Determination of passive mechanical properties of the superficial and deep digital flexor muscle-ligament-tendon complexes in the forelimbs of horses

Abstract

Objective - To determine the relative contributions of the muscles, tendons, and accessory ligaments to the passive force-length properties of the superficial (SDF) and deep digital flexor (DDF) myotendinous complexes. Sample Population - 8 cadaveric forelimbs from 6 adult Thoroughbreds. Procedure - In vitro, limb configurations during slack position and myotendinous lengths during subsequent axial loading of forelimbs were recorded before and after transection of accessory ligaments. Expressions were derived to describe the force-length behavior of each muscle, tendon, and accessory ligament-tendon unit; linear stiffness was computed for these components. The elastic modulus was established for the SDF and DDF tendons. Results - Linear stiffness was 2.80 ± 0.38 kN/cm for the SDF muscle, 3.47 ± 0.66 kN/cm for the DDF muscle, 2.73 ± 0.18 kN/cm for the SIDF tendon, 3.22 ± 0.20 kN/cm for the DDF tendon, 6.46 ± 0.85 kN/cm for the SDF accessory ligament, 1.93 ± 0.11 kN/cm for the SDF accessory ligament-tendon unit, and 2.47 ± 0.11 kN/cm for the DDF accessory ligament-tendon unit. The elastic modulus for the SDF and DDF tendons was 920 ± 77 and 843 ± 56 MPa, respectively. Conclusions and Clinical Relevance - Both the muscle-tendon and ligament-tendon portions of SDF and DDF myotendinous complexes had important roles in supporting the forelimb of horses. Although muscle tension can be enhanced by elbow joint flexion and active contraction, the accessory ligaments transmitted more force to the distal tendons than did the muscles under the conditions tested.