There are many muscles of the shoulder that attach the various bones that make up the shoulder joints, include the humerus, the scapula, and the clavicle. The muscles come together to form the outer contour of the underarm and shoulder. They help the shoulder joint travel over a wide range of motion and aid in the protection of the shoulder joint. This article will discuss these muscles and their function in detail.
Three Groups of Muscles of the Shoulder
Shoulder muscles are divided into three separate groups. They include the superficial muscles or "extrinsic muscles", the deep muscles or "intrinsic muscles", and the muscles that involve both the shoulder and the arm. The different muscle in different groups will be described specifically.
1. Superficial or Extrinsic Muscles
These include the following muscles:
Pectoralis major muscle. This muscle helps provide support and movement of the front part of the shoulder. It has two separate heads – there is a clavicular head that starts in the midline half of the collar bone and a sternocostal head that starts in the area of the chest plate. The muscle inserts into a part of the humerus, acting to draw the arm inward, flex and medially rotate the glenohumeral joint.
Trapezius muscle. This is a large muscle that starts at the back of the skull and along most of the spinal vertebrae. It travels to the collar bone, the acromion of the scapula and the spine of the scapula. It is a strong muscle that raises the shoulder and rotates the scapula so that you can reach upwards.
Latissimus dorsi muscle. This muscle starts at the spinous processes of the lower thoracic vertebrae as well as the iliac crest of the pelvic bone and the lower ribs. It inserts into the humerus at the intertubercular groove. It helps bring the arm close to the body and rotates the arm medially. It also brings down the shoulder and keeps the lower part of the scapula stuck against the wall of the chest.
Deltoid muscle. It gives the shoulder its shape and is the strongest muscle of all the muscles of the shoulder. There are three parts: the anterior, middle, and posterior parts. The anterior part starts at the anterior and upper part of the collar bone. The middle part starts at the acromion process, and posterior part starts from the lower part of the spine of the scapula. The various parts come together to insert into the humerus. Together, they flex and rotate the arm medially, bring the arm into the body and extend the arm backward. It contributes to lifting the arm after it comes away from the side of the body.
2. Deep or Intrinsic Muscles
This group includes many important muscles of the shoulder as listed below:
The front muscles. This includes the pectoralis minor muscle, which is a very small muscle that is deep to the pectoralis major muscle. It is connected to the 3rd through 5th rib and extends out to the coracoid process located on the scapula. It helps bring the scapula downward and forward, aiding in inspiration by raising the ribs. The subclavius muscle also lies deep to the pectoralis major muscle. It starts at the first rib and connects to the collar bone, helping the shoulder move downward and forward, keeping the collar bone stable.
The back muscles. These include the levator scapulae muscle, the rhomboid major and minor muscles, and the teres major muscle. The levator scapulae muscle starts in the vertebrae of the neck and attachs to the upper and medial part of the scapula, allowing it to rise up. The rhomboid muscles start in the vertebrae and travel to the middle part of the scapula, which help the scapula rise up. The teres major muscle starts from the lower part of the scapula and attaches to the anterior part of the humerus. It helps extend and rotate the arm medially.
The side muscles. This includes the serratus anterior muscle that starts on the first nine ribs and passes around the chest wall, attaching to the medial side of the scapula. It helps bring the scapula forward along the chest wall and rotates the scapula.
The rotator cuff muscles. These are four muscles that help the shoulder joint be stabilized. It consists of the supraspinatus muscle, the infraspinatus muscle, and the teres minor muscle. There is also the subscapularis muscle that forms the back wall of the axilla and inserts on a tuberosity of the humerus. These help rotate the arm medially and stabilizes the shoulder joint itself.
3. Muscles of the Shoulder and Arm
These include the following muscles:
The biceps brachii muscle. This has two heads. The long head starts at the supraglenoid tubercle of the scapula and the short head starts at the coracoid process of the scapula and both insert in the medial aspect of the forearm. The muscle helps flex the forearm and shoulder as well.
The coracobrachialis muscle. It starts at the coracoid process of the scapula and inserts along the humerus to help draw the arm toward the body. It also stabilizes the humerus.
The triceps brachii muscle. It has three heads. One head starts at the infraglenoid tubercle of the scapula, the second at the upper part of the back of the humerus, and the last one at the lower half of the back of the humerus. They end at the olecranon process of the ulna. It is responsible for extending the forearm at the elbow. It can also help draw the humerus into the body and extend the arm.