Range of motion (ROM) refers to the extent of movement possible around a joint. This measurement indicates the flexibility and mobility of the joint and surrounding soft tissues. Understanding how far a joint can move is fundamental in assessing injury, monitoring rehabilitation, and ensuring physical function. ROM is divided into two distinct types—active and passive—that reveal different information about the body’s physical capabilities. Distinguishing between these two measurements helps identify the source of movement limitation, whether it stems from muscle weakness or a structural issue within the joint.
Active Range of Motion: Voluntary Movement
Active Range of Motion (AROM) is the movement a person can achieve at a joint using their own muscle contraction. This movement relies entirely on the coordinated effort of the surrounding muscles, where one group contracts and the opposing group relaxes. Examples include lifting an arm overhead or bending a knee. AROM is a direct reflection of a person’s functional mobility and the controlled movement they can generate for daily activities.
The force driving the joint originates solely from the body’s musculature. AROM assesses the flexibility of the joint’s tissues, along with the strength and motor control of the muscles involved. Moving a joint through its full normal range indicates that the muscles are strong enough and the nervous system is coordinating the movement effectively. This measurement is used in rehabilitation to determine a patient’s ability to move independently.
Passive Range of Motion: Assisted Movement
Passive Range of Motion (PROM) is the maximum range a joint can be moved through when an external force provides the movement. The individual remains completely relaxed, contributing no muscle effort to the motion. This external force can be applied by a physical therapist, a partner, or a mechanical device, such as a continuous passive motion (CPM) machine.
Because the muscles are relaxed, PROM bypasses the need for muscle strength and motor control, assessing the structural limits of the joint instead. This movement directly reflects the extensibility of the joint capsule, ligaments, tendons, and surrounding soft tissues. A healthcare professional gently moves the limb until a natural resistance, known as the “end-feel,” is met. PROM is useful in the initial healing phases after an injury or surgery when active movement might be restricted or unsafe.
Interpreting the Discrepancy in Range of Motion
The comparison between AROM and PROM provides valuable diagnostic information about the cause of a movement restriction. For a healthy joint, the passive range of motion is typically slightly greater than the active range of motion. This difference occurs because an external force can stretch the soft tissues slightly further than a voluntary muscle contraction, which must maintain tension for stability.
A significant difference, where PROM is substantially greater than AROM, is known as the “ROM Gap” and suggests a problem with the motor unit. This large discrepancy indicates that the joint structures are flexible, but the person lacks the muscular power or control to achieve that motion voluntarily. Potential causes include muscle weakness, nerve impairment, or pain-related inhibition. Treatment in this scenario focuses on strengthening the muscles and improving motor control to close the gap.
If the AROM and PROM measurements are approximately equal, but both are significantly limited compared to expected values, it points to a structural restriction within the joint itself. The external force cannot push the joint any further than the person can move it actively, suggesting a mechanical block. This outcome is often associated with issues like joint capsule tightness, arthritis, scar tissue formation, or a bony obstruction.
These measurements are quantified using a goniometer, which is a protractor with two arms. A physical therapist aligns the goniometer’s axis over the joint, using the arms to measure the angle of movement in degrees. Measuring both AROM and PROM with this device helps the healthcare provider establish a diagnosis, create a targeted treatment plan, and track the patient’s progress.