Many movements of the arm and shoulder may seem complicated, yet there are surprisingly few muscles in each part, and most of these are muscle groups. The shoulder is most notable for the deltoids, of which there are three. The upper arm relies upon two muscle groups in the front and back, which work on the elbow. Several smaller muscles in the forearms and wrist are responsible for flexion and extension.
Wrists
There are just two muscle groups that work upon the wrist since the movement of the wrist is hinge-like. The flexors bring the back of the hand toward the forearm. The extensors pill the palm of the hand toward the forearm.
Forearms
The action of the forearm consists of three movements: flexion of the elbow, which is performed by the brachioradialis; the upward twisting of the forearm performed by the supinators; and the downward twisting of the forearm by the pronators. The supinators are in action when you turn your palm up to test for rain. The pronators are engaged when you turn your palm back down to face the ground.
Upper Arm
The upper arm holds two major muscle groups and a third minor muscle. The triceps brachii, located at the back of the arm, are three muscles that work together to extend the elbow and straighten the arm. In the front of the arm, the two muscles of the biceps brachii flex the elbow, bringing your hand to your shoulder. The brachialis, a minor muscle, assists the biceps brachii in elbow flexion.
Shoulder
The major muscles of the shoulder are the deltoids, of which there are three heads: the anterior (front), lateral (side), and anterior (rear). The anterior and lateral deltoids abduct the shoulder, i.e. lift the arm away from the body such as when you raise your arms while being frisked. The posterior deltoid extends the arm away from the chest as you do when cocking your arm back to throw a baseball. The other muscle of the shoulder is the supraspinatus, also known as the rotator cuff. This muscle, like the posterior deltoid, extends the arm away from the chest.
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