Contractile Properties of Muscle Fibers from the Forelimb Deep and Superficial Digital Flexors of Horses

Equine digital flexor muscles have independent tendons but nearly identical mechanical relationship to the main joint they act upon. Yet these muscles have remarkable diversity in architecture, ranging from long, unipennate fibers (‘short’ compartment of DDF) to very short, multipennate fibers (SDF). To investigate the functional relevance of the form of the digital flexor muscles, fiber contractile properties were analyzed in the context of architecture differences and in vivo function during locomotion. Myosin heavy chain (MHC) isoform fiber type was studied and in vitro motility assays were used to measure actin filament sliding velocity (Vf). Skinned fiber contractile properties (isometric tension (P0/CSA), velocity of unloaded shortening (VUS) and Force-Ca relationships) at both 10°C and 30°C were characterized. Contractile properties were correlated with MHC isoform and their respective actin filament sliding velocities. The DDF contained a higher percentage of MHC-2A fibers with myosin (heavy meromyosin; HMM) and Vf that was 2-fold faster than SDF. At 30°C P0/CSA was higher for DDF (103.5 ± 8.75 mN/mm) than SDF fibers (81.8 ± 7.71 mN/mm). Similarly, VUS (pCa 5, 30oC) was faster for DDF (2.43 ± 0.53 FL s) than SDF fibers (1.20 ± 0.22 FL s). Active isometric tension increased with increasing Ca concentration, with maximal Ca-activation at pCa 5 at each temperature in fibers from each muscle. In general, the collective properties of DDF and SDF were consistent with fiber MHC isoform composition, muscle architecture and the respective functional roles of the two muscles in locomotion.