The shoulder muscles are responsible for maintaining the widest range of motion of any joint in your body. This flexibility is also what makes the shoulder prone to instability and injury.

Muscles, tendons, and ligaments combine to keep your arm bone in your shoulder socket. They also protect the main shoulder joint, the glenohumeral.

About eight shoulder muscles attach to the shoulder blade (scapula), upper arm (humerus), and collar bone (clavicle). Many other muscles play a part in stabilizing and guiding the shoulder and its movements.

There are about 20 muscles supporting the shoulder and allowing it to turn and rotate in many directions.

These are the largest shoulder muscles:

  • Trapezius is a wide muscle that extends along the back part of your neck and shoulders and partway down your spine.
  • Deltoid is a large triangular muscle that covers the glenohumeral joint, where your upper arm inserts into your shoulder socket.
  • Pectoralis major is a large, fan-shaped muscle stretching from your collarbone to mid-chest.
  • Serratus anterior is a three-sectioned muscle starting at the shoulder blade and attached to the surface of the first eight ribs.
  • Rhomboid major is a flat trapezoid muscle in your back that reaches from the second, third, fourth, and fifth vertebrae to the shoulder blade.

Another four muscles comprise the shoulder rotator cuff:

  • Supraspinatus is a narrow triangular muscle at the rear of the shoulder blade.
  • Infraspinatus is a wide triangular muscle that attaches to the rear of the shoulder blade, below the supraspinatus.
  • Teres minor is a narrow muscle on the underside of the upper arm that connects the shoulder blade to the upper arm. It’s overlapped by the teres major and infraspinatus muscles.
  • Subscapularis is the largest and strongest of the rotator cuff muscles. It’s a triangular muscle on the front of the upper arm, beginning at the shoulder blade.

Other shoulder muscles include:

  • Pectoralis minor is a thin, flat muscle just under the pectoralis major that connects to the third, fourth, and fifth ribs.
  • Latissimus dorsi, known as lats, are large muscles in the middle of the back stretching from the backbone to the lower part of the shoulder blade.
  • Biceps brachii, or biceps, is a two-headed muscle that starts in two points at the top of the shoulder blade and comes together at the elbow.
  • Triceps is a long muscle that runs along the back of the upper arm, from the shoulder to the elbow.

Here are the normal ranges of motion for the shoulder:

  • Flexion is moving your arm from the side of your body then forward all the way over your head, normally up to 180 degrees.
  • Extension is moving your arm behind your back, normally 45 to 60 degrees.
  • Abduction is moving your arms from the sides of your body outward and up until your arms are parallel to the floor, up to 90 degrees.
  • Adduction is moving your arms from a position parallel to the floor to your sides, normally up to 90 degrees.
  • Medial or internal rotation is holding your arm at your side, bending the elbow forward 90 degrees, and then moving your lower arm toward your body.
  • Lateral or external rotation is holding your arm at your side, bending the elbow forward 90 degrees, and then moving your lower arm out away from your body.

Each muscle and muscle group plays a role in supporting your shoulder and allowing the wide range of motion of your arms and shoulder.

Larger shoulder muscles

The large shoulder muscles are responsible for most of the shoulder’s work.

  • Trapezius is responsible for elevating the shoulder blade and rotating it during arm abduction.
  • Deltoid is responsible for flexion and medial rotation of the arm. It’s also responsible for arm abduction, extension, and lateral rotation.
  • Pectoralis major is responsible for arm adduction and medial rotation of the arm. It’s also involved with air intake in breathing.
  • Rhomboid major helps keep the shoulder blade attached to the rib cage and allows you to pull your shoulder blades back.

Rotator cuff muscles

The four muscles of your rotator cuff keep the head of your upper arm bone, the humerus, from popping out of the socket of the shoulder blade.

  • Supraspinatus is responsible for beginning the upward motion of your arm. After about 15 degrees, the deltoid and trapezius muscles do the work. The technical term for the motion is horizontal abduction.
  • Infraspinatus mainly helps the rotation of your arm away from the center of your body. It’s the second most frequently injured shoulder muscle.
  • Teres minor helps with the lateral rotation of your arm.
  • Subscapularis helps stabilize the shoulder joint and allows it to rotate so the arm can turn in toward the midline of your body.

Other shoulder muscles

  • Pectoralis minor protects your shoulder blade and allows you to lower a shoulder.
  • Latissimus dorsi is responsible for extension, adduction, and the medial rotation of your upper arm.
  • Biceps brachii help keep your shoulder in place and are responsible for flexion and rotation of your lower arm.
  • Triceps help keep your shoulder in place and are responsible for extension of your lower arm.

Because your shoulder is so flexible in all ranges of motion, it’s a common site of muscle injury and pain.

According to the American Academy of Orthopedic Surgeons, most shoulder injuries involve the muscles, ligaments, and tendon, not the bones.

Sometimes shoulder pain can be referred pain, which results from an injury to your neck or another place. Usually, this type of pain doesn’t get worse when you move your shoulder.

Common shoulder muscle injuries include:

  • Sprains. These stretch or tear the shoulder ligaments, possibly resulting in a dislocation of the shoulder bones. Sprains range from mild to severe.
  • Strain. A shoulder strain stretches or tears a muscle or tendon. Strains range from mild to severe.
  • Labrum tear. This is a tear in the cartilage that lines the socket that holds the top of your upper arm bone. This can affect the rotator cuff and the biceps. When the tear is front to back, it’s known as a SLAP tear.
  • Spasm. These are a sudden tightening in the muscles.

Athletes are most at risk of shoulder muscle injuries. Older adults and those in occupations that involve repetitive or overhead motion or heavy lifting are also at risk.

A 2007 study found that shoulder pain was most prevalent in those age 55 to 64, affecting about 50 percent in that age group.

About 18 million Americans per year are affected by shoulder pain, according to a 2019 review. Rotator cuff tears are the most frequent cause.

Shoulder muscle injuries can result from:

  • trauma, such as a fall, a blow to the shoulder, or a car accident
  • age-related degeneration
  • overuse
  • sports that require the repeated overhead use of the shoulder, such as:
    • baseball
    • swimming
    • tennis
    • volleyball
    • golf
  • occupations that include repetitive overhead motion, vibration, or constant computer or phone use
  • bad posture

Treatment for muscle shoulder pain and injuries depends on the cause and severity of the pain or injury.

Conservative treatment is often effective. This can include:

Acupuncture is reported to improve pain and function for 2 to 4 weeks. In some cases, surgery may be necessary.

It’s best to see a doctor if you have persistent or acute shoulder pain.

Sudden shoulder pain may be a sign of a heart attack, requiring emergency medical attention.

It’s important to get a diagnosis and begin treatment as soon as possible. “Working through” shoulder pain, or continuing a sport or training despite the pain can make the pain or injury worse.

The shoulder is a complex joint with many muscles that control the shoulder’s wide range of motion.

This freedom of movement makes the shoulder vulnerable to injury and pain.

Shoulder pain is common in athletes and the general population. Prompt treatment and rest are effective conservative treatments.