Your range of motion — how far you can move a joint in various directions — is determined by many things, starting with the inner workings of the joints involved. Also important is the amount of tension in the muscles surrounding the joint, which can be affected by scarring or your habitual posture (passive factors), or by involuntary muscle spasms or purposeful muscle contractions (active factors).
Stretching exercises can help extend your range of motion. To understand how, it helps to know what joints, tendons, and ligaments do:
Joints are the junctions that link bones together. The architecture of each joint — that is, whether its structure is a hinge, pivot, or ball-in-socket — determines how the bones can move.
Tendons are flexible cords of strong tissue that connect muscles to bones.
Ligaments are tough, fibrous bands of tissue that bind bone to bone, or bone to cartilage, at a joint. An example is the anterior cruciate ligament (ACL), one of five ligaments that together control the movements of the knee. Among other things, the ACL keeps the knee joint from rotating too far.
When you stretch, you're working muscles and tendons rather than ligaments. Ligaments are not supposed to be elastic. An overly stretchy ligament wouldn't provide the stability and support needed for a safe range of movement.
For more on ways to improve your strength and flexibility, buy Stretching, a Special Health Report from Harvard Medical School.