Mechanics of tendon, from an engineering perspective

This is a review of the mechanics of tendon, with new observations on: (i) elastic modulus around zero stress and (ii) the mode of fatigue rupture. The collagenous building-block of tendon is the fibril, long and thin, and more-or-less circular in cross-section. The diameters of fibrils are well-known, but their lengths remain uncertain. The basic structural question is: do fibrils run the whole length of the tendon, or does each fibril have at least one end in the tendon? This is a simple question, with no simple answer. Difficulties in measuring elastic modulus, UTS and hysteresis stem from the problem of clamping the tendon into the test machine. This is circumvented by tests carried out in life. Fatigue is both the focus of this journal, and is also especially important for tendon. Susceptibility to fatigue is such that rupture is only avoided by ongoing repair, with each tendon being only just adequate for its habitual stresses. Fatigue rupture of tendon involves longitudinal cracks leaving long, thin interdigitating fibrous units. At the deepest level, the whole of biology involves forces acting on materials, which puts the Mechanics of Materials at the centre of Biology.