range of motion
/ ROM (say each letter: R-O-M) /
A door can swing wide open, or it can be jammed so it only opens a crack. Range of motion is exactly that idea for a joint: how far it can travel from one end of a movement to the other. A knee that bends fully and straightens fully has good range; a knee stuck part-way has lost range, and that lost arc is often what stops someone kneeling, climbing stairs, or even sitting comfortably.
Range of motion is the amount of movement available at a joint, measured in degrees and named for the movement (for example, knee flexion of 0 to 130 degrees). It comes in two important kinds. Active range is how far you can move the joint using your own muscles. Passive range is how far someone else can move it for you while you relax — and passive range is normally a little greater than active. The gap between them is diagnostic: if passive range is full but active range is short, the joint itself is fine and the problem is muscle or nerve (the joint can move, but you can't move it); if even passive range is blocked, something in the joint or soft tissue is physically limiting it, such as a contracture, swelling, or pain. Clinicians measure ROM with a goniometer, a simple protractor for joints.
Range is one of the first things rehabilitation protects and restores, because it is far easier to keep than to win back. When a joint is held still by a cast, paralysis, or spasticity, the surrounding tissues shorten and the joint can become permanently stiff — a contracture — sometimes within weeks. This is why gentle range-of-motion exercise begins so early, even in someone who cannot move themselves: maintaining range now prevents a fixed deformity later. The honest limit is that more range is not always the goal — some stability or even a degree of tightness can be functionally useful, so the aim is the right range for the task, not the maximum possible.
A patient cannot lift their arm overhead. The therapist relaxes the arm and moves it passively — it goes all the way up. Passive range is full but active range is not, so the joint is healthy; the problem is muscle or nerve weakness, and the plan is strengthening, not stretching.
Full passive but limited active range points to muscle or nerve, not the joint.
More range is not always better: a degree of tightness or stability can be functionally useful, so the goal is the range a task needs, not the maximum the joint could theoretically reach.