Hindlimb net joint energies during swing phase as a function of trotting velocity.

Net joint powers and energies have been described in walking horses during the swing phase of the stride in the fore- and hindlimb (Clayton et al. 2001). During trotting, swing phase net joint powers have been described in the forelimb but not in the hindlimb. The effects of velocity on power profiles and energy patterns are important in relation to locomotor energetics. The objective of this study was to evaluate velocity-dependent changes in hindlimb net energy profiles of the swing phase during trotting. Inverse dynamic analysis was used to calculate net joint energies at the hindlimb joints of 6 horses trotting overground at velocities ranging from 2.27-5.17 m/s. At all velocities, there was net energy generation at the hip and tarsus and net energy absorption at the stifle, fetlock and coffin joints. Velocity-dependent bursts of energy generation at the hip actively protracted the limb in early swing and initiated retraction in late swing. Synchronous with the bursts of energy generation at the hip were velocity-dependent bursts of energy absorption across the stifle that acted to control flexion in early swing and extension in late swing. The distal limb was raised and lowered by velocity-dependent bursts of energy generation that flexed the tarsus in early swing and extended it in late swing. The energy bursts in early swing increased linearly with velocity, whereas the energy bursts in late swing increased as a function of the square or cube of velocity. The results contribute to understanding the mechanisms used to accelerate and decelerate the limbs more rapidly as velocity increases, which is an important consideration in racing and sporting performance.