Tendons connect muscle to bone; ligaments connect bone to bone.

A contracting muscle produces a pull. A tendon transmits that force to a bone, allowing the muscle to move the skeleton rather than merely shorten in place. The Achilles tendon, which links the calf muscles to the heel bone, is a familiar example.

Ligaments span between bones and help guide or limit their relative movement. At a joint, they contribute stability without turning the joint into a rigid connection. Their position and tension help determine which movements are permitted and which are restrained.

Both tissues contain abundant collagen, but sharing a building material does not make their roles interchangeable. A tendon belongs to the force-transmission pathway from muscle to skeleton; a ligament belongs to the system that holds and guides the skeletal connection.

The National Institute of Arthritis and Musculoskeletal and Skin Diseases describes a joint as a place where two or more bones meet. Bones themselves do not bend to produce ordinary limb movement. Instead, motion occurs at their connections, with muscles supplying force and surrounding tissues controlling how that force moves the bones.

At a freely movable, or synovial, joint, cartilage covers the ends of the adjoining bones and reduces friction. The synovial membrane produces fluid that lubricates the joint. Ligaments connect the bones, and tendons attach the muscles that move them. These structures cooperate, but lubrication, force transmission, and stabilization are separate functions.

The knee illustrates the distinction. Muscles acting across the knee move the lower leg through their tendon connections, while ligaments help keep the bones aligned during movement. Articular cartilage provides a smoother contact surface. Pain near a knee therefore does not, by itself, identify whether a tendon, ligament, cartilage surface, or another structure is involved.

The distinction also appears in injury vocabulary. A sprain involves a stretched or torn ligament. A strain involves a muscle or tendon. Either can cause pain and swelling, so the labels describe the injured tissue rather than a reliable ranking of how painful or serious the injury must be.

Not every joint permits the same movement. The hip and elbow are freely movable joints, while connections in the spine allow more limited movement. Some skull connections are essentially immovable in adulthood. The tendon-versus-ligament comparison is most useful when considering how muscles act around movable joints.

Keeping joints healthy involves more than making muscles stronger. NIAMS recommends physical activity, maintaining a healthy weight, and using protective equipment where appropriate. Persistent pain, substantial swelling, or difficulty using a joint deserves assessment rather than a diagnosis based only on which tissue seems nearest the painful spot.

The NIAMS joint lesson separates the structures that supply force, guide movement, and reduce friction. That functional map is more useful than judging tissues by appearance alone. Tendons and ligaments can look similar because both contain collagen, but their attachments place them in different mechanical relationships. A joint needs coordinated movement and restraint, not merely a collection of equally strong connecting bands.