How does AI detect squat depth?

Form Detective reconstructs a 3D skeleton from your video and computes hip-crease vs knee-joint position on every frame. A rep is graded as ATG if the hip crease drops below the knee, parallel if hip and knee are level, and shallow if hip stays above knee.

What causes knee cave (knee valgus) in the squat?

Knee valgus is usually a glute-medius weakness or an ankle/hip mobility limitation, not a knee problem. Cues like 'spread the floor' help, but the long-term fix is targeted glute activation (banded glute bridges, Spanish squats) plus ankle dorsiflexion work.

Is butt wink dangerous?

A small posterior pelvic tilt at the very bottom of a deep squat is normal and not dangerous under moderate loads. It only becomes a concern when it happens above parallel, under heavy load, or with visible lower-back rounding throughout the rep.

What camera angle should I use to film a squat?

A 30-45° front-side angle is best. It gives the AI both depth visibility (side) and knee-tracking visibility (front). Pure side-on works for depth and forward lean but hides knee valgus; pure front-on works for knee valgus but hides depth.

Does it work with a low-bar squat?

Yes. Both high-bar and low-bar squats are supported. Forward lean is graded relative to your bar position, so a more horizontal torso on a low-bar squat is not flagged as a fault.

Is the squat form check really free?

Yes. Every account gets free daily form checks with no credit card. Heavier usage uses a low-cost credit pack — there is no subscription required.

Free AI Squat Form Check — Depth, Knee Cave & Butt Wink Detection

The 5 squat errors Form Detective detects automatically

Every error below is detected from a 3D skeleton reconstructed from your video — not from a single 2D angle. That means the detector still works when your phone isn't perfectly side-on.

Squat depth (parallel / ATG / shallow)

Hip-crease vs knee-cap position is computed on every rep, so depth is reported objectively instead of by eye.

Knee cave (valgus)

Frontal-plane knee tracking flags reps where your knees collapse inward at the bottom or on the way up.

Butt wink

Pelvis tilt at the bottom of the rep is measured against your spine angle to catch posterior tucking.

Forward lean

Excessive torso pitch — usually a quad/ankle mobility issue or weak upper back — is flagged with severity.

Heel lift

Foot pressure shift forward (heels coming up) is detected from ankle and heel keypoint positions over time.

Hip shift & uneven lockout

Side-to-side asymmetry is detected from left vs right hip and knee paths so you can spot a one-sided weakness.

How the analysis works

Form Detective runs YOLO pose detection on every frame, refines the keypoints with ViTPose, and lifts them into true 3D with VideoPose3D. Errors are scored against squat-specific biomechanical thresholds and presented to you as a written report plus a scrubbable 3D skeleton replay. Read the full pipeline overview →

What you get back

Per-rep score and a clear list of which reps had which issues
3D skeleton replay you can scrub and rotate
Specific written cues — e.g. "drive knees out at the bottom of rep 3"
A personalized warmup & cooldown routine that adds extra mobility drills for the muscles driving your recurring errors
An AI coach chat that remembers your past sessions and the issues that keep showing up

How to film a good squat video

Place the phone roughly 30–60° off-center, about hip-height, far enough to fit your full body
Film at 30fps or higher — anything modern works
One working set per upload is ideal (trim long videos before uploading)
Side-on works best for depth and forward lean; a slight 3/4 angle helps catch knee cave

Frequently asked questions

Does it tell me if I hit depth?

Yes. Hip-crease vs knee position is calculated on every rep and reported as Above parallel / Parallel / ATG.

Will it work if my knees cave only on heavy reps?

Yes — that's exactly what rep-by-rep analysis is for. The report lists which specific reps showed valgus and how severe it was.

How much does it cost?

Free for daily use. Heavier usage is covered by a one-time credit pack — no subscription required.

Also check your other lifts

Bench press form check · Deadlift form check · How the 3D analysis works · Lifting form blog