Conductive and convective heat flows of exercising humans in cold water

The apparent conductance (K(ss), in W · m^-2 · °C^-1) of a given region of superficial shell (on the thigh, fat + skin) was determined on four nonsweating and nonshivering subjects, resting and exercising (200 W) in water [water temperature (T(w)) 22-23°C] (K(ss) = H(ss)/(T(sf) - T(sk)) where H(ss) is the skin-to-water heat flow directly measured by heat flow transducers and T(sf) and T(sk) are the temperatures of the subcutaneous fat at a known depth below the skin surface and of the skin surface, respectively. The convective heat flow (q(c)) through the superficial shell was then estimated as q(c) = (T(sf) - T(sk)) · (K(ss) - K(ss,min)), assuming that at rest K(ss) was minimal (K(ss,min)) and resting q(c) = 0. The duration of immersion was set to allow rectal temperature (T(re)) to reach ~37°C at the end of rest and ~38°C at the end of exercise. Except at the highest T(w) used, K(ss) at the start of exercise was always K(ss,min) and averaged 51 W · m^-2 · °C^-1 (range 33-57 W · m^-2 · °C^-1) across subjects, and q(c) was zero. At the end of exercise at the highest T(w) used for each subject, K(ss) averaged 97 W · m^-2 · °C^-1 (range 77-108 W · m^-2 · °C^-1) and q(c) averaged 53% (range 48-61%) of H(ss) (mean H(ss) = 233 W · min^-2). The threshold for cutaneous vasodilation (TCV) was defined as the combinations of the highest T(re) and T(sk) at which K(ss) = K(ss,min) and q(c) = 0. T(sk) at the TCV was found to be negatively related to T(re): T(sk) = 285.9 - 6.88 T(re); r = -0.90; n = 11. Above the TCV, q(c) increased by 271 ± 84 W · min^-2 · °C^-1 of increase in T(sk). It is concluded that a given change of T(re) has 7- to 9-fold stronger effects than the same change of T(sk) on the TCV and on the increase of q(c) beyond the TCV.