Attenuation of the cutaneous blood flow response during combined exercise and heat stress

Skin blood flow (SkBF) was measured in six male subjects using laser-Doppler velocimetry, with zero-gradient auditory canal temperature (T_ac) used as an index of body core temperature (T_c). Subjects performed incremental, upright cycling commencing at 40% peak power ( _peak: 10 min), increasing every 4 min by 5 % _peak thereafter. Trials were conducted in hot (ambient temperature (T_a) 36.7 ±0.2°C, relative humidity (rh) 46.1 ±3.2%; _peak ±S.D.), and neutral environments (T_a 19.6 ±0.3°C, rh 50.2 ±1.4%). SkBF increased with T_ac in all subjects. Attenuation of SkBF occurred at the same T_ac, relative SkBF and cardiac frequency (f _c) between environments, but at a lower exercise intensity (40.8 ±0.8% versus 55.8 ±3.0% _peak) in the hot environment (p<0.05). Data indicate that T_c thresholds for SkBF attenuation may exist. However, it is suggested that attenuation thresholds coincided with a reduced central blood volume, which may occur at a critical level of cutaneous blood pooling.