Tendons transfer force from muscle to bone. Specific tendons, including the equine superficial digital flexor tendon (SDFT), also store and return energy. For efficient function, energystoring tendons need to be more extensible than positional tendons such as the common digital extensor tendon (CDET), and when tested in vitro have a lower modulus and failure stress, but a higher failure strain. It is not known how differences in matrix organization contribute to distinct mechanical properties in functionally different tendons. We investigated the properties of whole tendons, tendon fascicles and the fascicular interface in the highstrain energy-storing SDFT and low-strain positional CDET. Fascicles failed at lower stresses and strains than tendons. The SDFT was more extensible than the CDET, but SDFT fascicles failed at lower strains than CDET fascicles, resulting in large differences between tendon and fascicle failure strain in the SDFT. At physiological loads, the stiffness at the fascicular interface was lower in the SDFT samples, enabling a greater fascicle sliding that could account for differences in tendon and fascicle failure strain. Sliding between fascicles prior to fascicle extension in the SDFT may allow the large extensions required in energy-storing tendons while protecting fascicles from damage.

Age-related tendinopathy is common in both humans and horses; the initiation and progression of which is similar between species. The majority of tendon injuries occur to high-strain energy storing tendons, such as the human Achilles tendon and equine superficial digital flexor (SDFT). By contrast, the low-strain positional human anterior tibialis tendon and equine common digital extensor (CDET) are rarely injured. It has previously been established that greater extension occurs at the fascicular interface in the SDFT than in the CDET; this may facilitate the large strains experienced during locomotion in the SDFT without damage occurring to the fascicles. This study investigated the alterations in whole tendon, fascicle and interfascicular mechanical properties in the SDFT and CDET with increasing age. It was hypothesised that the amount of sliding at the fascicular interface in the SDFT would decrease with increasing horse age, whereas the properties of the interface in the CDET would remain unchanged with ageing. Data support the hypothesis; there were no alterations in the mechanical properties of the whole SDFT or its constituent fascicles with increasing age. However, there was significantly less sliding at the fascicular interface at physiological loads in samples from aged tendons. There was no relationship between fascicle sliding and age in the CDET. The increase in stiffness of the interfascicular matrix in aged SDFT may result in the fascicles being loaded at an earlier point in the stress strain curve, increasing the risk of damage. This may predispose aged tendons to tendinopathy.

Little is known about the rate at which protein turnover occurs in living tendon and whether the rate differs between tendons with different physiological roles. In this study, we have quantified the racemization of aspartic acid to calculate the age of the collagenous and non-collagenous components of the high strain injury-prone superficial digital flexor tendon (SDFT) and low strain rarely injured common digital extensor tendon (CDET) in a group of horses with a wide age range. In addition, the turnover of collagen was assessed indirectly by measuring the levels of collagen degradation markers (collagenase-generated neoepitope and cross-linked telopeptide of type I collagen). Tendons play a key role in locomotion by providing the mechanical link between muscle and bone. High mechanical strength is an important prerequisite as stresses of up to 50 MPa (1) can be imposed on the tendon, and this is provided largely by the highly organized collagen component. More specialized tendons, such as the Achilles tendon in humans and the superficial digital flexor tendon (SDFT) 2 in horses, in addition to positioning the limb play a vital role in energy storage and release thereby increasing the efficiency of locomotion by up to 36% (2). An appropriate compliance is required by the energystoring tendons to allow stretching and recoil to occur at a rate in keeping with the gait cycle. These energy-storing tendons have a higher non-collagenous protein content, predominately proteoglycan (3), which is thought to allow sliding movement between collagen fibrils (4). The fractional increase in D-Maintenance of both tendon material and structural properties is essential for function, and this is achieved by a balance between matrix synthesis and degradation. Recent studies have demonstrated that, contrary to previous thinking, collagen turnover in patellar tendons occurs at a rate comparable to that of collagen in metabolically active tissues such as muscle (5). However, degenerative changes are a common finding, and these changes are more frequent in older aged individuals and in specific tendons (6).Energy-storing tendons are subjected to much higher stresses and strains than tendons that are designed predominantly for limb placement (positional tendons) and therefore might be expected to experience higher levels of micro-damage thus requiring a greater capacity for matrix turnover. Unexpected findings from our previous work (3), however, suggest that the matrix of the high strain equine SDFT is turned over more slowly than in the low strain positional common digital extensor tendon (CDET). This conclusion was based upon a simple measurement of tissue-associated fluorescence; longlived proteins such as collagen are subjected to age-related glycation and subsequent spontaneous formation of advanced glycation end-products (AGEs), some of which fluoresce naturally. However, enzymatically derived cross-links, such as hydroxylysyl pyridinoline (HP) and lysyl pyridinoline (LP), which do not accumulate with age in mature equine tend...