Abstract
Tendons can broadly be categorized according to their function: those that act purely to position the limb and those that have an additional function as energy stores. Energy-storing tendons undergo many cycles of large deformations during locomotion, and so must be able to extend and recoil efficiently, rapidly and repeatedly. Our previous work has shown rotation in response to applied strain in fascicles from energy-storing tendons, indicating the presence of helical substructures which may provide greater elasticity and recovery. In the current study, we assessed how preconditioning and fatigue loading affect the ability of fascicles from the energy-storing equine superficial digital flexor tendon to extend and recoil. We hypothesized that preconditioned samples would exhibit changes in microstructural strain response, but would retain their ability to recover. We further hypothesized that fatigue loading would result in sample damage, causing further alterations in extension mechanisms and a significant reduction in sample recovery. The results broadly support these hypotheses: preconditioned samples showed some alterations in microstructural strain response, but were able to recover following the removal of load. However, fatigue loaded samples showed visual evidence of damage and exhibited further alterations in extension mechanisms, characterized by decreased rotation in response to applied strain. This was accompanied by increased hysteresis and decreased recovery. These results suggest that fatigue loading results in a compromised helix substructure, reducing the ability of energy-storing tendons to recoil. A decreased ability to recoil may lead to an impaired response to further loading, potentially increasing the likelihood of injury.
Highlights
Tendons provide the attachment from muscle to bone, facilitating movement of the limbs during locomotion
Our previous work has demonstrated relatively low levels of fibre sliding in the energy-storing SDFT; extension in this tendon type appears to be governed by unwinding of helical substructures, indicated by sample rotation, a mechanism that we propose provides greater elasticity [9]
Previous work has indicated the presence of helical substructures within fascicles from energy storing tendons, which are associated with a greater ability for fascicles to elastically stretch and recover [9]
Summary
Tendons provide the attachment from muscle to bone, facilitating movement of the limbs during locomotion. To store and release sufficient energy in a useable form, these tendons need to be more elastic than tendons with a purely positional function [3, 4]. Such differences in mechanical properties between tendon types must be conferred by differences in structural organisation and composition. All tendons can be considered as hierarchical fibre-composite materials, in which type I collagen molecules are grouped together in a highly ordered fashion, forming subunits of increasing diameter [5], the largest of which is the fascicle. While the basic structure of all tendons is similar, numerous studies have documented structural and compositional differences between energy-storing and positional tendons [7,8,9,10,11,12]
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