Rotator Cuff Muscles
Rotator Cuff Muscles
The rotator cuff is a set of four distinct muscles and their tendons that surround the shoulder. They provide strength and stability to the shoulder and allow normal, pain-free movement in all directions. Each of the rotator cuff muscles arises from the scapula (shoulder blade) and connects to the head of the humerus (upper arm bone). Together, the four muscles form a “cuff” or collar around the shoulder joint. In this article, we take a deep dive into the four muscles of the rotator cuff – the supraspinatus, infraspinatus, teres minor, and subscapularis (an easy way to remember the names is the mnemonic SITS). We explore the structure and function of each of these muscles and explain how they are evaluated during a physical examination. We also discuss the common symptoms of rotator cuff pathologies.
Supraspinatus
Origin and insertion:
The supraspinatus muscle originates from the supraspinatus fossa, a shallow depression on the upper backside of the shoulder blade. It travels under the acromion (bony tip of the shoulder) and is inserted on the greater tuberosity of the humerus (a prominent area at the top end of the upper arm bone).
Function:
The function of the supraspinatus is to abduct the arm from 0 to 15 degrees. Abduction is the movement of the arms away from the body. Beyond this range, up to 90 degrees, the supraspinatus assists the deltoid muscle in abduction. An additional important function of the supraspinatus is shoulder stability. It holds the head of the humerus firmly against the shoulder socket. The supraspinatus resists the force of gravity and the downward pull of the shoulder due to the weight of the upper limb.
Testing:
The empty can test can be used to assess the integrity of the supraspinatus muscle. The arm is fully abducted to 90 degrees with full internal rotation (thumb pointing down). This position simulates emptying a Coke can with an outstretched arm. The orthopedist applies downward pressure on the forearm, and the patient is asked to resist. If the patient gives way or reports pain or weakness, the empty can test is considered positive. It can imply injuries or tears of the supraspinatus muscle or tendon.
Infraspinatus
Origin and insertion:
The infraspinatus is a thick, triangular muscle that originates from the infraspinatus fossa, a large depression in the shoulder blade. It is inserted on the greater tuberosity of the humerus and the capsule of the shoulder joint.
Function:
The infraspinatus is the primary external rotator of the shoulder joint. External rotation is the movement away from the center of the body so that your thumb points to the side. It also assists in shoulder extension (sticking your arm behind you). Additionally, the infraspinatus provides stability to the shoulder joint.
Testing:
One of the tests an orthopedist can perform an infraspinatus pain test to see if your pain is coming from this muscle. With your elbow at your side, bend your arm 90 degrees with the palm facing up and your arm out in front of you. The doctor pushes against your arm while you rotate it outward. If this is painful, then it indicates an infraspinatus problem.
Teres Minor
Origin and insertion:
The teres minor is a narrow muscle that arises from the upper part of the outer border of the shoulder blade. It is inserted on and near the greater tubercle of the humerus, a prominence located to the back side of the humeral head.
Function:
Along with the infraspinatus, the teres minor performs the action of external rotation. It also assists in extension (sticking your arm behind you) and adduction (moving your arm towards the middle like hugging yourself). Along with the other muscles of the rotator cuff, the teres minor plays an instrumental role in providing shoulder stability.
Testing:
A test called the Hornblower’s sign can be used to test for a tear in the teres minor. The elbow is flexed at 90 degrees, and the arm is raised to 90 degrees. The orthopedists then asks you to externally rotate your arm against resistance (move the arm upward and backward against pressure by the examiner). If you are unable to do so, the test is considered positive. Tears in the trees minor are very uncommon.
Subscapularis
Origin and insertion:
The subscapularis is the largest, strongest, and most active rotator cuff muscle. This triangular muscle sits under the scapula (shoulder blade). It arises from the subscapular fossa, a broad depression in the shoulder blade. It is inserted on the lesser tuberosity of the humerus and in front of the joint capsule.
Function:
The main function of the subscapularis is internal rotation of the arm (movement in and towards the middle of the body). It also assists in adduction (lowering the arm toward the body) and extension (sticking the arm behind the body). As with the other rotator cuff muscles, the subscapularis plays a vital role in shoulder stabilization.
Testing:
The lift-off test can be used to see if the subscapularis is functional. You are asked to place your hand on your lower back with the palm facing out. The orthopedist will then ask you to lift your arm into shoulder extension and internal rotation. An inability to do so indicates dysfunction of the subscapularis. Another test for the subscapularis is the bear hug test. You are asked to place your hand on the opposite shoulder. The doctor will apply an external rotation force and ask you to try and maintain the arm position. Weakness compared to the other side indicates a problem with the subscapularis muscle.
Rotator Cuff Injuries
Injuries of one or more of the rotator cuff muscles are very common, especially in people over the age of 40. Athletes and people whose occupations or hobbies involve the repeated movement of the arms above the head are at increased risk of injuring the rotator cuff muscles. In most instances, however, conservative treatments are successful in controlling symptoms.