Knee flexion is the act of bending the knee joint, reducing the angle between the femur (thigh bone) and the tibia (shin bone). Measuring this motion is important for understanding joint mobility, tracking recovery following injury or surgery, and assessing overall joint health. The resulting measurement, known as the range of motion (ROM), provides objective data used by healthcare professionals to guide treatment plans and set functional goals.
The Required Tool: The Goniometer
The standard instrument for accurately quantifying joint angles is the goniometer, a device designed specifically for measuring range of motion. Unlike visual estimation, the goniometer provides a quantifiable degree measurement that is reproducible for consistent tracking over time. The universal goniometer, often a clear plastic model with long arms, is preferred for the knee joint as its large protractor body and long arms align well with the leg segments.
The instrument consists of three main parts: a body or protractor, a stationary arm, and a moving arm. The body is marked with a 360-degree scale for reading the angle. The stationary arm is fixed, while the moving arm pivots around the central point, or fulcrum, tracking the limb segment’s motion. This structure allows the practitioner to align the device with bony landmarks for precise measurement.
Step-by-Step Measurement Technique
Accurate measurement begins with proper positioning, typically with the patient lying on their back (supine position). The leg should be straight and relaxed, with the hip positioned neutrally, allowing the knee to start at zero degrees of extension. This starting position helps standardize the measurement and isolate movement to the knee joint.
The next step involves identifying three specific bony landmarks that serve as reference points for alignment. The goniometer’s fulcrum is placed directly over the lateral femoral epicondyle, the bony prominence on the outside of the knee joint. This point approximates the knee’s rotational axis.
The stationary arm is aligned with the lateral midline of the femur, pointing toward the greater trochanter, the large bony bump on the outer side of the hip. This arm establishes the fixed reference of the thigh segment. The moving arm is aligned with the lateral midline of the fibula, pointing toward the lateral malleolus, the prominent bone on the outside of the ankle.
Once aligned, the patient is instructed to bend their knee, drawing their heel toward their buttocks as far as possible. During this movement, the practitioner must maintain the goniometer’s axis over the lateral epicondyle and keep the arms aligned with the established landmarks. After the patient reaches maximum flexion, the final angle is read directly from the protractor scale.
What the Measurements Indicate
The numerical reading provides context for the patient’s joint mobility compared to expected values. For a healthy adult, the normal range for knee flexion is approximately 0 to 135-140 degrees. This range can vary depending on factors like age, sex, and physical activity level, with some individuals reaching up to 150 degrees.
The measurement can be taken in two ways: active range of motion (AROM) or passive range of motion (PROM). AROM is the angle achieved when the patient moves the joint independently using their own muscles. PROM is the angle achieved when an examiner gently moves the joint through its range without the patient’s muscular effort.
Comparing AROM and PROM is valuable for diagnostic purposes. If AROM is less than PROM, it may suggest muscle weakness or pain is limiting movement. Equal limitation in both suggests a structural issue, such as swelling or capsular tightness. Tracking these numbers monitors post-surgical recovery and determines if the patient is achieving the functional mobility needed for daily activities, which often requires at least 110 degrees of flexion.