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

The researchers in this study have investigated the physical properties of muscles, tendons, and accessory ligaments in the digital flexor of horses’ forelimbs. They found that these different parts of the horse’s forelimb all play significant roles in supporting its weight, with the accessory ligaments in particular providing a significant amount of force.

Research Methods

• The experiment used eight cadaveric forelimbs from six adult Thoroughbred horses.
• The position of the limbs and lengths of the myotendinous structures (muscle-tendon complexes) were recorded before and after the accessory ligaments were cut, during an in vitro experiment. This experiment enabled the researchers to see the effect of losing the accessory ligaments on the passive force-length properties.
• The researchers derived mathematical expressions to describe the physical (force-length) behavior of each structure – the muscles, tendons, and accessory ligament-tendon units.
• The team computed linear stiffness for each of these components – essentially measuring how resistant each structure is to deformation (change in shape).
• The elastic modulus – a measure of the stiffness of an elastic material – was established for two types of tendons, the superficial digital flexor (SDF) and deep digital flexor (DDF).

Results

• The team found different values of linear stiffness for each structure. The accessory ligament of the SDF showed the greatest stiffness (6.46 kN/cm), while the DDF accessory ligament-tendon unit had the least (1.93 kN/cm).
• The elastic modulus was slightly higher for the SDF tendons (920 MPa) than for the DDF tendons (843 MPa).

Conclusion

• The study concluded that each component of the SDF and DDF myotendinous complexes – muscle, tendon, and accessory ligaments – played a significant role in bearing the load of a horse’s forelimb.
• Under the conditions tested, the accessory ligaments transmitted more force to the distal (farthest) tendons than did the muscles.
• Therefore, while muscle tension can be increased by flexing the elbow joint and actively contracting the muscle, the accessory ligaments provide more support in the conditions of the experiment.