Tendon and Ligament

Tendons and ligaments are both made of dense, largely parallel bundles of type I collagen fiber, which is why they're often lumped together, but they connect different things and do different jobs. A tendon anchors muscle to bone and transmits the muscle's contractile force across a joint — the Achilles tendon transmitting calf-muscle force to the heel is the largest and most familiar example. A ligament connects one bone directly to another, stabilizing a joint and limiting how far it can move in a given direction, without any muscle in the chain at all — the ACL, restraining forward slide of the shin bone relative to the thigh bone, is the best-known example precisely because tearing it is common in sports involving sudden direction changes.

That functional difference explains the terminology split that trips people up: a sprain is a ligament injury (overstretched or torn), and a strain is a muscle or tendon injury — the words are not interchangeable even though casual usage often swaps them, and the distinction matters clinically because the two involve different tissue and different healing timelines.

Both tissue types share a frustrating property: they are relatively poorly vascularized compared to muscle, receiving far less blood supply, which is precisely why tendon and ligament injuries heal slowly and are prone to reinjury — repair depends on nutrients and cells arriving via blood flow, and there is comparatively little of it to work with. A muscle strain typically resolves faster than a comparable ligament sprain for exactly this reason. This same poor vascularity is why a torn ACL is frequently reconstructed with a graft rather than left to heal on its own — a full-thickness ligament tear often cannot bridge the gap through natural repair alone.