Knee flexion is the bending motion of the knee joint, which decreases the angle between the back of the thigh and the calf. This movement is a component of the overall knee Range of Motion (ROM), which also includes extension, the act of straightening the leg. Measuring knee flexion is important for assessing joint health, especially when tracking recovery from injury, surgery, or chronic conditions like osteoarthritis. Accurate measurement allows health professionals to establish a baseline for mobility and objectively monitor a patient’s progress.
The Standard Tool for Measurement
The instrument most commonly used to measure joint angles in a clinical setting is the universal goniometer. This device acts like a large protractor, providing a reliable and standardized method for quantifying the degrees of movement at a joint. The goniometer is composed of three primary parts: the body, the stationary arm, and the moving arm.
The body resembles a semicircle or a full circle marked with an angular scale. Its center point, the fulcrum or axis, must be aligned with the joint’s axis of rotation. The stationary arm remains fixed, aligning with the non-moving segment of the limb. The moving arm rotates freely around the fulcrum and tracks the movement of the limb’s distal segment. The long-arm universal goniometer is the accepted clinical standard for large joints like the knee due to its accuracy and reliability.
Preparing the Patient and Identifying Landmarks
Before taking any measurement, the patient must be positioned correctly to isolate the knee joint movement and ensure accuracy. A common position for measuring active knee flexion is having the patient lie face down, or prone, on a firm surface, keeping the hip neutral. This helps prevent compensatory movements, such as hip flexion or rotation, that could skew the reading. Alternatively, the patient may be seated with the leg hanging off the edge of a table.
Once positioned, three specific anatomical landmarks for goniometer alignment must be identified. The axis of the goniometer is aligned over the lateral femoral epicondyle, a bony prominence on the outside of the knee. The stationary arm is aligned parallel to the longitudinal axis of the femur, pointing toward the greater trochanter of the hip. The moving arm is aligned parallel to the midline of the fibula, pointing toward the lateral malleolus, the ankle bone on the outside of the lower leg.
Executing the Measurement Procedure
The actual measurement begins by stabilizing the patient’s thigh to ensure that only the knee joint moves. The examiner places the fulcrum of the goniometer directly over the lateral femoral epicondyle, which serves as the axis of rotation. The stationary arm is fixed along the lateral midline of the femur. This alignment establishes the zero-degree starting position, where the knee is fully extended.
The patient is instructed to actively bend their knee, bringing their heel toward their buttocks as far as they can comfortably go. As the patient moves, the examiner follows the motion with the moving arm of the goniometer. The moving arm must remain aligned with the lateral midline of the fibula, pointing toward the lateral malleolus, to track the lower leg’s movement. Once the patient reaches their maximum range of motion, the movement is held steady. The final number is read directly from the goniometer’s scale.
Understanding Range of Motion Results
The numerical result represents the maximum degree of knee flexion the patient achieved. A healthy adult knee exhibits a full range of motion for flexion between 135 and 150 degrees, though a functional range of 0 to 120 degrees is sufficient for most daily activities. The measurement is documented as a range, such as 0 to 135 degrees, indicating the patient can fully straighten their leg and bend it to 135 degrees.
A result significantly lower than the expected range suggests restricted movement, which may be due to swelling, muscle tightness, or joint pathology. Monitoring these measurements over time provides objective data on the effectiveness of rehabilitation and a patient’s progress toward regaining mobility.