Knee angle at the bottom of the pedal stroke

It's the angle between the thigh and the shin when the pedal is at its lowest point — bottom dead centre, the crank at six o'clock. Saddle height drives it most. Cleat position, how far the rider drops the heel and crank length move it too.

Fitters usually quote it as knee bend: 0° is a straight leg and the number is how far the knee is from straight. Markerless reports the angle between the two segments instead, so a straight leg reads 180°. Converting is a subtraction: bend = 180° − the Markerless reading.

The longest-standing guide is the Holmes method, from Holmes, Pruitt and Whalen's 1994 paper on overuse in cycling. It calls for 25–35° of knee bend with the pedal at the bottom, measured while the rider sits still. That's 145–155° in Markerless's convention, and it's the starting range the cycling view shows beside its figures.

Static has one great virtue: it's quick. Its weakness is that nobody rides with the crank parked at six o'clock. A pedalling rider moves the ankle, shifts on the saddle and drops the heel differently from how they sit for a measurement.

Millour and colleagues (2019) filmed 26 cyclists in 2D and compared the methods directly. With the pedal at the bottom, the knee was 8° more bent while pedalling than in the static position. It was also 3° more bent at bottom dead centre than at its straightest point in the stroke. Their conclusion was a separate dynamic range of 33–43° at the bottom of the stroke.

Ferrer-Roca and colleagues (2012) worked from a 30–40° range, taken at the knee's straightest point on 2D video of the rider pedalling. On video, 26% of their riders fell outside it, and the authors concluded that setting the saddle from inseam length alone may not put the knee inside it. A 2022 systematic review of 41 studies, by Bini and Priego-Quesada, found strong evidence for setting saddle height from dynamic knee-angle measurements.

As the pedal comes over the top, the knee is at its most bent and the hip at its most closed. Saddle height, setback and crank length all show here. There's no single published range for the top that compares with the ones above, so treat it as a number to track rather than to hit.

In our example clip the rider's knee read 158.4° at the bottom (21.6° of bend), ±2.2° across 33 strokes, and 74.6° at the top. Where that sits for this rider is a fitter's call, made with the rider in front of them.

1. Put the bike on a trainer, smart bike or rollers, so it stays in one place relative to the camera.
2. Phone on a tripod, side-on and square to the bike, at about hip height and level with the bottom bracket. If the wheels look round rather than oval, you're square.
3. Get the whole rider and bike in frame. Shorts or fitted kit make the knee and ankle easy to track.
4. Film the side you want to measure. The far leg is hidden behind the bike for much of the stroke.
5. Let the rider settle at their usual cadence and effort, then film 15–30 seconds of steady pedalling.

Frame rate matters less than you'd think. At 90 rpm the cranks turn 540° a second, so 30 fps gives a frame every 18° of crank rotation and 60 fps every 9°. The knee changes slowly near the bottom of the stroke, and the cycling view reads each bottom with a five-point fit through the frames around it, so 30 fps is usually enough. Use 60 fps for high cadences or for reading the ankle.

A short clip holds dozens of revolutions. The bottom of each one is the knee's peak, and the useful figures are the average across the steady strokes and the spread around it. The cycling view gives both, plus a chart of every stroke you can click to watch.

If you export the angles and smooth them yourself, choose the filter carefully. Smoothing shaves peaks, and this number is a peak. A moving average takes 2–3° off the knee at the bottom, which is a whole fitting decision. A zero-lag Butterworth or a Savitzky–Golay filter keeps peaks where they are. See smoothing filters for the trade-offs.

The strongest use of the number is comparison. Change one thing, such as saddle height, film again exactly the same way and put the two clips side by side. Filed under the rider in People, every fit keeps its history: what changed, when, and what the knee did.

Keep the camera where it was. A different height or angle changes the reading as surely as a different saddle height does.

• It's a 2D angle in the plane of pedalling. Knee tracking — the knee moving in and out over the pedal — happens in the frontal plane and needs a front-on view. The keypoints show its path there, but it isn't a single number we report.
• It isn't a goniometer on bony landmarks. A camera's angle and a goniometer's are different measurements, so compare against your own earlier clips filmed the same way, not against someone else's tape measure.
• It reads the near leg. The far leg is hidden behind the bike. Film each side separately if you want both.
• It doesn't decide the fit. Comfort, flexibility, injury history and goals matter as much as any number.