Ligament

Ligaments are bands of collagen connecting bones across a joint. Their job is passive stability: limiting the range and direction of movement so the joint stays where it should. Unlike tendons they do not attach to muscle and do not transmit voluntary force.

It matters because ligaments are even less well supplied with blood than tendons, so injuries to them heal slowly and sometimes incompletely. The anterior cruciate ligament is the well-known case, with poor intrinsic healing and frequent surgical reconstruction.

In practice you cannot strengthen a ligament the way you strengthen a muscle, but you can protect it. Building strength in the muscles crossing a joint and training proprioception and landing mechanics measurably reduces ligament injury rates, particularly for the ACL.