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Scapula (Shoulder Blade)
The scapula provides the articulation required for movement of the upper arm. It's flexibility with the ribs, collarbone, and humerus allows for all of the movement needed for throwing, from cocking the arm back to following through to the front.
The clavicle provides stability, motion, and muscular attachments for the throwing motion of the upper arm. This provides the attachment for ligaments that hold the shoulder in place.
The ribs provide a stable surface for the shoulder blade to slide on, without which the arm could only elevate to approximately 30 degrees above the shoulder.
Humerus (Upper Arm)
The humerus is the main bone moved in the upper arm. It's articulation with the shoulder and forearm make this a very important bone in this action. All of the main movers of the shoulder and elbow in this throwing motion, save for a few muscles on the wrist/hand and scapula, are attached to the humerus.
Radius and Ulna (Forearm Bones)
The radius (thumb side) and ulna (pinky side) are able to both twist among each other and act as a hinge at the elbow. The muscles and tendons of the wrist/hand muscles are nearly all attached to one of these bones. The throwing motion explained by this page is a straight pitch, so no turning of the wrist is occurring. On pitches such as a curveball, the pitcher must turn their thumb up, or "supinate" their hand as the ball is being released to spin it.
Carpals, Metacarpals, and Phalanges (Wrist, Hand, and Fingers)
The carpals, metacarpals, and phalanges do not play much of a role in the power of the pitch. However, they play a pivotal role in the accuracy of the pitch. They provide the force to the baseball and the attachments for muscles to extend the ball towards the plate.