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ChukCheck: estimate a bowler's elbow straightening from video
A one-hour Claude build that tracks shoulder, elbow and wrist in a phone clip and checks the result against cricket's 15 degree limit.
ChukCheck takes one side-on video of a bowler and estimates how much the elbow straightens during the delivery. In the reel, one bowler came out at 78 degrees. The limit is 15.
What chucking means
If the elbow snaps straight as you let go, that's throwing, not bowling. In cricket it's called chucking. ICC rules let a bowler straighten the elbow by up to 15 degrees, measured from when the bowling arm is level with the shoulder until the ball leaves the hand. More than that is a throw.
The app came out of a weekly challenge. A random spinner picks an app, and I get one hour to build it with Claude. This was Day 13, and the spinner landed on a chucking meter.
How you use it
Film one ball from the side. Upload the clip, choose side on, pick the bowling arm, and hit start.
The AI puts dots on the shoulder, elbow and wrist in every frame. From those three points it can work out the angle at the elbow, which is how bent the arm is.
How it measures the 15 degrees
The app checks the elbow angle at two moments. The first is when the upper arm is level with the ground. The second is when the ball leaves the hand. The difference between the two readings is the straightening.
If the second reading is 15 degrees more than the first, that's right on the limit. Anything past that is over.
Where it got fooled
On the first try, it got fooled. Jadeja lifts both hands above his head before he bowls, and the app called that the release. The arm was high, so the release detector took it for the real thing.
The fix was to make the app wait for the arm to whip over fast, not just go high. Height alone doesn't tell you much. Speed is what separates the delivery from the run-up.
What the numbers say
Back to the bowler from the opening. Before his coach fixed his action, the app read 78 degrees. After, it read 8.
There's a caveat. That clip was filmed from behind, so the exact number is rough. But every way I measured it, he was well over 15. Side-on footage is what the app is built for.
It's an estimate from one phone video, not an ICC test. Treat it as a way to start an argument with evidence, not to settle one officially.
Try it
Film a friend from the side, one ball, with the bowling arm clearly in view. Run the clip and see which side of 15 the reading lands on. If you want the app link, comment CHUCK on the reel and I'll DM it to you.
