Determination of muscle architecture and fiber characteristics of the superficial and deep digital flexor muscles in the forelimbs of adult horses.

OBJECTIVE: To provide a quantitative description of the architecture of superficial digital flexor (SDF) and deep digital flexor (DDF) muscles in adult horses to predict muscle-tendon behavior and estimate muscle forces. SAMPLE POPULATION: 7 forelimb specimens from 7 adult Thoroughbreds. PROCEDURE: Muscle and tendon lengths and volumes were measured from 6 fixed forelimbs. After processing, fiber bundle and sarcomere lengths were measured. Optimal fascicle lengths and muscle length-to-fascicle length, muscle length-to-free tendon length, and fascicle length-to-tendon length ratios were calculated, as were tendon and muscle physiologic cross-sectional areas (PCSAs). Pennation angles were measured in 1 embalmed specimen. RESULTS: The SDF optimal fascicle lengths were uniformly short (mean +/- SD, 0.8 +/- 0.1 cm), whereas DDF lengths ranged from 0.9 +/- 0.2 cm to 10.8 +/- 1.6 cm. The DDF humeral head had 3 architectural subunits, each receiving a separate median nerve branch, suggestive of neuromuscular compartmentalization. Pennation angles were small (10 degrees to 25 degrees). The PCSAs of the SDF and DDF muscle were 234 +/- 51 cm2 and 259 +/- 30 cm2, with estimated forces of 4,982 +/- 1148 N and 5,520 +/- 544 N, respectively. CONCLUSIONS AND CLINICAL RELEVANCE: The SDF muscle appears to provide strong tendinous support with little muscle fascicular shortening and fatigue-resistance properties. The DDF muscle combines passive and dynamic functions with larger tension development and higher shortening velocities during digital motion. Architectural parameters are useful for estimation of forces and have implications for analysis of muscle-tendon function, surgical procedures involving muscle-tendon lengthening, and biomechanical modeling.