Elastic Energy in the Limb
Adapted from Corel
The spring-like properties of the limbs moderate the mechanical energy cost through the storage and release of elastic energy. Some energy is lost from the system. Muscle and tendons have elastic properties. The tendon spring only return 90 -95 percent of the energy originally stored in them. Energy can also be lost to the environment during aerodynamic drag and ground deformation. The hindlegs, with a large proportion of muscle mass, replace some of this loss during stance periods.
The external work of locomotion is the work required to transport the center of mass of an animal. Limbed animals reach this external work requirement through cyclical movement of the limbs. The horse limbs represent a small motion of the total body mass. Therefore internal work is required for the change in rotational energies of the trunk around the center of mass. Symmetrical gaits can be compared to pendulum-like energy exchange. Asymmetrical gaits can be described as a combination of elastic and inverted pendulum energy-saving mechanisms (Pfau et al. 2006). The spring-like action in the elastic energy of the tendons results in the smooth gait (Back et al. 1995a).
In both the hindlimbs and forelimbs the distal joints function elastically. They are not meant for energy generation due to the small muscles with short, highly pinnate muscle fibers and long tendons (Dutto et al. 2006). The hindlimb increases the efficiency of the gait by using tendons as elastic springs to conserve energy. This stored elastic energy then aids propulsion and flexion of the limb during the swing phase. Some have even suggested that the swing of the hindlimb is passive as a result of this elastic energy in the tendons being released. The release of elastic energy in the swing phase also supports tarsal and stifle flexion (Back et al. 1995b).
The forelimb also uses the tendons as elastic springs. Extension and flexion of the fetlock and coffin help to load the ligaments with stored elastic energy. This increases the efficiency of locomotion through he role of inertia. In clinical experience horses with low radial nerve paralysis have been able to walk. This is due to the uses of elastic energy as a passive pendulum in forelimb motion (Back et al. 1995a).