Abstract
Introduction
Whilst all tendons connect muscle to bone, energy storing (ES) tendons, such as the equine superficial digital flexor tendon (SDFT) play an additional role, storing energy to improve locomotion efficiency. ES tendons experience significantly higher strains during locomotion than other positional tendons, such as the common digital extensor tendon (CDET). Our previous work has demonstrated that the interfascicular matrix (IFM) is more extensible in ES tendons, allowing ES tendons to stretch further during use. However, ES tendons must also recoil efficiently to perform their energy storing function. It has not been yet established if the IFM is able to recoil and recover after loading. Thus, this project aimed to determine the recoil capacity of the IFM in both the ES and positional tendons from young and old horses.
Materials and Methods
Five young (3–7 years) and five old (17–20 years) SDFTs and CDETs were dissected from the forelimbs of 10 euthanized horses. Groups of 2 intact fascicles (bounded by IFM) were dissected from each tendon. Using a custom-made dissection rig and a polarised light microscope, samples were dissected, and the opposing end of each fascicle was cut transversely, leaving a 10 mm length of IFM. IFM samples were tested in shear, by preconditioning with 10 loading cycles then pulling to failure. The hysteresis and stress relaxation that occurred during preconditioning were calculated.
Results
The IFM was able to recoil in both SDFT and CDET tendons. However, hysteresis and stress relaxation were both significantly higher in CDET than SDFT IFM samples. The SDFT IFM was less stiff than the CDET IFM, but the SDFT IFM stiffness increased with ageing.
Discussion
The IFM showed reversible deformation behaviour, with a greater ability to recoil in the SDFT. Changes with ageing were only evident in the SDFT, where the IFM became less fatigue resistant and stiffer. These results further indicate that ES tendons have a specialised IFM to facilitate efficient function, and changes in the mechanical properties of this matrix with ageing may predispose these tendons to injury.
