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Measuring movement from video
How pose models describe a body, how a joint angle is calculated from them, and what the numbers mean in a bike fit, on the rowing machine or in a golf swing. Written to be checked: the maths is shown, the sources are cited, and the limits are stated.
Pose estimation
Keypoints, coordinate systems and the angle maths, for anyone building on a pose model.
- MediaPipe pose landmarks: all 33 by indexThe 33 MediaPipe pose landmarks with their index numbers, names and a diagram — plus what x, y, z and visibility mean, and world vs image landmarks.
- Calculate joint angles from pose landmarksThree landmarks make a joint angle. The formula, Python and TypeScript code, and the two mistakes — normalized coordinates and lost direction.
- MediaPipe vs MoveNet, ML Kit and Apple VisionKeypoints, coordinates, platforms and gotchas for MediaPipe, MoveNet, ML Kit, Apple Vision and YOLO-pose: what changes between pose engines.
Bike fit
The angles a fit is built on, and how to measure them while the rider pedals.
- Knee angle at the bottom of the pedal strokeThe knee-angle ranges bike fitters start from at the bottom of the pedal stroke, static and while pedalling, and how to measure it from a phone video.
- How to do a bike fit from videoGoniometer, frame-by-frame video or automatic pose tracking: how to set up a video bike fit at home, which angles to read, and what they can't tell you.
Rowing
Erg technique read as numbers: the catch, the finish and the faults in between.
- The rowing catch and finish, as anglesBody angle, shins, knees and arms at the catch and finish of the rowing stroke: the coaching positions, and how to measure them from a side-on video.
- Common erg technique mistakes, in anglesShooting the slide, opening the back early, early arm bend, overreaching and more: what each erg fault looks like and which angles show it on video.
Golf
Posture, sway and tempo from one phone video, down the line or face on.
- Golf posture: spine angle at addressThe spine's forward bend and knee flex at address, holding posture to impact, and early extension — read from one down-the-line phone video.
- Golf sway and weight shift, face onHip sway, the pelvis moving over to the lead foot, and the head staying behind the ball — what a face-on phone video can measure, and what it can't.
- Golf swing tempo: the 3:1 ratioGolf tempo is backswing time over downswing time. Where 3:1 comes from, how to time it from a phone video, and why frame rate decides how exact it is.