Tendons are designed to withstand considerable loads. Mechanical loading of tendon tissue results in upregulation of collagen expression and increased synthesis of collagen protein, the extent of which is probably regulated by the strain experienced by the resident fibroblasts (tenocytes). This increase in collagen formation peaks around 24 h after exercise and remains elevated for about 3 days. The degradation of collagen proteins also rises after exercise, but seems to peak earlier than the synthesis. Despite the ability of tendons to adapt to loading, repetitive use often results in injuries, such as tendinopathy, which is characterized by pain during activity, localized tenderness upon palpation, swelling and impaired performance. Tendon histological changes include reduced numbers and rounding of fibroblasts, increased content of proteoglycans, glycosaminoglycans and water, hypervascularization and disorganized collagen fibrils. At the molecular level, the levels of messenger RNA for type I and III collagens, proteoglycans, angiogenic factors, stress and regenerative proteins and proteolytic enzymes are increased. Tendon microrupture and material fatigue have been suggested as possible injury mechanisms, thus implying that one or more 'weak links' are present in the structure. Understanding how tendon tissue adapts to mechanical loading will help to unravel the pathogenesis of tendinopathy.
Tendons are metabolically active and respond readily to both loading and unloading
Mechanical loading results both in protein synthesis and degradation of collagen
Without sufficient rest (24 h) after exercise, net loss of collagen might occur that leaves the tendon vulnerable to injury
Tendinopathy is associated with neovascularization, but newly formed blood vessels (and nerves) disappear during healing
The pathogenesis of tendinopathy can be accelerated by overloading
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The authors declare no competing financial interests.
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Magnusson, S., Langberg, H. & Kjaer, M. The pathogenesis of tendinopathy: balancing the response to loading. Nat Rev Rheumatol 6, 262–268 (2010). https://doi.org/10.1038/nrrheum.2010.43
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