Role of speed vs. grade in relation to muscle pump function at locomotion onset

We sought to clarify the roles of contraction frequency (speed) and contraction force (grade) in the rise in muscle blood flow at the onset of locomotion. Shoemaker et al. (Can J Physiol Pharmacol 76: 418-427, 1998) explored this relationship in human handgrip exercise and found that the time course of the rise in muscle vascular conductance was similar when a light weight was lifted in a fast cadence and a heavy weight was lifted in a slow cadence (total work constant). This indicates that muscle pumping (contraction frequency) was of limited importance in governing the time course. Rather, vasodilator substances released in proportion to the total work performed appeared to determine the pattern and extent of the rise in conductance. We hypothesized that conductance would rise faster during locomotion at a high speed (frequency) and low grade (force) than at a low speed and high grade, despite similar total increases in conductance, owing to more effective muscle pumping at faster contraction rates. Seven male rats performed nine 1-min bouts of treadmill locomotion across a combination of three speeds (5, 10, and 20 m/min) and three grades (-10, 0, and +15 degrees ) in random order. Locomotion at 10 m/min and 0 degrees grade and 20 m/min and -10 degrees grade led to an equal rise in terminal aortic vascular conductance. However, the equal rise was achieved more quickly at the higher running speed, suggestive of more effective muscle pumping. Across the nine combinations of exercise, speed began to exert a statistically significant influence on conductance by the 3rd s of locomotion. Grade did not begin to exert an influence until the 12th s of locomotion (similar to the delays reported for arteriolar dilation to muscle contraction). Additional experiments in dogs provided similar results. Thus the muscle pump appears to initiate the increase in blood flow in proportion to contraction frequency at locomotion onset.