What is the 15 degrees rule in cricket?

Overarm bowling in cricket was legalised in 1864. The deficit in biomechanical technology in that era made it difficult for the umpires to judge how fairly the overarm bowl is released. Visual judgement of the umpires was the final call. The laws of cricket back then completely banned any flexing of the arm when the bowler releases the ball. 

It was not until the 1990s that biomechanical tests in England revealed that it was impossible to bowl without some flexing of the arm. However, the extent to which the arm could be flexed from the elbow joint had to be determined for a fair delivery to be distinguished from a throw.

In 2003, a study was conducted which included 130 bowlers (a mix of pacers and spinners) to understand the flex of the elbow during a throw. It was eventually concluded that if the arm is flexed from the elbow joint for more than 15 degrees, it will be considered a throw.

Based on this, Clause 3 of Law 24 is as follows:
“A ball is fairly delivered in respect of the arm if, once the bowler’s arm has reached the level of the shoulder in the delivery swing, the elbow joint is not straightened partially or completely from that point until the ball has left the hand. This definition shall not debar a bowler from flexing or rotating the wrist in the delivery swing.”

Today, the average extension of the elbow for a legal delivery is between 8 to 10 degrees. The most common extension for a delivery that is tagged as a throw is between 20 to 30 degrees. The 15 degrees rule in cricket thus ensures that the bowler delivers a bowl well within the spirit of the game

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