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Supination

This term refers to the lateral rotation of the radius, a simple forearm twist at the radioulnar joint. In pitching, this action generally turns the palm toward the body and upward at release.

Supination is the opposite of pronation. Not only does supination move the radius in the opposite direction, it also has a small valgus force contribution.

Some supination prior to release is required to throw different types of pitches, but supination during release leaves the arm subject to forearm flyout, which can result in a collision between the ulna and the humerus as the elbow slams closed.

The biceps brachii and supinator are muscles that perform supination.