How to do a bike fit from video

A static fit measures the rider sitting still, with the crank parked at a set position and a goniometer on the joint. A dynamic fit measures them while they pedal, at their own cadence and effort.

The two don't give the same number. One study of 26 cyclists found the knee 8° more bent at the bottom of the stroke while pedalling than in the static position, so a range written for one method doesn't carry over to the other. A 2022 systematic review found strong evidence for setting saddle height from dynamic knee-angle measurements. Knee angle at the bottom of the pedal stroke goes through the published ranges.

Each has a place. A goniometer is quick and needs no camera. Placing points by hand puts you in full control of what was measured, but it costs time per frame, so most people measure a few strokes rather than all of them. Automatic tracking measures every stroke without the clicking, and the price is checking: read the tracked skeleton drawn over the video before you trust a figure, because you didn't place the points yourself.

1. Fix the bike. A turbo trainer, smart bike or rollers, with the bike level. It must stay in one place relative to the camera.
2. Place the camera side-on. Phone on a tripod at about hip height, level with the bottom bracket and square to the bike. Round wheels, not oval ones, mean you're square.
3. Frame the whole rider and bike, with a little room around them. A plain, contrasting background helps the tracking.
4. Wear fitted kit. Shorts or tights make the knee and ankle easy to see. Baggy clothing hides the joint the angle is measured at.
5. Film the near side. The far leg is hidden behind the bike for much of the stroke. Turn the bike round to film the other side.
6. Ride steady. Let the rider warm up at their usual cadence and effort, then film 15–30 seconds. 60 fps helps at high cadence. 30 fps is enough otherwise.

Markerless's cycling view finds every revolution and reads the knee at the bottom and top, the hip at the top, the ankle's range and the cadence on each one. The trunk and elbow come from the range-of-motion charts beside them. The trunk figure is its lean off vertical, where 180° is upright.

Video earns its keep in comparison. Change one thing, film again with the camera exactly where it was, and put the two clips side by side on the same moment of the stroke. Change two things at once and you can't tell which one moved the number.

Expect small changes to stay small. The same review found that saddle-height changes of under 4% of leg length made trivial to small differences to lower-limb loads, with no effect on oxygen uptake or efficiency. The angles will still show what changed. Whether it matters is the fitter's question.

File each fit under the rider in People and the history builds itself. Send the result with a view-only share link and the rider can open their own video and numbers without an account.

• How the position feels: saddle pressure, numb hands, a tight lower back.
• Flexibility, injury history and what the rider is training for.
• Power, pedal force or left/right balance. Those come from a power meter or smart trainer.
• Knee tracking over the pedal, which happens in the frontal plane. A front-on view shows the path, but not as a single angle.
• Anything filmed off-square. A camera at an angle to the bike measures a different angle from the one you meant.

Measured well, the angles are a consistent record of what the camera saw. That makes them strongest for comparing a rider with their own earlier sessions, and that's most of what a fit needs.