HERMANSON, J.W.*; DUCHARME, N.G.; BUTCHER, M.T.; BERTRAM, J.E.A.; Cornell Univ., Ithaca; Cornell Univ., Ithaca; Florida State Univ., Tallahassee; Florida State Univ., Tallahassee: Activity and function of equine digital flexor muscles.

The function and morphology of equine forelimb digital flexor muscles was studied to assess their role in locomotion. Equine superficial (SDF) and deep (DDF) digital flexor muscles were studied using standard histo- and biochemical techniques. Muscle architecture exhibited complex patterns correlated with muscle fiber type and potential function. SDF contained nearly 60% type I fibers suggestive of fatigue-resistant “postural” fibers. The superficial portion of DDF was nearly identical to that of SDF in fiber type composition and in the presence of short (3-5 mm long) myofibers arranged in a complex pinnate fashion. Deeper regions of DDF were subdivided into 3 compartments containing long myofibers and 60-70% type II, fast twitch fibers. Function at walk and trot was assessed with surgically implanted bipolar EMG electrodes and sonomicrometry crystals in muscles, and strain sensors on SDF and DDF tendons. There was significant shortening of myofibers in DDF coincident with onset of EMG activity during the earliest portion of stance phase in walking and trotting. DDF muscle fiber contraction occurred during the final 60% of stance phase, coincident with large amplitude EMG activity, and continued through the ensuing swing phase. Tendon strain occured immediately after hoof contact followed by a period of gradual (stance phase) and then increased (swing phase) tendon shortening. SDF EMG and strain patterns were similar in timing to those of DDF. Analysis of SDF strain data is in progress. Supported by NSF IBN9819985.