AI Bike Fit Analyzer

Film yourself pedaling from the side, pick road or TT, and the analyzer reads the same joint angles a fitter measures with a goniometer: knee bend at the straightest point of the stroke, torso angle, elbow bend, shoulder angle and hip angle at the top. Saddle height gets a graded range, the one number the research backs. The rest is reported against typical values with a plain flag when something is clearly off. The pose model runs in your browser. The video never leaves your phone or computer.

How to film it

Put the bike on a trainer. Set the camera square to the bike at hip or crank height, far enough back that the whole near leg and arm stay in frame. Hands where you ride: hoods or drops on a road bike, forearms on the pads for a TT bike. Fitted kit, so the knee and hip stay visible. Pedal at your normal cadence for 20-30 seconds in steady light, then stop. If you change something and refilm, keep the camera exactly where it was. That is what makes two clips comparable.

What the numbers mean

Knee bend at max extension. Measured hip to knee to ankle at the point in the stroke where the leg is straightest, just past the bottom, and averaged over your pedal cycles. Fitters measuring this dynamically aim for about 30-40 degrees of bend. Under that, the leg nearly locks out and the saddle is likely too high; over it, the saddle is likely too low. Move the saddle 3-5 mm at a time, refilm the same way, and compare. A dropped heel adds bend without the saddle moving, so glance at your ankle before you touch the seatpost. Saddle height is the one setting that changes everything else, so fix it first.

Torso angle. The hip to shoulder line against the horizontal. Around 40-50 degrees on the hoods is a fitter's convention rather than a research finding, and the same rider sits 8-10 degrees lower in the drops. Lower is more aerodynamic and harder on the neck, hands and lower back. A TT position sits far lower, which is why the tool has a separate profile.

Elbow bend. Riders on the hoods span anything from 3 to 45 degrees, and the bend changes with effort, so the number is reported rather than graded. Locked elbows are the thing worth flagging: they send every bump into your shoulders and usually mean the reach is long or the bars low.

Shoulder angle. The angle between torso and upper arm. Fitters often aim near 80-95 degrees on a road bike, but no published reference range exists, so it is reported for your own comparison. Closed reads cramped, open reads stretched. A stem change is the usual fix either way.

Hip angle at the top and knee bend at the top are reported for reference only. They depend on flexibility, crank length and saddle setback, published numbers disagree by definition alone, and a side camera cannot see pelvic tilt.

What a phone video cannot tell you

A 2D side view reads hip and knee angles to roughly 5-8 degrees in the lab and worse between days, so the tool grades one number in a wide band and reports the rest. Read it as a way to compare your own clips filmed the same way, not as your true joint angle. It cannot see saddle pressure, cleat position, pelvic tilt, or what happens under load on the road. Persistent pain or numbness, or a big position change, is a job for a fitter in person.

Use it with your coach

In the Movement Rebels app the same analysis lives next to your training data. Send the result to your coach and it reads the angles alongside your Garmin or Apple Health rides, so a saddle that is likely low shows up in your next brief instead of in a sore knee three weeks later. Check your power numbers with the FTP calculator and power zones once the position is set.