Knee flexion is the act of bending the leg, a fundamental movement required for walking, sitting, and climbing stairs. Measuring this motion, known as the range of motion (ROM), provides objective data on the health and mobility of the knee joint. For those recovering from an injury or surgery, monitoring flexion at home is a practical way to track progress between physical therapy sessions and identify changes in joint performance.
Essential Tools and Preparation for Measurement
The most precise instrument used by clinicians to measure joint angles is the goniometer, which resembles a large protractor with two movable arms. While a dedicated goniometer can be purchased, accessible alternatives exist for home measurement, such as specialized smartphone applications. These apps utilize the phone’s internal gyroscope and accelerometer to function as a digital inclinometer, measuring the angle of the lower leg as it moves.
Before starting, prepare a flat, firm surface, like a bed or the floor, to lie down comfortably. Wear loose-fitting clothing and ensure the room is well-lit. Position yourself lying flat on your back (supine position) with the leg fully extended and relaxed to establish the zero-degree starting point. Having a partner present to assist with aligning the tool and reading the measurement will yield more consistent results.
Step-by-Step Home Measurement Procedure
Accurate measurement relies on correctly identifying three specific anatomical points on the leg. Locate the lateral epicondyle, the bony prominence on the outside of the knee joint, as this spot serves as the central axis or fulcrum of the measuring device. The stationary arm of the goniometer must then be aligned with the lateral midline of the thigh, pointing upward toward the greater trochanter of the hip.
The movable arm is then aligned with the lateral midline of the lower leg, tracking the fibula and pointing down toward the lateral malleolus of the ankle. Once the tool is correctly positioned, slowly and smoothly bend the knee, bringing the heel toward the buttocks as far as is comfortable. It is important to avoid pushing past the point of noticeable pain during this active movement.
As the knee reaches its maximum bend, the partner holds the leg steady and reads the degree measurement indicated by the movable arm. Record this reading immediately, ensuring the central axis of the tool has not shifted off the lateral epicondyle during the movement. The entire process should be repeated several times to ensure the consistency of the data collected.
Interpreting Your Knee Range of Motion
A healthy, fully mobile knee achieves a flexion measurement between 135 and 140 degrees. If you are measuring a knee recovering from injury, compare the result to the reading taken from the unaffected leg, as this provides a personalized baseline for your body.
Consistency in tracking is important, so compare your current measurement against previous readings to observe any trend of improvement or decline over time. The measurement you take by moving your leg yourself is known as the active range of motion. If a partner gently assists the leg to bend further, the resulting, often slightly higher, number is the passive range of motion. Tracking both active and passive measurements can help determine if joint stiffness or muscle weakness is limiting mobility.
Knowing When to Consult a Professional
Home range of motion measurements are intended for personal tracking and should never replace professional medical guidance. If the act of measuring causes significant or sharp pain, stop immediately and seek advice from a doctor or physical therapist.
A substantial inability to achieve a functional angle, such as less than 90 degrees of flexion, especially following a surgery, warrants professional attention.
If your measurements are consistently far outside the 135-140 degree range, or if you observe a sudden, unexplained regression in mobility, discuss this with a healthcare provider. These measurements provide data points, but only a medical professional can diagnose the underlying reason for limited or excessive joint movement.