Meeting Abstract

54.5  Jan. 6  Equine digital flexor muscle contractile function studied in vivo BUTCHER, M.T.*; HERMANSON, J.W.; DUCHARME, N.G.; BERTRAM, J.E.A.; Univ. of Calgary; Cornell Univ.; Cornell Univ.; Univ. Calgary mbutche@clemson.edu

The forelimb digital flexor muscle complex of the horse displays a remarkable range of architectural diversity despite each muscle having basically the same mechanical advantage across the same functionally important joint. Previous research has focused on muscle-tendon unit architecture, contractile properties of the muscle fibers, and fiber MHC composition and motility as a basis for understanding muscle function. Current research focuses on in vivo whole muscle function of the short compartment deep digital flexor (DDFsc) and the superficial digital flexor (SDF). Direct recordings of muscle force (via tendon strain gages) and muscle fascicle length (via sonomicrometry crystals) were made over a range of gaits and speeds. The DDFsc fascicles shorten during the contact period in walking (1.8 ms^-1), trotting (4.1 ms^-1) and cantering (7.0 ms^-1) while producing low force, resulting in a modest amount of positive net work generation. The SDF fascicles initially shorten following hoof contact, but eccentric contraction predominates during the contact period while force is high, leading to substantial negative net work generation. Findings suggest that the long fiber, unipennate DDFsc muscle supplements power during running whereas the short fiber, multipennate SDF muscle is specialized for economical force production and enhanced elastic energy storage. Apparent in vivo functions are largely consistent with the contractile physiology of the fibers from each muscle and likely indicates a specialized limb-spring design for locomotion economy using a bouncing trot as the primary running gait.