Normal values for knee flexion and extension, how to line up a goniometer, and how to measure from a camera or a photo.
Knee flexion is one of the most measured numbers in rehab, especially after surgery, and one of the easiest to measure well from the side. The landmarks are clear and the motion stays in one plane.
Normal knee range of motion
AAOS reference values, with the CDC average for healthy adults aged 20 to 44. Full extension is zero, and a degree or two of hyperextension is normal.
| Motion | AAOS | CDC average women / men |
|---|---|---|
| Flexion | 0–135° | 142° / 138° |
| Extension CDC values past 0° are hyperextension | 0° | 2° / 1° |
By age and sex
From the CDC Normal Joint Range of Motion Study: more than 600 healthy people measured by physical therapists with universal goniometers. Women had more range than men in nearly every joint, and range fell with age in both.
| Motion | Women 20–44 | Men 20–44 | Women 45–69 | Men 45–69 |
|---|---|---|---|---|
| Flexion | 141.9° | 137.7° | 137.8° | 132.9° |
| Extension | 1.6° | 1.0° | 1.2° | 0.5° |
How to measure knee range of motion with a goniometer
Standard goniometric alignment for both directions. A long-arm goniometer is more precise than a short-arm one at the knee.
Flexion
- Position
- Supine, hip allowed to flex
- Axis
- Lateral femoral epicondyle
- Stationary arm
- Lateral midline of the femur, toward the greater trochanter
- Moving arm
- Lateral midline of the fibula, toward the lateral malleolus
- Watch for
- Hip rotating or the thigh drifting off line
Extension
- Position
- Supine, heel supported so the knee can straighten
- Axis
- Lateral femoral epicondyle
- Stationary arm
- Lateral midline of the femur, toward the greater trochanter
- Moving arm
- Lateral midline of the fibula, toward the lateral malleolus
- Watch for
- Hip flexing to hide a flexion contracture
Measuring knee range of motion with a camera
Knee flexion reads best side-on with the camera level with the knee, either during a heel slide lying down or seated. In the photo goniometer, tap the knee, the hip and the ankle, and read Bend from straight.
- From a photo: the free online goniometer lets you tap the axis and both segments on a picture and read the angle. Nothing is uploaded.
- Live: MotionArc tracks the joints from any phone, tablet or laptop camera and measures the movement as it happens, in the clinic or on a video call.
Why two clinicians get different numbers
A goniometer reading depends on the person holding it. Finding the bony landmarks, lining up the arms and positioning the patient all add variation, and that variation is larger between two clinicians than for one clinician measuring twice.
- Small changes between two clinicians may not be real. At the shoulder, a true change measured by two different therapists needs about 15°, compared with 6° to 11° when the same therapist measures both times. Agreement between testers is weakest for shoulder extension and internal rotation.
- The instrument matters. At the knee, the smallest difference you can trust is about 6° with an inclinometer, 10° with a long-arm goniometer, 12° with a smartphone app and 14° with a short-arm goniometer or a visual estimate.
- Video can work. Shoulder angles measured over a video call with an on-screen protractor came within 0.2° to 5° of goniometer readings on average, and most smartphone apps studied show adequate reliability.
- What helps: a written protocol for patient position and landmarks, the same instrument each time, the same clinician where possible, and a note of how the measurement was taken.
Measuring from a camera takes out the hand-held step. Nobody has to line up the goniometer arms by hand, and the same view can be set up visit after visit.
Early access is limited, and we set up every account personally.
More reference values: normal range of motion chart for every joint. Other joints: Shoulder · Hip · Ankle · Neck · Low back.
Sources
- American Academy of Orthopaedic Surgeons. Joint Motion: Method of Measuring and Recording. Chicago: AAOS; 1965.
- AAOS values as tabulated in a Jones & Bartlett Learning clinical handbook, chapter 1 (sample chapter). Link
- Hendriks et al. Plastic and Reconstructive Surgery Global Open. 2020. Supplement: normal range of motion values according to the AAOS. Link
- Soucie JM, Wang C, Forsyth A, et al. Range of motion measurements: reference values and a database for comparison studies. Haemophilia. 2011;17(3):500-507. Reference values from the CDC Normal Joint Range of Motion Study. Link
- Norkin CC, White DJ. Measurement of Joint Motion: A Guide to Goniometry. 5th ed. Philadelphia: F.A. Davis; 2016.
- University of West Alabama Athletic Training. Goniometry lab: joint measurement procedures. Link
- Watkins MA, Riddle DL, Lamb RL, Personius WJ. Reliability of goniometric measurements and visual estimates of knee range of motion obtained in a clinical setting. Phys Ther. 1991. Link
- Mullaney MJ, et al. Physiother Theory Pract. 2010;26:327 (reliability of shoulder range of motion measurement). Link
- Riddle DL, Rothstein JM, Lamb RL. Goniometric reliability in a clinical setting: shoulder measurements. Phys Ther. 1987;67:668. Link
- Hancock GE, et al. J Exp Orthop. 2018 (reliability of knee range of motion measurement methods). Link
- Sahu et al. J Shoulder Elbow Surg. 2022;31:201 (shoulder range of motion by video conference versus goniometer). Link
- Keogh JWL, et al. Reliability and validity of clinically accessible smartphone applications to measure joint range of motion: a systematic review. PLOS ONE. 2019;14:e0215806. Link
Measure knee range of motion from any camera.
No goniometer, no wearables. The same movement, measured the same way, every visit.
Straight answers. If yours is not here, request access and ask us anything.
Healthy adult knees straighten fully to 0° and bend to about 135° by most AAOS tables. In a CDC study of healthy adults aged 20 to 44, average knee flexion was about 142° in women and 138° in men.
With the patient supine, center the axis on the lateral femoral epicondyle, align the stationary arm with the femur toward the greater trochanter and the moving arm with the fibula toward the lateral malleolus. A long-arm goniometer is more precise.
Yes. Take the photo side-on with the camera level with the knee, then tap the knee, hip and ankle in the free online goniometer and read Bend from straight.
Yes. MotionArc measures range of motion, form and balance from any phone, tablet or laptop camera, in the clinic or live on a video call. Early access is open to a small group of professionals.
Clinicians sign a business associate agreement (BAA) with us before any real patient data is entered. The free goniometer on this site runs in your browser and never uploads photos.