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The shoulder is a complex ball-and-socket joint that allows for a wide range of motion. This freedom of movement is made possible by the many muscles that surround the shoulder joint. The shoulder muscles are also responsible for maintaining the shape of the shoulder and underarm and protecting the shoulder joint. There are about 20 muscles supporting the shoulder, including the deltoids, rotator cuff muscles, scapular muscles, and more. These muscles are prone to injury and pain due to their flexibility and the amount of wear and tear they undergo.
| Characteristics | Values |
|---|---|
| Number of muscles supporting the shoulder | 20 |
| Number of muscles attaching to the shoulder blade (scapula), upper arm (humerus), and collar bone (clavicle) | 8 |
| Shoulder flexion | Movement of the shoulder in a forward motion |
| Shoulder extension | Movement of the arm directly behind the body |
| Shoulder adduction | Lowering the arm towards the body |
| Shoulder abduction | Lifting the arm away from the body |
| Shoulder external rotation | Movement seen in a tennis backhand stroke |
| Shoulder internal rotation | Opposite of external rotation, e.g. reaching into a back pocket |
| Rotator cuff muscles | Supraspinatus, infraspinatus, teres minor, and subscapularis |
| Shoulder muscles | Deltoids, trapezius, serratus anterior, rhomboids, levator scapulae, latissimus dorsi, teres major, coracobrachialis, triceps, pectoralis major, pectoralis minor, subclavius |
| Shoulder injuries | Bursitis, frozen shoulder, rotator cuff strains, shoulder impingement syndrome |
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What You'll Learn
Deltoids
The deltoid muscle, also known as the 'common shoulder muscle', forms the rounded contour of the human shoulder. It is a main muscle group in the upper body and is made up of three distinct sets of muscle fibres: the anterior or clavicular part (commonly known as the front delt), the posterior or scapular part (commonly known as the rear delt), and the intermediate or acromial part (commonly known as the side delt). The deltoid muscle is involved in several movements of the shoulder and arm.
The anterior deltoid is used when moving the arm forward in front of the body. It also works with the subscapularis, pecs, and lats to internally (medially) rotate the humerus. The anterior fibres assist the pectoralis major to flex the shoulder.
The lateral deltoid is the largest of the three deltoid muscles and is used when moving the arms out to the side.
The posterior deltoid is used when reaching backward or rotating the shoulder. These are often the smallest of the three deltoid muscles as they may get less use than the other two.
The deltoid muscle is involved in shoulder abduction, which is lifting the arm to the front, side, and back. It is also involved in shoulder extension, which is the movement of the arm directly behind the body. The deltoid is also one of the main abductors of the shoulder, along with the supraspinatus.
The deltoid muscle is easy to locate as it is close to the surface of the skin. To find it, touch the side of your neck and follow your shoulder line outward to the point where your arm slants downward.
The deltoid muscle is very mobile and can move in almost any direction without restrictions. This mobility also makes it prone to injury.
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Rotator cuff muscles
The rotator cuff is a group of four distinct muscles and their tendons that provide stability and strength to the shoulder complex during motion. These muscles are the supraspinatus, infraspinatus, teres minor, and subscapularis. They are also referred to as the SITS muscle, with reference to the first letter of each muscle's name.
These muscles originate from the scapula and connect to the head of the humerus, forming a cuff around the glenohumeral joint. The rotator cuff muscles are responsible for stabilising the shoulder joint by providing "fine-tuning" movements of the head of the humerus within the glenoid fossa. They keep the head of the humerus within the small glenoid fossa of the scapula, enlarging the range of motion in the glenohumeral joint and avoiding mechanical obstruction.
The rotator cuff muscles are essential for almost every type of shoulder movement, including flexion, abduction, internal rotation, and external rotation. For example, the deltoid, teres minor, and infraspinatus muscles are primarily responsible for external rotation, as seen in a tennis backhand stroke. The supraspinatus is the only muscle of the rotator cuff that is not a rotator of the humerus.
The rotator cuff muscles are common sites of injury in adults, particularly among those who perform overhead motions repeatedly, such as throwing a baseball or painting a ceiling. These muscles have tendons that run under the acromion, a bony prominence at the distal end of the scapula, leaving them vulnerable to compression and pinching, which can result in an injury known as shoulder impingement syndrome.
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Scapular muscles
The scapula, or shoulder blade, is the sturdy, flat, triangular bone that sits above the rib cage in the upper back. It forms the posterior of the shoulder girdle and creates the shoulder joint where it meets with the head of the humerus, the bone of the upper arm. The scapula provides attachment points for several groups of muscles, including the intrinsic and extrinsic muscles.
The intrinsic muscles of the scapula include the rotator cuff muscles: the teres major, subscapularis, teres minor, supraspinatus, and infraspinatus. These muscles attach to the scapular surface and assist with abduction and external and internal rotation of the glenohumeral joint. The supraspinatus stretches from the top of the scapula to the top of the humerus, at the ball of the shoulder joint. The infraspinatus stretches from the bottom of the scapula and connects to the humerus behind the supraspinatus. The subscapularis attaches to the middle part of the scapula and stretches to the bottom part of the ball of the humerus. The teres minor attaches to the side of the scapula and attaches to the humerus beneath the infraspinatus. The rotator cuff muscles are common sites of injury in adults, particularly those who perform overhead motions repeatedly.
The extrinsic muscles of the scapula include the triceps, biceps, and deltoid. The deltoid muscle is on the outside of the shoulder and helps move the arm forward, backward, and to the side. The supraspinatus and deltoid are the two main abductors of the shoulder. The teres major attaches at the inferior angle of the scapula.
The third group of muscles includes the levator scapulae, trapezius, rhomboids, and serratus anterior. These muscles are responsible for rotational movements and stabilization of the scapula. The rhomboids help lift the shoulder blade. The trapezius and serratus anterior are the muscle synergists for upward rotation of the scapula. The scapula must upwardly rotate, posteriorly tilt, and externally rotate for full arm elevation.
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Shoulder flexion
The deltoid muscle, located on the outside of the shoulder, plays a crucial role in shoulder flexion. It helps move the arm forward, backward, and to the side. During shoulder flexion, the anterior fibers of the deltoid muscle contract concentrically to flex and internally rotate the shoulder. The deltoid muscle is also involved in abduction, the motion of lifting the arm away from the body.
The pectoralis major muscle, situated in the chest, is another key contributor to shoulder flexion. In addition to flexion, this muscle is also responsible for internal rotation of the shoulder and adduction, which is the action of lowering the arm towards the body.
The coracobrachialis muscle, found on the inside of the upper arm, is integral to shoulder flexion. During this movement, the coracobrachialis muscle concentrically contracts, contributing to both flexion and abduction of the shoulder.
The biceps brachii muscles, located on the front of the upper arm, assist in shoulder flexion. The long head of the biceps brachii is specifically involved in flexion and abduction, while the muscle as a whole aids in elbow flexion and extension. However, it is important to note that the biceps brachii only weakly assists in shoulder flexion compared to the other muscles mentioned.
The ability to perform shoulder flexion and the range of motion can be assessed through manual muscle testing. Examiners can passively place the shoulder in a starting position with slight flexion and then guide the patient through the movement. The patient is then asked to actively perform the motion to ensure a full range of motion.
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Shoulder adduction and abduction
The shoulder joint is a complex ball-and-socket joint, formed by the head of the humerus, the clavicle, and the scapula. It is one of the most freely moveable areas in the human body, with a wide range of motion. This comes at the expense of decreased stability, making the shoulder prone to dislocation and injury.
The shoulder muscles aid in movement and help to protect and maintain the shoulder joint. They also provide structural support to the shoulder joint and allow for a wide range of movements, from throwing a ball to reaching for an item on a shelf.
The deltoid muscle is on the outside of the shoulder and helps move the arm forward, backward, and to the side. It is involved in shoulder flexion (forward motion) and extension (backward motion). Shoulder flexion is also achieved by the pectoralis major muscle in the chest, the coracobrachialis muscle in the upper arm, and the biceps brachii muscles in the upper arm. Extension of the shoulder is accomplished by the latissimus dorsi, teres major, and triceps, in addition to the deltoid.
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Frequently asked questions
There are about 20 muscles that support the shoulder and allow it to turn and rotate in many directions. The main ones are the deltoids, the rotator cuff muscles, the scapular muscles, and the pectoralis major.
The deltoid muscles sit on top of the rotator cuff muscles on the front, side, and back of each shoulder. They are the shoulder's prime movers, allowing us to move our arms forward, backward, and to the side.
The four rotator cuff muscles are the supraspinatus, infraspinatus, teres minor, and subscapularis. They stabilize the humeral head (the ball portion of the ball-and-socket shoulder joint).
The scapular muscles, including rhomboids (major and minor), trapezius, levator scapulae, and serratus anterior, sit on or around the shoulder blade and assist in rotating the bone.
Shoulder injuries are common in people who use their shoulders a lot for overhead motions, such as athletes or swimmers. Common injuries include shoulder bursitis (inflammation of fluid-filled sacs in the shoulder joint), frozen shoulder (stiffness in the shoulder joint), and rotator cuff strains.
