What is the human body's largest joint?

The knee, and it is not the simple hinge it looks like. It rolls, glides and rotates slightly as it bends, the contact point moving backwards through the movement, which is why it is so hard to replicate mechanically and why knee replacements took decades longer to get right than hips.

Its stability comes almost entirely from soft tissue rather than from bone. A hip is a ball in a deep socket and is stable by shape; a knee is essentially a rounded end resting on a nearly flat plateau, held together by four ligaments and two cartilage wedges. That is why a twist can wreck it, why the anterior cruciate ligament is the most famous injury in sport, and why it happens disproportionately to women - a wider pelvis changes the angle of load on the joint.

It also takes forces far above body weight. Going downstairs loads the knee at several times what you weigh, and running considerably more, which is the arithmetic behind advice to lose weight for knee pain: a kilogram off is several kilograms per step. The joint is a compromise between the stability of a hinge and the mobility of a ball, and it fails at the compromise more often than any other joint in the body.