Abstract
Some tendons, such as the human Achilles and equine superficial digital flexor tendon (SDFT), act as energy stores, stretching and recoiling to increase efficiency during locomotion. Our previous observations of rotation in response to applied strain in SDFT fascicles suggest a helical structure, which may provide energy-storing tendons with a greater ability to extend and recoil efficiently. Despite this specialization, energy-storing tendons are prone to age-related tendinopathy. The aim of this study was to assess the effect of cyclic fatigue loading (FL) on the microstructural strain response of SDFT fascicles from young and old horses. The data demonstrate two independent age-related mechanisms of fatigue failure; in young horses, FL caused low levels of matrix damage and decreased rotation. This suggests that loading causes alterations to the helix substructure, which may reduce their ability to recoil and recover. By contrast, fascicles from old horses, in which the helix is already compromised, showed greater evidence of matrix damage and suffer increased fibre sliding after FL, which may partially explain the age-related increase in tendinopathy. Elucidation of helix structure and the precise alterations occurring owing to both ageing and FL will help to develop appropriate preventative and repair strategies for tendinopathy.
Highlights
Tendons provide an attachment from muscle to bone, transferring force generated by muscle contraction to the skeleton and facilitating movement
Our previous work has demonstrated that ageing results in decreased rotation in superficial digital flexor tendon (SDFT) fascicles under monotonic loading and that this is accompanied by decreased recovery and increased hysteresis [7]. These findings suggest that ageing results in alterations to the helix substructure, resulting in a decreased ability to recoil and recover, which may predispose aged tendons to injury
Our results show that in samples from young horses, levels of fibre extension were not altered by fatigue loading (FL), suggesting that fibre integrity was maintained throughout the test procedure
Summary
Tendons provide an attachment from muscle to bone, transferring force generated by muscle contraction to the skeleton and facilitating movement. The ability to withstand large unidirectional forces is provided by their structure; tendons are hierarchical fibre-composite materials, in which type I collagen molecules are grouped together, forming subunits of increasing diameter, the largest of which is the fascicle [1]. Energy-storing tendons must withstand large, repetitive stresses and strains during exercise. In the Achilles tendon, which is the predominant energy store in humans, strains in excess of 10% have been recorded during hopping exercise [13]. The energy-storing equine superficial digital flexor tendon (SDFT) has a similar function to the human Achilles [3,7,14,15] and experiences similar high strains, which can reach 16% during galloping [16]. The mechanisms that enable these high levels of extension in energy-storing tendons are yet to be determined
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