Muscle contraction duration and fibre recruitment influence blood flow and oxygen consumption independent of contractile work during steady-state exercise in humans

We tested the hypothesis that, among conditions of matched contractile work, shorter contraction durations and greater muscle fibre recruitment result in augmented skeletal muscle blood flow and oxygen consumption () during steady-state exercise in humans. To do so, we measured forearm blood flow (FBF; Doppler ultrasound) during 4 min of rhythmic hand-grip exercise in 24 healthy young adults and calculated forearm oxygen consumption () via blood samples obtained from a catheter placed in retrograde fashion into a deep vein draining the forearm muscle. In protocol 1 (n= 11), subjects performed rhythmic isometric hand-grip exercise at mild and moderate intensities during conditions in which timetension index (isometric analogue of work) was held constant but contraction duration was manipulated. In this protocol, shorter contraction durations led to greater FBF (184 +/- 25 versus 164 +/- 25 ml min-1) and (23 +/- 3 versus 17 +/- 2 ml min-1; both P < 0.05) among mild workloads, whereas this was not the case for moderate-intensity exercise. In protocol 2 (n= 13), subjects performed rhythmic dynamic hand-grip exercise at mild and moderate intensities in conditions of matched total work, but muscle fibre recruitment was manipulated. In this protocol, greater muscle fibre recruitment led to significantly greater FBF (152 +/- 15 versus 127 +/- 13 ml min-1) and (20 +/- 2 versus 17 +/- 2 ml min-1; both P < 0.05) at mild workloads, and there was a trend for similar responses at the moderate intensity but this was not statistically significant. In both protocols, the ratio of the change in FBF to change in was similar across all exercise intensities and manipulations, and the strongest correlation among all variables was between and blood flow. Our collective data indicate that, among matched workloads, shorter contraction duration and greater muscle fibre recruitment augment FBF and during mild-intensity forearm exercise, and that muscle blood flow is more closely related to metabolic cost (C) rather than contractile work per se during steady-state exercise in humans.