The Range of Motion (ROM) of the knee refers to the extent a joint can move, measured in degrees of a circle. This measurement is a reliable indicator of the overall health and functional capacity of the knee joint. The ability to move the knee fully is directly related to performing daily tasks, such as walking, sitting, and climbing stairs. ROM provides a baseline for identifying potential limitations or joint issues.
Defining Normal Knee Movement
A healthy, uninjured knee joint is capable of two primary movements: flexion and extension. Flexion is the action of bending the knee, bringing the heel toward the buttocks, while extension is the action of straightening the leg. For most adults, the maximum degree of knee flexion ranges between 135 and 150 degrees. Achieving at least 135 degrees of flexion is generally considered sufficient for most functional activities, such as standing up from a chair or walking.
Normal knee extension is defined as zero degrees. This zero-degree mark represents a straight line formed by the femur and tibia bones. Some individuals, particularly younger people, may exhibit a slight ability to straighten the knee beyond the zero-degree mark. This movement is known as hyperextension, and a range of up to 5 to 10 degrees is considered a normal variation.
How Range of Motion is Measured
The most common instrument used for assessment is the goniometer, which is a specialized protractor with two movable arms. The goniometer’s axis is placed over the knee joint, and its arms are aligned with specific bony landmarks on the thigh and lower leg. Measurements are recorded in degrees, providing an objective numerical value for both flexion and extension.
The assessment process distinguishes between Active ROM and Passive ROM. Active ROM measures the extent of movement a person can achieve independently by contracting their own muscles. Passive ROM involves the examiner gently moving the joint further, without the person’s muscle involvement, to determine the maximum possible movement. Passive ROM is typically slightly greater than Active ROM because it bypasses any muscle guarding or weakness.
Factors Influencing Knee Mobility
Age is a common variable, as joint flexibility and muscle elasticity naturally decline over the years. Older adults often show a gradual reduction in maximum flexion compared to younger individuals.
Previous injuries, such as ligament tears or fractures, can also significantly restrict movement. The formation of scar tissue following a serious injury or surgery may physically limit the joint’s ability to move through its full arc. Furthermore, underlying medical conditions like osteoarthritis cause inflammation, pain, and stiffness, which actively reduce the joint’s capacity for movement.
Maintaining and Improving Knee ROM
Maintaining a consistent level of physical activity is important for preserving the knee’s range of motion over time. Low-impact exercises, such as walking, swimming, or cycling on a stationary bike, help lubricate the joint and keep surrounding muscles flexible. Regular movement prevents the stiffness that occurs when the joint remains inactive for long periods.
Specific stretches and mobility exercises target the muscles that influence the knee joint, including the quadriceps and hamstrings. Movements like heel slides, where the heel is slid toward the buttocks while lying down, are simple yet effective ways to encourage flexion. Another exercise involves propping the heel up to allow gravity to assist in achieving full extension. If a person experiences severe pain or a significant, persistent limitation in movement, consulting a physician or physical therapist is advisable. A physical therapist can provide guided, progressive exercises and techniques to safely restore lost mobility.