Contractile behavior of the forelimb digital flexors during steady-state locomotion in horses (Equus caballus): an initial test of muscle architectural hypotheses about in vivo function.

This study investigates the distinct contributions of two muscles in a horse’s limb during locomotion – the short compartment deep digital flexor (DDF(sc)) and the superficial digital flexor (SDF). The study finds that DDF(sc), a long, unipennate muscle, generates low force but performs positive work, while the shorter, multipennate SDF generates high force and accomplishes negative work.

Overview of the Research

• This research focuses on the different roles and contributions of the DDF(sc) and SDF muscles during the locomotion of horses. Despite both muscles operating across the fetlock joint and being part of the digital flexor system, they display great variation in their muscle architecture.
• Employing direct methods of measurement through tendon strain gauges and sonomicrometry crystals, the researchers captured data on muscle force and muscle fascicle length while horses performed different gaits (walking, trotting and cantering) on a motorized treadmill.

Findings of the Research

• The results from the tests showed that DDF(sc) fascicles shortened while producing low force and generated moderate positive work across all the gaits and speeds tested.
• On the contrary, the SDF fascicles initially shortened but then lengthened and produced high levels of force, resulting in a significant amount of negative net work.

Meaning of the Research

• The observations and results from the study shed light on the underlying in vivo functions of these two muscles during the locomotion of horses.
• The DDF(sc) with its long fibers and unipennate architecture, aids in mechanical work during running, while the shorter, multipennate SDF specializes in high force production and enhanced elastic energy storage for economical performance.
• The distinct in vivo functions match well with the different architectural features of the DDF(sc) and SDF muscles, implying that muscle architecture plays an essential role in function.