This is the muscle that fielders use when throwing the ball and is very important as it holds the arm in the shoulder when you release the ball.

The subscapularis is one of the rotator cuff muscles that originates from the underside of the shoulder blade and inserts at the front of the upper arm (humorous). It is the powerful muscle that rotates the arm inwards, and therefore fielders are the most prone to injuring the subscapularis.