Range of motion · Elbow
Elbow range of motion: normal values
The elbow is the easiest joint there is to measure from video: a hinge, moving in one plane, between three points a pose model finds reliably. Here are the published reference values, how they translate into Markerless's numbers, and the one part of the elbow's movement a camera cannot see.
The movements
- Flexion — bending the elbow, bringing the hand towards the shoulder.
- Extension — straightening it. Some people straighten a few degrees past straight, which is recorded as extension beyond zero.
- Pronation and supination — turning the forearm so the palm faces down or up. This happens along the forearm, below the elbow hinge, and is usually measured with the elbow.
Reference values
| Motion | Women 20–44 | Men 20–44 | Women 45–69 | Men 45–69 |
|---|---|---|---|---|
| Elbow flexion | 150.0° | 144.6° | 148.3° | 143.5° |
| Elbow extension | 4.7° | 0.8° | 3.6° | -0.7° |
| Forearm pronation | 82.0° | 76.9° | 80.8° | 77.7° |
| Forearm supination | 90.6° | 85.0° | 87.2° | 82.4° |
The study's elbow extension figures are small numbers either side of zero: on average the elbow straightens to about straight, and a negative mean means slightly short of it. Pronation and supination are in the table because the study measured them; the section below explains why video does not.
How it is measured with a goniometer
The pivot goes at the elbow, one arm along the upper arm and the other along the forearm. Fully straight is 0°, and flexion is read as the elbow bends. Forearm rotation needs a different set-up, usually with the elbow bent at the side and the goniometer following the wrist or a held pencil.
How Markerless measures the elbow
The elbow angle is made from the shoulder, the elbow and the wrist: the angle at the elbow between the upper arm and the forearm. A straight arm reads 180°.
To convert, take the distance from 180°. An elbow bent 150° from straight — about the study's average for women aged 20 to 44 — reads 30°, or 330° if the sweep runs the other way for the side you filmed. An elbow that straightens a few degrees past straight crosses 180° and reads a few degrees on the other side of it.
Film side-on, with the arm moving in the plane the camera faces, and the reading is flexion and extension. Front-on, the forearm swings towards the camera as the elbow bends and the angle reads short — useful only for movements that happen out to the side.
Filmed front-on, a person's two sides are mirror images in the picture, so the same movement reads x on one side and 360 − x on the other. The report flips the mirrored side back so left and right sit on the same scale; the downloads keep the engine's own clockwise angles.
How to film it
- Stand side-on to the camera, the measured arm nearest it.
- Put the camera at about elbow height, square to the arm.
- Keep the shoulder, elbow and wrist in frame through the whole movement.
- Bend and straighten the elbow in the plane the camera faces, with the upper arm still.
- Wear short or fitted sleeves so the model can find the elbow.
- Film the same way each time to compare one session with another.
What video cannot measure
Pronation and supination turn the forearm about its own length. The shoulder, elbow and wrist barely move while it happens, so no angle made from them can see it, and Markerless does not report it. The same goes for anything that carries the forearm towards or away from the camera: it reads short.
The wrist point is the model's estimate of the wrist, not a bony landmark, so a hand that is blurred or hidden behind the body costs accuracy on the elbow too.
Questions
What is normal elbow range of motion?
In the CDC's reference study, mean elbow flexion was 150.0° for women and 144.6° for men aged 20 to 44, with extension close to 0°: on average the elbow straightens to about straight.
How do I measure elbow range of motion without a goniometer?
Film the arm side-on with a phone, bending and straightening it in the plane the camera faces. Markerless reports the angle between the upper arm and forearm on every frame; the distance of that reading from 180° is the elbow's flexion.
How accurate is a elbow range of motion app?
It depends on the app, the joint and above all the filming. Markerless has not published a validation study against a goniometer or marker-based capture, so treat its elbow angles as a consistent measure of what the camera saw rather than a clinical absolute. They are strongest for comparing someone with their own earlier clips, filmed the same way.
Does a elbow reading outside the reference range mean something is wrong?
No. Reference values are averages from one study's sample, and individuals vary well beyond them. A number from a video is a measurement, not a diagnosis. If movement is painful or has changed, that is a question for a qualified clinician, not for a chart.
References
Soucie, J. M., Wang, C., Forsyth, A., Funk, S., Denny, M., Roach, K. E., Boone, D., & the Hemophilia Treatment Center Network (2011). Range of motion measurements: reference values and a database for comparison studies. Haemophilia, 17(3), 500–507.
The CDC's Normal Joint Range of Motion Study: goniometer measurements of people without conditions affecting joint mobility, published by sex and age group. Every reference value on these pages is copied from its tables.
doi:10.1111/j.1365-2516.2010.02399.x
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