Muscle architecture and fiber characteristics of the superficial and deep digital flexor muscles in adult Thoroughbred horses.

Introduction: Muscle architecture is optimized for muscle function and affects both the magnitude of force that a muscle can generate and the muscle’s shortening velocity. Therefore, in order to correctly predict the behavior of muscles a detailed quantitative description of skeletal muscle parameters is needed. The architectural features of the two forelimb digital flexor muscles, superficial and deep digital flexor (SDF, DDF), were determined in six adult equine limbs fixed in a physiological postural stance immediately post mortem. Material and Methods: Seven thoracic limbs were harvested below mid-humerus from healthy skeletally-mature Thoroughbreds. All animals were euthanatized for reasons unrelated to musculoskeletal disorders and heparin sodium (200 IU/kg IV) was administered prior to euthanasia with an overdose of barbiturate. The limbs were exanguinated and perfused with 10 % formaldehyde via the brachial artery while the limbs were secured in a custom-built frame to reproduce the physiological postural stance (140º elbow dorsi-flexion, 180º antebrachio-carpal/carpo-metacarpal extension, 150º metacarpo-phalangeal dorsi-flexion). Six specimens were used for morphological measurements of muscle and tendon length, muscle mass and muscle volume. The specimens were then soaked in 0.9% saline solution and digested in nitric acid at decreasing concentrations (25%, 5%, and 2.5%) to allow for fiber bundle (fascicle) and sarcomere length determinations. One specimen was used to determine fiber pennation angles. Optimal fascicle length (the fascicle length at which the muscle generates maximum tension), muscle length/fiber length ratio, muscle length/free tendon length ratio, fiber length/tendon length ratio, muscle physiologic cross-sectional areas (PCSAs; the ratio of muscle volume to optimal fascicle length), and tendon cross sectional areas (CSAs; the ratio of tendon volume to tendon length) were calculated from these measurements. The performance of each muscle was predicted with respect to its force production and velocity of shortening potentials. Results: There was a marked uniformity of very short optimal fascicle length with a mean (SD) of 0.8 (0.1) cm throughout the SDF muscle while the DDF muscle showed high variation in optimal fascicle length among the three heads (humeral, ulnar, radial) and inside the humeral head. Three distinct muscle units were identified in the DDF humeral head with mean (SD) optimal fascicle lengths of 10.8 (1.6) cm, 2.6 (0.3) cm and 1.4 (0.2) cm, respectively. A primary nerve branch originating from the median nerve was identified for each subdivision, suggesting that these units function as muscle compartments. The DDF ulnar head showed a mean optimal fascicle length of 4.0 (0.5) cm, while the DDF radial head showed one of 0.9 (0.2) cm. Angles of fiber pennation were relatively small (10-25º) and generally consistent throughout the SDF muscle and the DDF’s heads and compartments. The PCSAs of the SDF and DDF muscle were 234 (51) cm2 and 259 (30) cm2, respectively. Discussion: The present study reports the first measurements of sarcomere and fascicle length for SDF and DDF muscles in adult horses. Our fascicle length data differ from those previously reported in ponies or small-size horses especially for the DDF muscle1-3. Variations in geometrical characteristics of the SDF and DDF muscles were consistent with the role of these muscles during posture and high speed locomotion. The SDF muscle seems to be designed for tension production with high passive capacity over prolonged periods as it occurs during posture and stance phase of locomotion. The DDF muscle fiber architecture appears to be consistent with an intramuscular division of labor combining postural and dynamic functions with larger tension development and shortening speed during digital flexion. The architectural parameters reported in this study provide the basis for estimation of the force generating potential of the SDF and DDF muscles, and can be integrated in future musculoskeletal limb models for Thoroughbred horses.