Logan Steele
The internal rotator muscles are those that facilitate the movement of a limb toward the midline along a vertical axis. The subscapularis muscle, which is the largest and strongest cuff muscle, is the primary internal rotator of the shoulder. It is also assisted by the pectoralis major, latissimus dorsi, teres major, and the anterior deltoid. In the thigh or femur, the tensor fasciae latae, gluteus medialis, gluteus maximus, adductor longus, and adductor brevis are responsible for internal rotation. The popliteus, semimembranosus, semitendinosus, and sartorius are the internal rotators of the leg at the knee. Lastly, the superior rectus muscle and superior oblique muscle are responsible for the internal rotation of the eyeball, also known as intorsion or incyclotorsion.
| Characteristics | Values |
|---|---|
| Muscles that perform internal rotation of the arm/humerus at the shoulder | Anterior part of the deltoid muscle, subscapularis, teres major, latissimus dorsi, pectoralis major |
| Muscles that perform internal rotation of the thigh/femur at the hip | Tensor fasciae latae, gluteus generalis, anterior fibers of gluteus meralis, adductor longus, adductor brevis |
| Muscles that perform internal rotation of the leg at the knee | Popliteus, semimembranosus, semitendinosus, sartorius |
| Muscles that perform internal rotation of the eyeball | Superior rectus muscle, superior oblique muscle |
| Shoulder internal rotators | Subscapularis, pectoralis major, latissimus dorsi, teres major, anterior deltoid |
| Largest and strongest cuff muscle, providing 53% of total cuff strength | Subscapularis |
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What You'll Learn
Shoulder internal rotators
The internal rotators of the shoulder, or the shoulder's internal rotators, are the muscles that enable the internal rotation of the arm or humerus at the shoulder. This involves the limb rotating toward the midline along a vertical axis, with a normal range of motion of 70º to 90º.
The subscapularis muscle is the main internal rotator of the shoulder. It is the largest and strongest cuff muscle, providing 53% of total cuff strength. The subscapularis consists of superficial and deep fibres that surround the bicipital groove, forming its "roof" and "floor", respectively. These fibres merge with those from the supraspinatus and the superior glenohumeral ligament/coracohumeral ligament complex.
The other muscles that act as internal rotators of the shoulder are the pectoralis major, latissimus dorsi, teres major, and the anterior deltoid. The pectoralis major and latissimus dorsi are also involved in posterior dislocation of the shoulder joint, which is caused by indirect mechanisms such as electric shock or convulsions. The teres major and anterior deltoid are also involved in shoulder flexion, along with the coracobrachialis and biceps brachii.
The rotator cuff muscles play a crucial role in stabilising the glenohumeral joint, which permits a vast range of motion across multiple planes. The rotator cuff is composed of four muscles: the supraspinatus, infraspinatus, subscapularis, and teres minor. The subscapularis, as mentioned, is the main internal rotator, while the infraspinatus and teres minor contribute to external rotation.
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Subscapularis
The subscapularis is a large and powerful triangle-shaped muscle that fills the subscapular fossa and is covered by a dense fascia that attaches to the scapula. It is the largest and strongest part of the four rotator cuff muscles, with the other three being the supraspinatus, infraspinatus, and teres minor muscles. The subscapularis muscle derives its name from its orientation under the scapula, with "sub" meaning under and "scapula" meaning wing bone.
The subscapularis muscle's primary function is the internal rotation of the humerus, but it can also aid in adduction. Arm position has a significant effect on the actions caused by this muscle. When the arm is raised, the subscapularis pulls the humerus forward and downward. When the humerus is in a fixed position, the insertion of the subscapularis can act as an origin, producing abduction of the inferior border of the scapula. As part of the rotator cuff, the subscapularis plays a crucial role in stabilising the shoulder.
The subscapularis tendon lies approximately 3 to 5 cm under the surface, which is quite deep for ultrasonography. However, a detailed examination of the muscle can be facilitated by using a highly penetrative 5 MHz linear applicator. The subscapularis nerve innervates the muscle, and the subscapular artery, a branch of the axillary artery, provides its primary blood supply.
Injuries to the subscapularis tendon are most common in young males, and patients typically report a history of forced external rotation. The patient will primarily complain of pain in front of the shoulder, and a physical exam will include inspection, palpation, range of motion, strength, and special tests. The "Gerber" or lift-off test is a useful clinical test for examining the subscapularis, and the bear hug test is also sensitive for detecting subscapularis muscle tears.
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Pectoralis major
The pectoralis major is a thick, fan-shaped muscle that contributes to thoracobrachial movement. It is the largest and most superficial muscle in the chest area, commonly referred to as the "pecs" or "chest muscle". The pectoralis major has a broad origin and is divided into three parts: the clavicular part, the sternocostal part, and the abdominal part. All three parts converge laterally and insert onto the greater tubercle of the humerus.
The pectoralis major's primary functions are flexion, adduction, and internal rotation of the humerus at the shoulder joint. It acts as a strong adductor and internal rotator of the humerus, with the clavicular portion of the muscle flexing the humerus up to 90 degrees in a horizontal plane. The sternocostal portion produces the antagonistic movement, extending the humerus back to the anatomical position.
The pectoralis major is innervated by both the medial and lateral pectoral nerves. The clavicular head receives innervation from the C5 and C6 nerve roots, while the sternal head receives input from the C7, C8, and T1 nerve roots. The sensory feedback from the pectoralis major follows a reverse path, returning via first-order neurons to the spinal nerves.
The absence or malformation of the pectoralis major can result in difficulties performing certain movements. Poland syndrome, for example, is a congenital anomaly characterised by the absence or malformation of the pectoralis major on one side of the body, along with other upper extremity defects. Individuals with Poland syndrome may experience challenges in adduction and internal rotation of the arm, and may also have visibly distorted breasts due to the aplasia of the pectoralis major muscle.
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Latissimus dorsi
The latissimus dorsi muscle is a large, flat muscle that covers the lower and lateral sides of the back (lumbar region) and the lateral part of the chest. It is one of the muscles responsible for internal rotation, especially at the shoulder, where it is the most important internal rotator. It also plays a role in adduction and extension of the arm at the glenohumeral joint.
The latissimus dorsi muscle originates from the spinous processes of the thoracic and lumbar vertebrae, the supraspinous ligament, the medial sacral crest, and the posterior third of the external lip of the iliac crest. Some of its fibres also originate from the outer face of the lower ribs. The muscle fibres are oriented in different directions, with the uppermost fibres being almost horizontal and the lower fibres being more vertical on the thorax.
Functionally, the latissimus dorsi muscle is involved in scapular motion. It can pull the inferior angle of the scapula in various directions, producing movements at the shoulder joint. This includes internal rotation, adduction, and extension of the arm. The muscle also works collaboratively with the teres major and pectoralis major muscles to perform actions of the upper extremity, including adduction, medial rotation, and extension of the arm.
The latissimus dorsi muscle is important for various activities and movements. For example, during deep inspiration and forceful respiratory functions such as coughing and sneezing, the muscle is active. It also enables the trunk to move forward and upward when the upper extremities are fixed overhead, as in climbing or performing a chin-up. In individuals with paralysis of the lower body, the latissimus dorsi muscle allows for movement of the pelvis and trunk by attaching to the pelvis and receiving innervation.
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Anterior deltoid
The deltoid muscle is a large, triangular-shaped muscle that covers the shoulder joint, giving it its rounded contour shape. It is composed of three distinct sections: the anterior or clavicular part (commonly known as the front delt), the posterior or scapular part (or rear delt), and the intermediate or acromial part (or side delt). The deltoid muscle is involved in both internal and external rotation of the shoulder, with the anterior section typically responsible for internal rotation.
The anterior deltoid is involved in horizontal adduction, which involves rotating the arm towards the centre of the body. It works in tandem with the subscapularis, pecs, and lats to internally (or medially) rotate the humerus. The anterior fibres also assist the pectoralis major in flexing the shoulder. When all the deltoid fibres contract simultaneously, the muscle becomes the prime mover of arm abduction along the frontal plane. For maximum effect, the arm must be medially rotated.
The deltoid muscle is an important stabiliser of the humeral head and helps prevent its dislocation when carrying heavy loads. It also assists in forward elevation and abduction of the shoulder. The anterior deltoid fibres originate from the clavicle and the anterior third of the lateral acromion. These fibres arise from the anterior border and upper surface of the lateral third of the clavicle, lying adjacent to the lateral fibres of the pectoralis major muscle.
The deltoid muscle is a defining feature of the human physique and is involved in various movements of the arm. It is a very powerful muscle, used in many everyday activities such as putting on clothes and carrying shopping bags, as well as athletic activities like swimming and water polo. The deltoid compensates for the loss of strength in the rotator cuff, although tears in the deltoid muscle can lead to functional issues in the shoulder complex.
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Frequently asked questions
The muscles involved in internal rotation include the subscapularis, pectoralis major, latissimus dorsi, teres major, and the anterior deltoid.
The subscapularis muscle is the main internal rotator of the shoulder. It is the largest and strongest cuff muscle, providing 53% of total cuff strength.
Internal rotation involves limb rotation toward the midline along a vertical axis. The normal range of motion is 70º to 90º.
