Cartilage

Cartilage is firm but flexible connective tissue. The articular cartilage capping bone ends in a joint is remarkably low-friction, distributes load and absorbs shock. It contains no blood vessels and is nourished by the movement of fluid within the joint.

It matters because that lack of blood supply means damage repairs poorly, and its progressive loss is the defining feature of osteoarthritis. But the widespread belief that exercise wears it out has it backwards: cartilage depends on cyclical loading to be nourished, and inactivity is worse for it than activity.

In practice, running has not been shown to increase knee osteoarthritis risk in recreational runners — cohort data generally find lower rates than in sedentary controls — and exercise is a first-line treatment for existing osteoarthritis, not a risk to be avoided.