INTRODUCTION

Tendon is a hierarchically structured collagenous tissue which has outstanding mechanical properties. A sketch of the stress-strain curve is shown in Figure 9.1. Most remarkably, the stiffness increases with strain up to an elastic modulus in the order of 1 to 2 GPa. Moreover, tendons are viscoelastic, and their deformation behaviour depends on the strain rate, as well as on the strain itself. In vivo, it is very likely that tendons are always somewhat prestrained (even if the muscles are at rest); hence, they are normally working in the intermediate (‘heel’; see Figure 9.1) and high modulus regions (Vincent, 1990). In this context, it is also interesting to compare the maximum stress generated in muscle [in the order of 300 kPa (Abe et al., 1996)] to the strength of tendon which is about 300 times larger. This explains why tendons and ligaments can be much thinner than muscle. Obviously, the remarkable mechanical properties of the tendons are linked to their complex hierarchical structure (Figure 9.2).

This chapter reviews some of the well-known (Diamant et al., 1972; Kastelic and Baer, 1980; Mosler et al., 1985; Sasaki et al., 1996, 1999; Misof et al., 1997; Fratzl et al., 1997) and more recently discovered (Puxkandl et al., 2001) structural principles giving rise to the mechanical behaviour of tendons.

DEFORMATION MECHANISMS OF COLLAGEN FIBRILS

The stress-strain curve of tendons usually shows three distinct regions (Vincent, 1990) which can be correlated to deformations at different structural levels (Figure 9.3).