORY RESPONSES DURING PROLONGED WALKING IN WATER AT25, 30 AND 35-DEGREES-C

Eight healthy and physically well-trained male students exercised on a treadmill for 60 min while being immersed in water to the middle of t he chest in a laboratory flowmill. The water velocity was adjusted so that the intensity of exercise correspond to 50% maximal oxygen uptake of each subject, and experiments were performed once at each of three water temperatures: 25, 30, 35 degrees C, following a 30-min control period in air at 25 degrees C, and on a treadmill in air at an ambient temperature of 25 degrees C. Thermal states during rest and exercise were determined by measuring rectal and skin temperatures at various p oints, and mean skin temperatures were calculated. The intensity of ex ercise was monitored by measuring Oxygen consumption, and heart rate w as monitored as an indicator for cardiovascular function. At each wate r temperature, identical oxygen consumption levels were attained durin g exercise, indicating that no extra heat was produced by shivering at the lowest water temperature. The slight rise in rectal temperature d uring exercise was not influenced by the water temperature. The temper atures of skin exposed to air rose slightly during, exercise at 25 deg rees C and 30 degrees C water temperature and markedly at 35 degrees C . The loss of body mass increased with water temperature indicating th at both skin blood flow and sweating during exercise increased with th e rise in water temperature. The rise in body temperature provided the thermoregulatory drive for the loss of the heat generated during exer cise. Heart rate increased most during exercise in water at 35 degrees C, most likely due to enhanced requirements for skin blood flow. Alth ough such requirements were certainly smallest at 25 degrees C water t emperature, heart rate at this temperature was slightly higher than at 30 degrees C suggesting reflex activation of sympathetic control by c old signals from the skin. There was a significantly greater increase in mean skin and rectal temperatures in subjects exercising on the tre admill in air, compared to those exercising in water at 25 degrees C.