Blood flow to fast-twitch red (FTR), fast-twitch white (FTW), and slow-twitch red (STR) muscle fiber sections of the gastrocnemius-plantaris-soleus muscle group was determined using 15 +/- 3-microns microspheres during in situ stimulation in pentobarbital-anesthetized rats. Steady-state blood flows were assessed during the 10th min of contraction using twitch (0.1, 0.5, 1, 3, and 5 Hz) and tetanic (7.5, 15, 30, 60, and 120/min) stimulation conditions. In addition, an earlier blood flow determination was begun at 3 min (twitch series) or at 30 s (tetanic series) of stimulation. Blood flow was highest in the FTR (220-240 ml X min-1 X 100 g-1), intermediate in the STR (140), and lowest in the FTW (70-80) section during tetanic contraction conditions estimated to coincide with the peak aerobic function of each fiber type. These blood flows are fairly proportional to the differences in oxidative capacity among fiber types. Further, their absolute values are similar to those predicted from the relationship between blood flow and oxidative capacity found by others for dog and cat muscles. During low-frequency contraction conditions, initial blood flow to the FTR and STR sections were excessively high and not dependent on contraction frequency. However, blood flows subsequently decreased to values in keeping with the relative energy demands. In contrast, FTW muscle did not exhibit this time-dependent relative hyperemia. Thus, besides the obvious quantitative differences between skeletal muscle fiber types, there are qualitative differences in blood flow response during contractions. Our findings establish that, based on fiber type composition, a heterogeneity in blood flow distribution can occur within a whole muscle during contraction.
Pre-training Skeletal Muscle Fiber Size and Predominant Fiber Type Best Predict Hypertrophic Responses to 6 Weeks of Resistance Training in Previously Trained Young Men
