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Effect of contraction frequency on the contractile and noncontractile phases of muscle venous blood flow

¹Department of Medicine, University of California, San Diego, La Jolla, California 92093-0623; ²Department of Health and Human Performance, Auburn University, Auburn, Alabama 36849-5323; ³Dipartimento di Scienze e Tecnologie Biomediche, Universita' degli Studi di Milano, I-20090 Segrate (MI); and ⁴Dipartimento di Medicina DiMCO, Universita di Milano, I-20142 Milano, Italy

Submitted 3 March 2003 ; accepted in final form 26 May 2003

The purpose of this study was to test the hypothesis that increasing muscle contraction frequency, which alters the duty cycle and metabolic rate, would increase the contribution of the contractile phase to mean venous blood flow in isolated skeletal muscle during rhythmic contractions. Canine gastrocnemius muscle (n = 5) was isolated, and 3-min stimulation periods of isometric, tetanic contractions were elicited sequentially at rates of 0.25, 0.33, and 0.5 contractions/s. The O₂ uptake, tension-time integral, and mean venous blood flow increased significantly (P < 0.05) with each contraction frequency. Venous blood flow during both the contractile (106 ± 6, 139 ± 8, and 145 ± 8 ml·100 g^-1·min^-1) and noncontractile phases (64 ± 3, 78 ± 4, and 91 ± 5 ml·100 g^-1·min^-1) increased with contraction frequency. Although developed force and duration of the contractile phase were never significantly different for a single contraction during the three contraction frequencies, the amount of blood expelled from the muscle during an individual contraction increased significantly with contraction frequency (0.24 ± 0.03, 0.32 ± 0.02, and 0.36 ± 0.03 ml·N^-1·min^-1, respectively). This increased blood expulsion per contraction, coupled with the decreased time in the noncontractile phase as contraction frequency increased, resulted in the contractile phase contribution to mean venous blood flow becoming significantly greater (21 ± 4, 30 ± 4, and 38 ± 6%) as contraction frequency increased. These results demonstrate that the percent contribution of the muscle contractile phase to mean venous blood flow becomes significantly greater as contraction frequency (and thereby duty cycle and metabolic rate) increases and that this is in part due to increased blood expulsion per contraction.

oxygen uptake; exercise; muscle pump

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