BUTCHER, M.T.*; CHASE, P.B.; HERMANSON, J.W.; BERTRAM, J.E.A.; CLARK, A.N.; Florida State University; Florida State University; Cornell University; Florida State University; Florida State University: Force and work output characterizations of equine forelimb digital flexor muscle fibers

The equine digital flexor complex is composed of several muscles that exert their influence almost exclusively around the fetlock joint with essentially the same mechanical advantage. Yet these muscles display a diversity of architecture, from large muscles with extremely short fibers to small muscles with relatively long fibers. As part of ongoing research to investigate the function of the digital flexors in equine locomotion, mechanical properties of individual muscle fibers are currently being examined and compared within the muscle complex. Fascicles from the deep digital flexor (DDF) and superficial digital flexor (SDF) were dissected from fresh cadaver horses and chemically skinned to remove cell membranes. Individual muscle fibers were separated from fascicles and attached to a force transducer at one end and a motor at the other via Al foil clips wrapped around chemically fixed ends of the fiber. Fiber mechanics were studied at maximum Ca²⁺ activation (pCa 5) and in relaxing conditions (pCa 9) at 10°, 20° and 30° C. Preliminary results show that force (in mN/mm²) was greatest at pCa 5 and generally increased with temperature. On average, maximum force output was greater for DDF fibers compared with SDF fibers. In a limited subset of fibers, unloaded shortening velocity (V_US) estimated using slack tests showed that DDF fibers were approximately 5-7-fold faster than SDF fibers at each temperature. Later work will focus on a more complete characterization of the force/Ca²⁺ response in addition to work output capacity of fibers through low and high frequency length oscillations of stretch and shortening cycles using work loop analysis.