Regional heat flows of resting and exercising men immersed in cool water

Trunk (H(T)), limb (H(L)), and whole-body (H(DIR) = H(T) + H(L) + H(forehead)) skin-to-water heat flows were measured by heat flow transducers on nine men immersed head out in water at critical temperature (T(cw) = 30 ± 2°C) and below [overall water temperature (T(w)) range = 22-32°C] after up to 3 h at rest and exercise. Body heat flow was also determined indirectly (H(M)) from metabolic rate corrected for changes in heat stores. At rest at T(cw) [O2 uptake (V̇O2) = 0.33 ± 0.07 l/min, n = 7], H(T) = 52.3 ± 14.2 (SD) W, H(L) = 56.4 ± 14.6 W, H(DIR) = 120 ± 27 W, and H(M) = 111 ± 29 W (significantly different from H(DIR)). T(w) markedly affected H(DIR) but only slightly affected H(M) (n = 22 experiments at T(w) different from T(cw) plus 7 experiments at T(cw)). During light exercise (3 MET) at T(cw) (V̇O2 = 1.06 ± 0.26 l/min, n = 9), H(T) = 122 ± 43 W, H(L) = 130 ± 27 W, H(DIR) = 285 ± 69 W, and H(M) = 260 ± 60 W. During severe exercise (7 MET) at T(cw) (V̇O2 = 2.27 ± 0.50 l/min, n = 4), H(T) = 226 ± 100 W, H(L) = 262 ± 61 W, H(DIR) = 517 ± 148 W, and H(M) = 496 ± 98 W. Lowering T(w) at 7-MET exercise (n = 9, plus 4 at T(cw)) had no effect on H(DIR) and H(M). In conclusion, resting H(L) and H(T) are equal. At T(w) < T(cw) at rest, H(DIR) > H(M), showing that unexpectedly the shell was still cooling. During exercise, H(L) increases more than H(T) but less than expected from the heat production of the working limbs. Therefore some heat produced by the limbs is probably transported by blood to the trunk. During heavy exercise, H(DIR) is constant at all considered T(w); apparently it is regulated by some thermally dependent mechanism, such as a progressive cutaneous vasodilation occurring as T(w) increases.