Range of motion · Low back
How to measure low back range of motion
Lumbar range of motion is the movement of the lower spine itself, and a pose model has no landmarks on the spine. What video can measure well is how far the whole trunk leans from vertical. This page is plain about the difference, because a number that looks like lumbar flexion and isn't is worse than no number.
The movements
- Flexion — bending forwards, as in reaching for the toes.
- Extension — bending backwards.
- Lateral flexion — bending to the side, the hand sliding down the leg.
- Rotation — twisting the trunk to look behind.
A forward bend is shared between the spine and the hips: much of a toe touch is the pelvis tipping forward over the thighs. That sharing is the heart of why lumbar range of motion is hard to measure from the outside.
Reference values
No reference values on this page
The CDC study behind the reference values on the other joint pages did not measure the lumbar or thoracolumbar spine. Rather than borrow a figure from a different method and a different sample, this page quotes none.
How it is measured clinically
Lumbar movement is usually measured with inclinometers placed on the skin over the spine, or with a tape measure — the Schober-type tests mark points on the lower back and measure how far they move apart in a forward bend. Both aim to separate the spine's movement from the hips', which a single angle of the whole trunk cannot do.
How Markerless measures the low back
The low back angle is made from the midpoint of the shoulders, the midpoint of the hips, and a point straight down from the hips. It is the trunk's inclination from vertical, taken at the hips: upright reads 180°, and the distance from 180° is how far the trunk leans.
Trunk lean, not lumbar flexion
There are no spine landmarks between the shoulders and the hips, so the curvature of the lower back cannot be seen, and we do not claim to measure it. The figure is the lean of the trunk as a whole — the spine and hips together — and the definition ships inside every download so nobody downstream mistakes it for the other thing.
Side-on, it shows the trunk leaning forwards and back; front-on, leaning to the side. It is most useful as a check on everything else: in the shoulder abduction example the low back moves just 0.8° while both arms go overhead, which is how you know the shoulders did the work. On the rowing machine it is the body's swing from the catch to the finish.
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
- Side-on for bending forwards and back, front-on for side bending.
- Put the camera at about hip height, square to the person.
- Keep the shoulders and hips in frame for the whole movement.
- Use a level camera: the angle is measured against the picture's vertical, so a tilted phone tilts the reading.
- Wear fitted clothing at the waist and shoulders.
- Film the same way each time.
What video cannot measure
- Lumbar curvature — no landmarks between the hips and the shoulders.
- Spine against hips in a forward bend — the trunk angle includes both.
- Rotation — the shoulders turn towards and away from the camera.
- Anything out of the filmed plane, which reads short.
Questions
What is normal lower back range of motion?
The CDC reference study quoted on the other joint pages did not measure the spine, so we quote no figure. Published lumbar norms come from inclinometer and tape-measure methods that a trunk angle from video does not reproduce.
Can a video measure lumbar range of motion?
Not the lumbar spine's own movement — a pose model has no spine landmarks. It can measure the trunk's lean from vertical on every frame, which is useful for technique and for checking that the trunk stayed still while another joint moved.
How accurate is a low back 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 low back 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 low back 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
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