By use of successive increments of discontinuous work with an arm-leg cycle ergometer the VO2, Q, SV, and HR were studied in six male subjects at rest and during exercise in air and in water at 18, 25, and 33 degrees C. The Q values obtained by CO2 rebreathing were reproducible. VO2 was linearly related to work with the plots for air and 33 degrees C water being similar. However, during work in 25 and 18 degrees C water, the VO2 averaged 9.0% (150 ml) and 25.3% (400 ml) higher, respectively, than values observed in 33 degrees C water, with the largest differences observed in leaner subjects. The plot of HR-VO2 was linear and almost identical during work in air and 33 degrees C water, but shifted significantly to the right in cooler water. VO2 averaged 250-700 ml higher in cold water compared to air and 33 degrees C water at a given mean heart rate. The Q vs. VO2 line was similar during work in air and in water with no effect of water or temperature. At similar levels of VO2, SV was significantly larger (P less than 0.05) in 25 and 18 degrees C water than in air or 33 degrees C water. Consequently, the reduction in heart rate during work in cold water was entirely compensated for by a proportionate increase in the SV of the heart. Q was therefore maintained at similar levels of energy expenditure in air and in 18, 25, and 30 degrees C water.
Thermoregulatory responses were studied in 10 men and 8 women at rest in air and during 1-h immersion in water at 20, 24, and 28 degrees C. For men of high body fat (27.6%), rectal temperature (Tre) and oxygen consumption (VO2) were maintained at air values at all water temperatures (Tw). For men of average (16.8%) and low (9.2%) fat the change in Tre (delta Tre) was inversely related to body fat at all Tw with VO2 increasing to 1.07 l X min-1 for a -1.6 degrees C delta Tre for lean men. For women of average (25.2%) and low (18.5%) fat Tre decreased steadily during immersion at all Tw. The greatest changes occurred at 20 degrees C with little differences in delta Tre and VO2 noted between these groups of women. In comparison with males of similar percent fat, Tre dropped to a greater extent (P less than 0.05) in females at 20 and 24 degrees C. Stated somewhat differently, lean women with twice the percentage of fat have similar delta Tre as lean men at all Tw. For delta Tre greater than -1.0 degree C men showed significantly greater (P less than 0.05) thermogenesis compared with women. The differences in thermoregulation between men and women during cold stress at rest may be due partly to the sensitivity of the thermogenic response as well as the significant differences in lean body weight and surface area-to-mass ratio between the sexes.
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