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

This research paper presents a quantitative description of the architecture of superficial digital flexor (SDF) and deep digital flexor (DDF) muscles in adult horses. The research provides insights into the muscle-tendon behavior and muscle forces, offering valuable information for surgical procedures, biomechanical modeling, and the analysis of muscle-tendon function.

Methods

The research sampled seven forelimb specimens from seven adult Thoroughbreds.

• Measurements of muscle and tendon lengths and volumes were taken from six fixed forelimbs.
• Following adequate processing, fiber bundle and sarcomere lengths were measured. Estimates of optimal fascicle lengths and muscle length-to-fascicle length, muscle length-to-free tendon length, and fascicle length-to-tendon length ratios were calculated.
• The physiologic cross-sectional areas (PCSAs) of the tendons and muscles were determined.
• Pennation angles (the angles between the muscle fibers and the direction of pull of the muscle) were measured in one embalmed specimen.

Results

The findings highlighted differences between the SDF and DDF muscle architectures.

• The optimal fascicle lengths in the SDF muscle were uniformly short (average: 0.8 cm), whereas DDF lengths varied more, ranging from 0.9 cm to 10.8 cm.
• The DDF’s humeral head displayed three architectural subunits, each receiving a separate branch of the median nerve. This is indicative of neuromuscular compartmentalization.
• Pennation angles across the samples were small, ranging between 10 degrees to 25 degrees.
• The PCSAs for the SDF and DDF muscles were 234 cm² and 259 cm² respectively, with estimated forces of 4,982 N and 5,520 N.

Conclusions

The results suggest a perspective on the varying functionalities of the two muscles.

• The SDF muscle provides strong tendinous support with little muscle fascicular shortening and fatigue-resistance properties, implying it primarily supports the weight and motion of the horse.
• In contrast, the DDF muscle combines passive and dynamic functions offering larger tension development and higher shortening velocities during digital motion, suggesting its key role in active movement.
• These architectural parameters could be critical for predicting forces and understanding muscle-tendon function, which could further inform surgical procedures and biomechanical modeling.