One letter higher: checking my saddle height with a side-view video, in the browser

Description and use

Saddle Check prototype measures knee bend and recommends saddle height adjustment from a side-view phone video, all computed locally in the browser.

Problem

Determine if a bike saddle is at the correct height using only a side-view phone video, without sending data to external servers.

Outcome

The prototype accurately detected a 10 mm saddle change as a 4.8° reduction in knee bend, matching the predicted 3.4° from the leg‑triangle model. Repeatability across three videos at the same height was 0.7°, and sensitivity to a 10 mm change was within 5 % of the expected value.

Workflow

• 1. Record a side-view video of pedalling for ~2 minutes.
• 2. Upload the video to the Saddle Check web page.
• 3. The page uses WebCodecs to read frames and MediaPipe Pose Landmarker to detect body landmarks.
• 4. The page calculates knee, hip, trunk, shoulder, elbow, and ankle angles per stroke, compares knee bend to published ranges (33‑43°).
• 5. It estimates the saddle change needed to reach the middle of the range and outputs millimetres and letter notches.
• 6. The user downloads the report and can bring it to a bike shop or use it for follow‑up adjustments.

Evidence

The page reads the video frame by frame with WebCodecs, the browsers' decoding API, fed by a small reader for MP4 and QuickTime boxes: each frame is decoded once, in order.

Source

The page calculates knee, hip, trunk, shoulder, elbow, and ankle angles per stroke, compares knee bend to published ranges (33‑43°).

Source

Five videos at 60 frames per second, tripod and bike taped to the floor: V1, V2 (after getting off and back on) and V4 (saddle set back to its letter) at the same height; V3 one letter higher, 10 mm; S, still, crank down, to compare with an inclinometer.

Source

Repeatability. Three videos at the same height, one after getting off the bike and one after resetting the saddle: 0.7° between the lowest and the highest.

Source

Sensitivity. One letter higher, the knee bends 4.8° less (95 % interval 4.3 to 5.4). The leg triangle predicted 3.4°. The direction and the size are right.

Source