Extension is a physical position that decreases the angle between the bones of the limb at a joint. It occurs when muscles contract and bones move the joint into a bent position. The opposite movement, flexion, bends the joint so that the joint angle shortens.
Each joint has a normal range of motion. However, the amount of movement in a joint varies greatly between individuals. Joints maintain a balanced range of motion by regular use and stretching of the surrounding soft tissues (muscles, tendons and ligaments).
Injuries to the soft tissues surrounding a joint often reduce range of motion due to swelling and tissue damage. Regaining range of motion in a joint is one of the first phases of injury rehabilitation, and physical therapists often prescribe specific ROM exercises for each joint.
Also see: Limited range of motion
Physical Therapy (PT). Merck Manual Professional. November 2005. http://www.merck.com/mmpe/sec22/ch336/ch336b.html