Can a phone check whether a bowling action is legal?

Bowling-action legality concerns elbow extension during the relevant part of the delivery. That is a three-dimensional joint-motion question. The apparent bend in a two-dimensional image changes with camera position, upper-arm rotation, clothing, landmark visibility and the exact frames chosen for comparison.

• Depth is projected onto a flat image, hiding motion toward or away from the lens.
• A small camera-angle error can change the apparent elbow angle.
• Automatic pose landmarks estimate joint centres; they are not calibrated lab markers.
• Motion blur and an obscured arm can move the estimated wrist, elbow or shoulder.
• An official protocol controls capture, calibration, processing and review.

A clear clip can support an initial coaching conversation, document how the action looks from a particular view and help an athlete prepare questions for an appropriately accredited assessment. It can also analyse other observable patterns when the camera is suitable, such as delivery events, front-knee behaviour and release repeatability.

Speak with a qualified coach and follow the assessment process of the governing body under which you compete. Do not attempt to redesign an action around a number produced by an uncalibrated phone app, particularly when pain is present.

The ICC publishes a standardised suspected-illegal-bowling-action assessment protocol with controlled capture and measurement requirements. Read the official ICC protocol.

Even a recent two-camera markerless system showed substantial error and lost trials when compared with marker-based motion capture during fast bowling. That system used stronger instrumentation than one uncalibrated side-on phone. Read the open-access validation study.