Comparison of the forearm and calf blood flow response to thermal stress during dynamic exercise

To examine the hypothesis that the skin blood flow response to body heating is not uniform over the entire body surface, we compared forearm (FBF) and calf (CBF) blood flow responses to an increase in core temperature (esophageal temperature, Tes) during dynamic exercise. We studied 13 physically active men during semi-recumbent one leg exercise and/or intermittent supine cycle exercise at 35°C. During 30 min of one leg exercise, Tes FBF, and CBF in the nonactive leg increased from 36.94 ±0 09°C, 5.7 ± 1.2, and 5.6 ± 0.6 ml-(min -100 ml)-1 at rest to 37.97 ± 0.10°C, 27.0 ± 2.4, and 11.1 ± 0.8 ml (min 100 ml)-1, respectively. The increase in blood flow per unit increase in Te, was much less in the calf than in the forearm. The ratio of the peak to resting blood flow averaged 6.5 in the forearm and 2.5 in the calf. During 60 min of intermittent supine two leg exercise, Tes FBF, and CBF increased from 36.96 ± 0.06°C, 7.9 ± 1.5, and 5.6 ± 0.7 ml-(min-100 ml)-1 at rest to 37.91 ± 0.07°C, 23.6 ± 3.0, and 11.4 ± 1.9 ml-(min-100 ml)-1, respectively. Skin blood flow (SkBF) in the forearm and calf was estimated by using a simple cylindrical model, assuming skin thickness and resting muscle blood flow to be 0.2 cm and 2 ml • (min • 100 ml)-1, respectively. Using this model, forearm and calf SkBF were 38 ± 12 and 51 ± 8 ml (min-100 ml)-1, respectively, at rest and 260 ± 25 and 127 ±11 ml-(min -100 ml)-1, respectively, after 30 min of one leg exercise. Manipulation of model parameters (skin thickness and resting muscle blood flow) made the calf SkBF response similar to the forearm SkBF response to body heating, but the ratio of peak to resting SkBF was always lower for the calf than the forearm. This analysis suggests that SkBF response to an increase in core temperature is less in the calf than the forearm. These data support the hypothesis that the skin blood flow response to body heating is not uniform over the entire body.