The design of soft collagenous load-bearing tissues

Tendon, articular cartilage and the human heel pad are all soft load-bearin g collagenous tissues but are designed according to utterly different micro mechanical principles. Tendon is (probably) a fibre-reinforced composite ma terial. The mechanical properties of cartilage depend on osmotic pressure d eveloped within an aqueous proteoglycan gel and resisted by tension in a co llagenous network. The micromechanics of the heel pad have not previously b een described quantitatively. Order-of-magnitude calculations are introduce d to assess a model based on a fluid-filled cushion. The processes of biolo gical design are illustrated by considering tendon. Structural design deter mines 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, St ress-in-life varies widely between tendons. Material design includes the de velopment of compressive stiffness in the regions where transverse loads ar ise. 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 o f fatigue damage, Homeostasis requires that routine repair can keep up with this rate of damage.