Knee range of motion: normal values

• Flexion — bending the knee, bringing the heel towards the buttock.
• Extension — straightening it. Some knees straighten a little past straight, which is recorded as extension beyond zero.

The knee also rotates a little when it is bent, but flexion and extension are what range-of-motion measurements of the knee are about.

The extension figures are small: on average a knee straightens to within a degree or two of straight.

The pivot sits at the knee, one arm along the thigh towards the hip and the other along the shin towards the ankle. Straight is 0°, and the angle is read as the knee bends. Flexion is often measured lying on the back with the heel sliding towards the buttock, and whether the hip is bent at the same time changes how far the knee will go.

The knee angle is made from the hip, the knee and the ankle: the angle at the knee between the thigh and the shin. A straight leg reads 180°.

To convert, take the distance from 180°. A knee bent 140° from straight — close to the study's average for women aged 20 to 44 — reads 40°, or 320° if the sweep runs the other way for the side filmed. A knee that straightens past straight crosses 180° and reads a few degrees on the other side of it.

In the indoor cycling example, the knee reads 158.4° at the bottom of the pedal stroke — about 22° short of straight — and 74.6° at the top. The same arithmetic turns any reading into the clinical figure.

Side-on is the view for flexion and extension. Front-on, the same three points show how far the knee sits in or out of the line from hip to ankle — which is a different thing, and on a bent knee it mixes with the bend itself, so treat it as a picture of alignment rather than a clinical angle.

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.

1. Film side-on, the measured leg nearest the camera.
2. Put the camera at about knee height, square to the leg.
3. Keep the hip, knee and ankle in frame through the whole movement.
4. Wear shorts or fitted leggings, so the model can find the knee.
5. For a full bend, lie or sit side-on; for a movement like a squat or a pedal stroke, film the movement itself.
6. Film the same way each time, so sessions compare like with like.

Anything out of the filmed plane reads short: a knee pointing towards the camera bends without appearing to. Rotation of the shin under the thigh is invisible to three points. And the points are the model's estimates of the joint centres, not palpated bony landmarks, so a reading is not quite the same measurement as a goniometer placed by hand.