HORSE SPECIES SYMPOSIUM: Biomechanics of the exercising horse.

In spite of having large height and body mass, horses are cursorial animals with an extensive gait repertoire and considerable athletic abilities. The limbs have evolved so that the heavy musculature is confined to the proximal limbs while the distal limbs are light in weight with a single functional digit and long, lightweight tendons to move and support the distal joints. These adaptations reduce the moment of inertia and decrease the energy expended to protract and retract the limbs during locomotion. There is a division of labor between the forelimbs, which have a pillar-like construction specialized for weight bearing, and the hind limbs, in which the more angulated joints provide leverage for the generation of propulsion. Each gait is characterized by a repeated, rhythmic pattern of limb movements with a single repetition of the pattern being a stride. Limb movements are coordinated by central pattern generators in the spinal cord that determine the rhythmic patterns of flexion and extension in the joints during the swing and stance phases. The patterns can be modified by commands descending from the motor cortex or in response to proprioceptive feedback that provides awareness of body and limb position and movements. Musculoskeletal pathologies are a common problem in equine athletes. Lame horses reduce the load-bearing responsibilities of the painful limb by adapting the movement pattern with the goal of transferring vertical force from the lame limb to the compensating limbs. When the horse is trotting, this is associated with asymmetrical movements of the poll, withers, and croup on the 2 diagonals. The lame limb can be identified, and the degree of lameness can be assessed qualitatively by visual assessment or quantitatively using an inertial sensor system.