The implications of the adaptable fatigue quality of tendons for their construction, repair and function.

Different tendons are (i) subject to very different stresses from their muscles and (ii) differ in their susceptibility to fatigue damage. The fatigue quality of each tendon is matched to the stress it experiences, so that, in life, all tendons are similarly prone to damage. On-going damage must be routinely repaired to maintain homeostasis and prevent damage from becoming symptomatic. The discovery of major differences in fatigue quality among tendons, which had previously seemed fairly similar in their mechanical properties, raises a wide range of new questions. (A) What structural and chemical differences underlie the variations in fatigue quality? (B) What molecular structure in the tendon is damaged and how is repair organised? (C) Is fatigue quality adaptable and if so what is the trigger for adaptation? Putting these questions into context leads to an integrated review of tendon, including structure and chemistry, the turnover of proteins, the cross-linking of collagen and the response of tenocytes to load on the tendon.