The muscle pump, What question do we want to answer?

M. Harold Laughlin

This letter is in response to the Point-Counterpoint series “The muscle pump is/is not an important determinant of muscle blood flow during exercise” that appeared in the July issue (vol. 99: 371–375, 2005; doi:10.1152/japplphysiol.00381;

Letter to the Editor: Our group has a long-standing interest in the muscle pump (MP) hypothesis of Pollack, Folkow, and colleagues to explain interactions of gravity and muscle contractions on venous pressure and blood flow (BF) in human limbs (2). The MP may provide up to 60% of the driving force for BF in humans during locomotion (1, 2, 4), yet there is a growing number of types of contractile activity where there is no MP effect. I wish a full accounting of mechanisms producing the following two observations could be provided without invoking the MP: 1) How can human and animal muscle BFs be as high as 3–5 ml·min−1·g−1 during locomotion (1)?, and 2) How can going from supine to the upright posture produce a twofold increase in exercise BF in humans (4)? However, a demonstration that the MP has little or no effect on BF in different forms of muscle contraction or models of exercise [even from our laboratory (3)] will not answer the fundamental question: does the MP contribute importantly to muscle BF during locomotory exercise (1)? All agree that a number of types of muscle contraction exhibit no MP effect. Whether rhythmic muscle contraction produces a MP effect depends on complex interactions of a number of mechanical factors (2). Finally, if the MP aids venous return it must also aid muscle BF.