Muscle blood flow in humans: how high can it go?

The question is whether skeletal muscle blood flow (MBF) in humans can reach the peak values of 300-400 ml.100 g-1.min-1 seen in dogs, ponies, and rats. The answer depends on the total mass of active muscle. Total MBF has a fixed upper limit or maximal value set by maximal cardiac output--normally about 25 1.min-1. In a 75-kg man with 30 kg of muscle, total MBF is 22 1.min-1 or 73 ml.100 g-1.min-1. As the mass of active muscle decreases, MBF available per 100 g of muscle increases, but a "maximal" value can no longer be defined because it becomes a function of blood pressure and muscle vascular conductance (and not maximal cardiac output) for which "maximal" values are also undefined. This paper emphasizes three key points. First, pumping capacity of the human heart is much smaller per kg body weight than in many other species. Second, human cardiac pumping capacity is exceeded by the vasodilator capacity of skeletal muscle; thus, so-called "peak MBF" often occurs when metabolic vasodilator and blood pressure-regulating vasoconstrictor mechanisms are competing. Third, in order to observe a peak MBF, one must maximally activate a mass of muscle that is too small to overwhelm cardiac pumping capacity. This experimental approach has revealed that MBF can exceed 300 ml.100 g-1.min-1 in small masses of active human muscle. The upper limit for flow, which depends on pressure, and for vascular conductance is still unknown.