One of the most remarkable aspects of muscle blood flow is its close matching with muscle metabolism. Blood flow to active skeletal muscle increases at the onset of exercise and remains elevated throughout the duration of an exercise bout. For example, blood flow through the femoral artery of humans is approximately 0.3L/minute at rest and increases in an intensity-dependent manner during leg extension exercise up to 6 to 10L/minute . One result of the close matching of blood flow to metabolism is that blood flow during exercise is not homogeneous in its distribution across different muscles or muscle regions. This regional heterogeneity of muscle blood flow appears to result from two interrelated factors: first, muscle fiber type and second, exercise intensity and recruitment order. Under resting conditions where postural maintenance is the only muscular activity, slow-twitch oxidative (SO) fibers are primarily recruited and blood flow is directed primarily to regions of muscle containing these fibers. During low-intensity exercise, SO and fast-twitch oxidative-glycolytic (FOG) fibers are recruited and blood flow is directed to regions containing these two fiber types. As exercise intensity increases up to maximal, fast-twitch glycolytic (FG) fibers are increasingly recruited and blood flow to these fibers also increases. However, even during maximal exercise, blood flow to FG fibers is never as high as flow to more oxidative fibers. In fact, the magnitude of the increase in blood flow to a particular region of muscle during exercise is directly correlated to its FOG fiber content . Thus the regional distribution of blood flow within and among skeletal muscles is determined by exercise intensity and by the oxidative capacities of the fibers in various regions of muscle.
Skeletal muscle blood flow is also not constant across time, and this temporal heterogeneity takes two forms. First, blood flow increases markedly with the first contraction and then gradually levels off to a steady rate during the first 15 to 60 seconds of exercise at a given intensity. One implication of this is that the initial hyperemic response at the onset of muscle contraction may result from different vascular control mechanisms than the maintained steady-state hyper-emia of continued exercise. Second, because of the mechanical effect of muscle contraction "squeezing" and occluding blood vessels located within the muscle, blood flow to contracting muscle follows a cyclical pattern in which almost all blood flow enters the muscle during the relaxation phase and very little blood flow enters the muscle during the actual contractions. Venous outflow of blood from contracting muscle follows the exact opposite pattern. Muscle contraction compresses veins, forcing blood to move, and venous valves ensure that the direction of movement is out of muscle and toward the heart. Thus most blood flow into muscle occurs during the relaxation phase of muscular contraction while most venous outflow from muscle occurs during the contractile phase.
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