PAYNE, R. C. *; WATSON, J.; HUTCHINSON, J. R.; ROBILLIARD, J. J. ; WILSON, A. M.; Royal Veterinary College, London, UK; Royal Veterinary College, London, UK; Royal Veterinary College, London, UK; Royal Veterinary College, London, UK; Royal Veterinary College, London, UK: Functional specialisation of the thoracic and pelvic limb in horses
Proximal-to-distal functional specialisation of muscle architecture has been observed in a wide range of mammals (e.g. humans, horses, camels and goats), where proximal limb muscles are specialised for doing work and distal limb muscles are specialised for force generation. Horses, and other quadrupeds, also have scope for functional specialisation between limbs. In horses, 57% of the total vertical ground reaction force impulse is directed through the thoracic limb. However, horizontal (acceleratory) forces are greatest in the pelvic limb. We expected these functional differences to be reflected in muscle-tendon architecture.
The thoracic and pelvic limbs of six Thoroughbred cadavers were dissected, obtaining data on muscle mass, fibre length and pennation angle. Fibre length predicts relative contraction velocity; mass predicts volume and hence power output; muscle force is estimated from PCSA (volume/fibre length). With substantial volumes and long parallel fibers, proximal thoracic limb muscles are appropriate for the negative work of slowing the trunk as it passes over the fixed limb and slowing the swinging limb for touch-down. The exceptions were serratus ventralis thoracis and biceps brachii which had short pennate fibers and a substantial tendinous element. These muscles will contribute to the elastic properties of the limb and likely function in support. Proximal pelvic limb muscles were comparatively large, indicating a high potential power output. Distal limb muscles were similar in both thoracic and pelvic limbs. However, the pelvic limb deep digital flexor had shorter fibres and thus a greater capacity for force generation than its thoracic limb equivalent.
