Sensitivity of maximum sprinting speed to characteristic parameters of the muscle force-velocity relationship.

An accumulation of evidence suggests that the force-velocity relationship (FVR) of skeletal muscle plays a major role in limiting maximum human sprinting speed. However, most of the theories on this limiting role have been non-specific as to how the FVR limits speed. The FVR is characterized by three parameters that each have a different effect on its shape, and could thus limit sprinting speed in different ways: the maximum shortening velocity V(max), the shape parameter A(R), and the eccentric plateau C(ecc). In this study, we sought to determine how specifically the FVR limits sprinting speed using forward dynamics simulations of human locomotion to examine the sensitivity of maximum speed to these three FVR parameters. Simulations were generated by optimizing the model's muscle excitations to maximize the average horizontal speed. The simulation's speed, temporal stride parameters, joint angles, GRF, and muscle activity in general compared well to data from human subjects sprinting at maximum effort. Simulations were then repeated with incremental and isolated adjustments in V(max), A(R), and C(ecc) across a physiological range. The range of speeds (5.22-6.91 m s⁻¹) was most sensitive when V(max) was varied, but the fastest speed of 7.17 m s⁻¹ was attained when A(R) was set to its maximum value, which corresponded to all muscles having entirely fast-twitch fibers. This result was explained by the muscle shortening velocities, which tended to be moderate and within the range where A(R) had its greatest effect on the shape of the FVR. Speed was less sensitive to adjustments in C(ecc), with a range of 6.23-6.70 m s⁻¹. Increases in speed with parameter changes were due to increases in stride length more so than stride frequency. The results suggest that the shape parameter A(R), which primarily determines the amount of muscle force that can be produced at moderate shortening velocities, plays a major role in limiting the maximum sprinting speed. Analysis of muscle force sensitivity indicated support for previous theories on the time to generate support forces in stance (Weyand et al., 2000, Journal of Applied Physiology, 89, 1991-1999) and energy management of the leg in swing (Chapman & Caldwell, 1983, Journal of Biomechanics 16, 79-83) as important factors in limiting maximum speed. However, the ability of the knee flexors to slow the rotational velocity of the leg in preparation for footstrike did not appear to play a major role in limiting speed.