Range of motion
Video goniometer: joint angles from video
A goniometer is a protractor with two long arms: line them up with the segments either side of a joint and read the angle. A video goniometer does the same job from a recording — and, done automatically, it does it for every joint on every frame, so you get the whole movement rather than one held position.
What a goniometer measures
The pivot sits over the joint, one arm along each segment, and the reading is the angle between them. It is quick, cheap and the everyday standard for range of motion. It is also a measurement with its own error: where the arms are placed, how the limb is positioned and who is holding it all move the number, which is why reliability between testers is studied as carefully as the instrument itself.
Two limits come with the method. It reads a joint while it is held still, so the angle at the bottom of a squat or a pedal stroke has to be reproduced statically. And it reads one joint at a time, so the hip and the knee in the same instant are two separate measurements.
Three kinds of goniometer app
| Kind | How it works | Reads | Good at |
|---|---|---|---|
| Photo protractor | You take a photo or pause a video and place the lines yourself | One joint, one frame | A quick static check with a record of it |
| Inclinometer | The phone's motion sensors measure its tilt, with the phone held or strapped to the limb | One joint, live | Movements the camera can't see well, such as rotation |
| Automatic pose tracking | A pose model finds the body in every frame and the angles are computed from it | Every joint, every frame | Movement as it happens, and comparing clips over time |
What a video adds
- The movement, not a pose. The angle at the real bottom of a squat, the real top of an arm raise, measured while it happens rather than reconstructed afterwards.
- All the joints at once. Fourteen angles on the same frame, so you can see the knee bend and the trunk lean together.
- Minimum, maximum and range. Taken from every frame of the movement, not from the moment someone chose to hold still.
- A record you can check. The skeleton is drawn over the video, so anyone can see what was measured — and measure it again later, the same way.
- Left against right, and today against last month. The same clip setup gives numbers that compare like with like.
How Markerless works as a video goniometer
- Film the movement on a phone, square-on to the plane you care about — side-on for bending and straightening, front-on for raising out to the side.
- Upload it in the browser. A pose model finds 33 points on the body in every frame.
- Check the skeleton drawn over the video before you read a number.
- Read fourteen joint angles frame by frame, with the minimum, maximum and range of each — and download them as CSV or JSON.
Joint by joint, how each angle is made and how to film it: shoulder, elbow, wrist, hip, knee, ankle, neck and low back. The shoulder abduction example shows a finished one.
Reading the numbers like a goniometer
A goniometer calls a straight knee 0°. Markerless reports the angle between the two segments, so a straight knee reads 180° and a knee bent 40° reads 140° — the distance from 180° is the goniometer's figure. The neutral position is not always a straight line, so range of motion by joint gives the conversion for each, alongside published reference values.
What a camera cannot do
- See out of its plane. A limb moving towards the lens reads short. Rotation — turning the forearm, the hip turning in or out — is mostly invisible to a single camera.
- Feel for bony landmarks. A clinician places a goniometer on landmarks they can palpate; a pose model estimates joint centres from the image. The two are not quite the same measurement.
- Replace a validation study. Markerless has not published one against a goniometer or marker-based capture. Its angles are best used as a consistent measure — the same person, filmed the same way, over time.
- Diagnose. A number from a video is a measurement. What it means for a particular person is a clinician's judgement.
Questions
Is there a goniometer app that works from video?
Yes. Some let you pause a video and place the lines by hand; others, Markerless among them, track the body automatically and measure the angles on every frame. Markerless runs in the browser, so it works from a phone or a computer with a clip you have already filmed.
Is a goniometer app as accurate as a goniometer?
It depends on the app, the joint and the filming, and a goniometer has its own error too. Published testing of smartphone apps varies by app and joint. Markerless has not published a validation study, so treat its angles as a consistent measure of what the camera saw rather than a clinical absolute.
Can I measure range of motion from a video I already have?
If the joint is in view and the camera is roughly square-on to the movement, yes. Upload the clip and every frame is measured. Footage filmed at a steep angle to the movement will read short.
How much does it cost?
Plans start at $29 a month, with range of motion, tags, people and shareable links on every plan. Side-by-side comparison and smoothing filters come with Pro.
References
Gajdosik, R. L., & Bohannon, R. W. (1987). Clinical measurement of range of motion. Physical Therapy, 67(12), 1867–1872.
A review of goniometry with the emphasis on its reliability and validity: a goniometer reading is itself a measurement with its own error, not a ground truth to compare everything else against.
doi:10.1093/ptj/67.12.1867
Keogh, J. W. L., Cox, A., Anderson, S., Liew, B., Olsen, A., Schram, B., et al. (2019). Reliability and validity of clinically accessible smartphone applications to measure joint range of motion: A systematic review. PLOS ONE, 14(5), e0215806.
A systematic review of how smartphone apps for measuring joint range of motion have been tested for reliability and validity — the place to start if you are choosing one.
doi:10.1371/journal.pone.0215806