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Rotator Cuff Muscles
The teres major, teres minor, supraspinatus, and subscapularis together form the "rotator cuff." These muscles, when they are activated, stabilize the shoulder. Upon activation, they contract and pull the top of the humerus horizontally towards the body. These muscles also can be torn, which is a major injury.
Horizontal Abductors: Orienting the Scapula
During horizontal abduction, the shoulder blade has to move to make it possible for the humerus to move above the head. The muscles that do that pull the top of the shoulder blade down and inside and the bottom of it up and outside. The lower part of the trapezius muscle pulls the top in, while the serratus anterior muscles pull the bottom out and up.
Horizontal abductors: Moving the Arm
These muscles actually do the work to move the humerus. The anterior (back side) of the deltoid, superior (upper) trapezius, middle trapezius, rhomboid muscles, and the infraspinatus pull the shoulder blade and humerus back to the "cocked" phase.
The horizontal adductors work to counteract the movement into the cocked phase. These muscles pull the humerus through to the front. These muscles will work throughout the final phases of the throwing motion, including the follow through. They are the anterior (front) deltoid, pectoralis major, and pectoralis minor. The latissimus dorsi and teres major work to pull the arm downward.
The external rotators are responsible for rotating the arm outward to generate more torque capacity for the forward movement. The external rotators are the infraspinatus and teres minor.
The internal rotators are responsible for getting the torque delivered from the elbow. They are the subscapularis, teres major, latissimus dorsi, and the pectoralis major.
The elbow flexors bend the arm. With the arm bent, there is much more forward torque to be generated. The elbow flexors are the biceps brachii, brachialis, and brachioradialis. Depending on the angle of the hand and shoulder, each one of these three muscles will be the dominant mover. However, at nearly all potential angles, they contribute to the motion.
The triceps brachii is the primary elbow extensor. This is a large muscle that can produce a significant amount of force at the elbow.
These muscles flex the hand and fingers so that a strong grip can be attained. There must be control to the grip, however. This is the main job of the flexors- force control. These muscles include palmaris longus, flexor carpi radialis, flexor digitorum superficialis, and flexor carpi ulnaris.
These muscles are the ones which allow the position of the ball to be determined before release. They work in cooperative opposition with the flexors. They need to work to control the balance between the position of the ball and the coordination with the rest of the arm to let the ball roll off the fingertips. These muscles include the extensor digitorum, extensor carpi radialis brevis, extensor carpi radialis longus, and extensor carpi ulnaris.