The role of thoracic limb muscles in equine locomotion

Until this study, none of the thoracic limb models have focused on muscles proximal to the elbow joint. Mainly because most injuries occur to the distal limb, however extrinsic muscles are also likely to be important in the prevention of injury. The extrinsic muscles appear to be large with long and parallel fascicles. The only exclusion was serratus ventralis thoracis which had very short fascicles and implies a force-generating, anti-gravity role. The muscle was squeezed between two thick sheets of aponeurosis and may participate to the overall elastic properties of the thoracic limb.

Horses do not have clavicles. Their thoracic limb is attached to the trunk with only muscle attachments and no bony articulation, called: synsarcosis. This sets free motion of the limb and increase limb length. Long legs are useful as they use energy more economical. It is the role of the extrinsic muscle group to attach the thoracic limb to the trunk (See figure). Ground reaction force is transferred via these muscles to the trunk.

At the beginning of stance the short tendon fibers are stretched, storing elastic energy and releasing it later, functioning as a passive spring. Tendon springs are important in running locomotion, because of the exchange of kinetic and elastic energy. This reduces the amount of work that muscles need to perform in order to move the limb and body mass.

The full article describes the exact function, anatomy and biomechanics of all extrinsic muscles of the thoracic limb. Full-text

> R. Payne et al., The role of the extrinsic thoracic limb muscles in equine locomotion,  J. Anat. (2005) 206, pp193–204. All rights reserved to Anatomical Society of Great Britain and Ireland 2005. Click here for the Pubmed summary

Watch this episode of racehorse anatomy to understand the function of the passive spring system in the thoracic limb