Muscle blood flow during locomotory exercise.

Mammalian skeletal muscles are composed of three primary muscle fiber types, FOG, FG, and SO. These fiber types are distributed within and among muscles in predictable patterns. Current data indicate that blood flows to inactive muscles composed of the various fiber types are approximately equal. Total muscle blood flow increases when the animal stands up. When maintaining posture (standing), the muscular force is provided by the SO fibers. These fibers receive the highest blood flows under these conditions. When the animal walks the SO fibers remain active and the additional muscular force is provided by FOG fibers. The increased muscle blood flow during exercise is primarily directed to the active FOG fibers. At fast running speeds, FG fibers are additionally recruited and blood flow increases in muscle areas composed of FG fibers. However, the FG blood flows per gram of tissue are much less than in the oxidative muscles. Thus, muscle blood flow is primarily directed to the active high oxidative muscle fibers within and among the muscles during normal activities. Much is known about factors believed to link blood flow to metabolism in skeletal muscle and about the reflex control of skeletal muscle vascular beds. However, the mechanisms responsible for blood flow control during locomotory exercise are yet to be established. It appears that the different muscle fiber types may have qualitatively similar blood flow control mechanisms with quantitative differences in relation to each fiber type. The capacity for blood flow in skeletal muscle is related to the oxidative potential of the muscles. Blood flows in high-oxidative muscles may reach 400-600 ml/min/100 g, which is considerably higher than the commonly accepted values for maximal muscle blood flow. Also, muscle blood flows are higher during locomotory exercise in conscious animals than in in situ electrical stimulation experiments. Chronic exercise training does not appear to change total muscle blood flow during locomotory exercise. However, the distribution of blood flow within and among muscles changes so that the deep red muscle fibers have higher flows during exercise. While data exist suggesting that blood flow capacity is increased with exercise training, this remains controversial.