Muscle contraction-blood flow interactions during upright knee extension exercise in humans

Abstract

To test for evidence of a muscle pump effect during steady-state upright submaximal knee extension exercise, seven male subjects performed seven discontinuous, incremental exercise stages (3 min/stage) at 40 contractions/min, at work rates ranging to 60-75% peak aerobic work rate. Cardiac cycle-averaged muscle blood flow (MBF) responses and contraction-averaged blood flow responses were calculated from continuous Doppler sonography of the femoral artery. Net contribution of the muscle pump was estimated by the difference between mean exercise blood flow (MBFM) and early recovery blood flow (MBFR). MBFM rose in proportion with increases in power output with no significant difference between the two methods of calculating MBF. For stages 1 and 5, MBFM was greater than MBFR; for all others, MBFM was similar to MBFR. For the lighter work rates (stages 1-4), there was no significant difference between exercise and early recovery mean arterial pressure (MAP). During stages 5-7, MAP was significantly higher during exercise and fell significantly early in recovery. From these results we conclude that 1) at the lightest work rate, the muscle pump had a net positive effect on MBFM, 2) during steady-state moderate exercise (stages 2-4) the net effect of rhythmic muscle contraction was neutral (i.e., the impedance due to muscle contraction was exactly offset by the potential enhancement during relaxation), and 3) at the three higher work rates tested (stages 5-7), any enhancement to flow during relaxation was insufficient to fully compensate for the contraction-induced impedance to muscle perfusion. This necessitated a higher MAP to achieve the MBFM.

Regulation of Increased Blood Flow (Hyperemia) to Muscles During Exercise: A Hierarchy of Competing Physiological Needs

“…This shows that the skeletal muscle pump operating in isolation can generate some blood flow, but this increase in flow is modest at best compared with the large increases in skeletal muscle blood flow associated with exercise, but interpretation is not completely straightforward in the absence of normal perfusion pressure and gas exchange. When various maneuvers like unloaded cycling or leg kicking, very light exercise, or measurements of the decay in blood flow immediately after exercise are examined, the relative contribution of the muscle pump can be variable (289,439). During upright exercise, the rapid reduction in venous pressure can account for 67% of the rise in femoral blood flow seen during the first 10 s of cycling at a low power output (516).…”

“…During upright exercise, the rapid reduction in venous pressure can account for 67% of the rise in femoral blood flow seen during the first 10 s of cycling at a low power output (516). However, during more prolonged heavy exercise, the contribution of the muscle pump is more modest and accounts for only a small fraction of the flow, and during heavy exercise, the net effect of higher muscle forces impeding blood flow might offset the flow-promoting effects of the muscle pump (289).…”

Do vasoregulatory mechanisms in exercising human muscle compensate for changes in arterial perfusion pressure?

“…In submaximal smallmuscle-mass exercise, where arterial oxygen content does not change, muscle blood flow determines oxygen delivery and steady-state muscle blood flow is linearly related to exercise intensity (21,22). This is commonly described as a matching of muscle blood flow to metabolic demand and is thought to be accomplished via a combined effect of a number of vasodilatory mechanisms [for review, see Clifford and Hellsten (4)] and a potential contribution of the muscle pump, depending on exercise intensity and mode (17,25,34).…”

Muscle Oxygen Saturation Measured Using “Cyclic NIR Signals” During Exercise

“…Previous research showed that muscle blood flow in both the artery and vein is decreased during muscle contraction, and increased during muscle relaxation due to changes in intramuscular pressure, a mechanism known as the "skeletal muscle pump" [6][7]. The main assumption in our technique is that the cyclic NIRS signal Δ is predominantly caused by absorption changes due to a portion of blood shifting in and out of the muscle during exercise.…”