Thermoregulatory-induced compromise of muscle blood flow in ponies during intense exercise in the heat: a contributor to the onset of fatigue?


University Veterinary Centre, Faculty of Veterinary Science, University of Sydney, Camden, New South Wales 2570, Australia.


Cardiac output (Q) and tissue blood flows (BF) were measured in four ponies at rest, after 30 min of moderate exercise (ME) (∼30% V̇O2max) and following moderately intense exercise (M-IE) (∼65%) until fatigue, in a mildly hot (MH) environment (dry bulb temperature = 41°C, wet bulb temperature = 27.5°C). Exercise at both intensities resulted in increases in Q (15.6 ± 1.7 [rest] vs. 35.8 ± 2.6 [ME] vs. 48.6 ± 3.9 [M-IE] l/min) and V̇O2 (4.7 ± 0.9 vs. 24.5 ± 1.2 vs. 55.8 ± 5.8 ml/kg/min). When compared to ME, M-IE resulted in decreases in BF to the skin (∼15%), nonworking muscle (46%), fat (93%), gut (56%), kidneys (57%), CNS (31–66%) and an increase in BF to respiratory (68%), limb (98.5%) and cardiac (50%) muscles. In contrast, BF to working muscle during ME in thermoneutral (TN) conditions (McConaghy et al. 2002) was higher than found during ME in MH. We calculated that limb BF was ∼25% lower for ME in MH vs. ME in TN. Similarly, we estimated that limb BF during M-IE in MH was ∼25% lower than for M-IE in TN. Attenuation of the increases in muscle BF during exercise in the heat may be a determinant of the onset of fatigue.