Progressive elevations in muscle blood flow during prolonged exercise in swine

Distribution of muscle blood flow has not been measured in man during prolonged exercise, but progressive elevations in skin flow coupled with constant cardiac output (QT) have suggested muscle blood flow may be compromised. However, previous experiments with rats demonstrated progressive increases in muscle blood flow over time during prolonged submaximal exercise. The present study was performed to study muscle blood flow in miniature swine during long-term exercise to shed light on this apparent anomaly. QT and distribution of QT were studied with radiolabeled microspheres while pigs ran on a level treadmill at a speed (10.5 km/h) requiring 71 +/- 4% of maximal O2 consumption (VO2 max). QT increased 23% from the 5th to the 30th min of exercise, whereas total skeletal muscle flow increased by 49%. Increases in flow in the muscles resulted from decreased resistance, since mean arterial pressure declined over this time period (-7%). In addition, the proportional increases in muscle flow were similar within synergistic muscle groups independent of fiber type composition (e.g., elbow extensors: 59-78%; elbow flexors: 26-40%). The factor that limited continued exercise appeared to be body temperature. Colonic temperature rose in linear fashion over time; the animals became exhausted at approximately 42 degrees C. These flow data are similar to previous findings in rats and indicate that during prolonged treadmill locomotion in quadrupedal animals muscle blood flow increases over time to near maximal levels.