The muscles and tendons of the complex shoulder joint must stabilize your arm while permitting a wide range of motion. A pitcher's shoulder can maintain the flexibility to aim a pitch at a bat while withstanding the torque created when throwing a fastball. Likewise, your shoulder maintains range of motion and strength to perform a variety of daily activities, from pulling out your chair to opening your computer to scrolling down your keypad to the thousands of other movements you make during a typical day.
Rotator Cuff
Often treated as one entity, the rotator cuff actually consists of a group of muscles and tendons that interact to stabilize the ball of your humerus in your shoulder socket. The four main muscles of the rotator cuff are the supraspinatus, infraspinatus, teres minor and subscapularis. You use the supraspinatus, which abducts your shoulder, to hold an object away from your body. The infraspinatus and teres minor are involved in external rotation, abduction and extension of your shoulder and are required for your tennis backhand swing or rubbing your hand through your hair. The subscapularis assists in internal rotation and helps you reach around behind you.
Trapezius
The trapezius is a triangular muscle that attaches at the base of your skull, your clavicle, upper scapula and down to your mid-vertebrae. This large muscle performs a variety of functions. The upper portion of the trapezius acts on the scapula so you can shrug your shoulders and lift your arm to the side. The upper trapezius is also responsible for moving your neck so you can look up. The middle trapezius permits you to hold your child. The lower portion assists in moving your shoulders down and lifting your arm to the side.
Levator Scapulae
For a relatively small muscle, the levator scapulae gives you a lot of mobility. The levator scapulae runs along the side of your neck and attaches to your upper vertebrae and scapula border. The muscle helps you to bring your arm toward your body so you can carry heavy bags. The levator scapulae also allows you to move your head to the side.
Rhomboids and Deltoids
The minor and major rhomboids run from the upper thoracic spine to the middle edge of the scapula. These muscles stabilize the scapula and assist in retraction and elevation of the inside scapula border. The rhomboids' movements support a pulling motion so you can open drawers. The deltoid muscle flexes and medially rotates your arm, and so permits you to raise a glass for a celebratory toast. In addition, the deltoid assists you in lifting your arm to the side.
Pectoral Muscles
The pectoral major and minor, or pec muscles, are the muscles that comprise your chest. The pectoral major is a large, strong muscle with a variety of important functions. As it relates to the shoulder joint, the pectoral major assists in arm flexion and medial rotation so you can brush your teeth and adduction so you can pull a chair toward you from the side. The pectoral minor attaches at the sternum and scapula and permits you to bring your scapula down and forward so you can push on the arms of a chair to stand up.
