Operating length and velocity of human M. vastus lateralis fascicles during vertical jumping

Humans achieve greater jump height during a counter-movement jump (CMJ) than in a squat jump (SJ). However, the crucial difference is the mean mechanical power output during the propulsion phase, which could be determined by intrinsic neuro-muscular mechanisms for power production. We measured M. vastus lateralis (VL) fascicle length changes and activation patterns and assessed the force-length, force-velocity and power-velocity potentials during the jumps. Compared with the SJ, the VL fascicles operated on a more favourable portion of the force-length curve (7% greater force potential, i.e. fraction of VL maximum force according to the force-length relationship) and more disadvantageous portion of the force-velocity curve (11% lower force potential, i.e. fraction of VL maximum force according to the force-velocity relationship) in the CMJ, indicating a reciprocal effect of force-length and force-velocity potentials for force generation. The higher muscle activation (15%) could therefore explain the moderately greater jump height (5%) in the CMJ. The mean fascicle-shortening velocity in the CMJ was closer to the plateau of the power-velocity curve, which resulted in a greater (15%) power-velocity potential (i.e. fraction of VL maximum power according to the power-velocity relationship). Our findings provide evidence for a cumulative effect of three different mechanisms-i.e. greater force-length potential, greater power-velocity potential and greater muscle activity-for an advantaged power production in the CMJ contributing to the marked difference in mean mechanical power (56%) compared with SJ.