How stretching keeps your joints moving

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 joint and the structures surrounding it. Stretching exercises can help extend this range of motion. To understand how, it helps to be familiar with these structures and how they can help — or hinder — a joint's flexibility:

• 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.
• Muscles surround joints and provide the energy used to move them. The amount of tension in the muscles surrounding a joint is a key factor in how big of a range of motion that joint can achieve. Muscle tension can be affected both by passive factors, such as tissue scarring or your habitual posture, and by active factors, such as involuntary muscle spasms or purposeful muscle contractions.
• Tendons are flexible cords of strong tissue that connect muscles to bones and make movement possible. When a joint moves, energy from the muscles is transferred into the tendons, which tug on the 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.