The design of soft collagenous load-bearing tissues.

Tendon, articular cartilage and the human heel pad are all soft load-bearing collagenous tissues but are designed according to utterly different micromechanical principles. Tendon is (probably) a fibre-reinforced composite material. The mechanical properties of cartilage depend on osmotic pressure developed within an aqueous proteoglycan gel and resisted by tension in a collagenous network. The micromechanics of the heel pad have not previously been described quantitatively. Order-of-magnitude calculations are introduced to assess a model based on a fluid-filled cushion. The processes of biological design are illustrated by considering tendon. Structural design determines the tendon's cross-sectional area relative to that of its muscle and, hence, the maximum stress to which the tendon may be subjected in life. Stress-in-life varies widely between tendons. Material design includes the development of compressive stiffness in the regions where transverse loads arise. More generally, the fatigue quality of each tendon is adjusted to suit its stress-in-life. The correlation between fatigue quality and stress-in-life means that every tendon is subject, on average, to a comparable rate of fatigue damage. Homeostasis requires that routine repair can keep up with this rate of damage.