Logan Steele
The internal rotator muscles are those that facilitate the movement of a limb towards the midline along a vertical axis. The muscles that perform internal rotation vary depending on the body part in question. For the arm or humerus at the shoulder, the subscapularis, pectoralis major, latissimus dorsi, teres major, and anterior deltoid are the muscles responsible for internal rotation. The subscapularis is the main internal rotator of the shoulder, providing 53% of total cuff strength. In the thigh or femur at the hip, the tensor fasciae latae, gluteus medialis, anterior fibres of the gluteus medialis, adductor longus, and adductor brevis are the muscles that perform internal rotation. Finally, for the leg at the knee, the popliteus, semimembranosus, semitendinosus, and sartorius are the internal rotator muscles.
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What You'll Learn
Shoulder internal rotators
The internal rotators of the shoulder, or the glenohumeral joint, include the subscapularis, pectoralis major, latissimus dorsi, teres major, and the anterior deltoid. The subscapularis is the main internal rotator of the shoulder and is the largest and strongest cuff muscle, providing 53% of total cuff strength. The upper 60% of the insertion is tendonous, and the lower 40% is muscle. 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 rotator cuff muscles act as the glenohumeral joint's dynamic stabilizers. The rotator cuff is composed of four muscles: the supraspinatus, infraspinatus, subscapularis, and teres minor. The primary biomechanical function of the rotator cuff is to stabilize the glenohumeral joint by compressing the humeral head against the glenoid. The infraspinatus and teres minor contribute to external rotation, while the subscapularis facilitates internal rotation.
The glenohumeral joint permits a wide range of motion across multiple planes. Understanding the contribution of various muscles to these movements is essential for differentiating shoulder pathologies. Internal rotation involves limb rotation toward the midline along a vertical axis. The normal range of motion for internal rotation is 70º to 90º.
Posterior dislocation of the glenohumeral joint is often the result of indirect mechanisms, such as electric shock or convulsions, causing contraction of the relatively stronger internal shoulder rotators, including the latissimus dorsi, pectoralis major, and subscapularis.
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Subscapularis
The subscapularis is a large and powerful triangle-shaped muscle that fills the subscapular fossa and inserts into the lesser tubercle of the humerus. The subscapularis is the largest and strongest muscle of the rotator cuff, with the other three muscles being the supraspinatus, infraspinatus, and teres minor. The subscapularis derives its name from its orientation under the scapula, with "sub" meaning under and "scapula" meaning wing bone.
The subscapularis is covered by a dense fascia that attaches to the scapula at the margins of the muscle's attachment, or origin, on the scapula. The muscle's fibres pass laterally from its origin before converging into a tendon of insertion. The tendon intertwines with the glenohumeral (shoulder) joint capsule. The subscapularis (supraserratus) bursa separates the subscapularis from the serratus anterior. The muscle's primary function is the internal rotation of the humerus, but it can also aid in adduction.
Arm position affects the actions caused by the subscapularis 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 may contain up to three trigger points, with the two most common occurring near the outside edge of the muscle.
Injuries to the subscapularis tendon are most common in young males, and patients typically report a history of forced external rotation. Pain is usually felt in front of the shoulder, and a physical exam may reveal weakness or pain during internal rotation at 0 degrees, with increased passive external rotation. The Gerber Lift-off test is the established clinical test for examining the subscapularis, while the bear hug test is used to detect muscle tears.
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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 of the arm/humerus at the shoulder.
At the level of the shoulder, the latissimus dorsi muscle is the most important internal rotator. It is also crucial for the adduction and flexion of the shoulder. This muscle can pull the inferior angle of the scapula in various directions, resulting in internal rotation, adduction, and extension of the arm.
The latissimus dorsi works collaboratively with the teres major and pectoralis major muscles to perform actions of the upper extremity. Together, these muscles adduct, medially rotate, and extend the arm at the glenohumeral joint. The latissimus dorsi is also involved in extending the humerus.
The origin of the latissimus dorsi muscle is through the posterior leaflet of the lumbodorsal fascia, arising from the spinous processes of the last six thoracic vertebrae and the lumbar vertebrae. It also originates from structures like the supraspinous ligament, the medial sacral crest, and the posterior third of the external lip of the iliac crest. Some of its fibers originate from the outer face of the lower ribs.
The muscle fibers of the latissimus dorsi are oriented in different directions. The uppermost fibers are almost horizontal, while the lower fibers are more vertically oriented on the thorax. This muscle is also involved in activities like climbing, coughing, and sneezing, demonstrating its versatility and importance in various bodily functions.
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Pectoralis major
The pectoralis major is a thick, fan-shaped muscle that contributes to thoracobrachial motion. It is the largest and most superficial muscle in the chest area, commonly referred to as the "pecs" or "chest muscle". The pectoralis major is responsible for four actions that are primarily responsible for the movement of the shoulder joint.
Firstly, it facilitates the flexion of the humerus, as seen in underhand throwing or lifting a child. Secondly, the pectoralis major adducts the humerus, as when flapping the arms. Thirdly, it medially rotates the humerus, as occurs during arm-wrestling. Finally, the pectoralis major is responsible for keeping the arm attached to the trunk of the body. The muscle has two distinct parts, each with its own functions. The clavicular part, located near the deltoid muscle, contributes to flexion, horizontal adduction, and inward rotation of the humerus. When at an angle of approximately 110 degrees, it also assists in humerus adduction. The sternocostal part acts antagonistically to the clavicular part, facilitating the downward and forward movement of the arm and inward rotation during adduction.
The pectoralis major originates from the clavicle and sternum, costal cartilages of the true ribs, and the aponeurosis of the abdominal external oblique muscle. It inserts onto the lateral lip of the bicipital groove and the greater tubercle of the humerus. The muscle receives dual motor innervation from the medial pectoral nerve and the lateral pectoral nerve, also known as the lateral anterior thoracic nerve.
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 physical abnormalities. Individuals with Poland syndrome may experience challenges in adduction and medial rotation of the arm due to the missing or defective muscle. However, compensatory mechanisms involving other muscles, such as the latissimus dorsi and teres major, may mitigate these difficulties to some extent.
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Anterior deltoid
The deltoid muscle is a large, triangular-shaped muscle that forms the rounded contour of the human shoulder. It is also known as the 'common shoulder muscle'. The deltoid muscle 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 anterior deltoid is involved in internal rotation, horizontal adduction, and forward elevation of the shoulder. It works in tandem with the subscapularis, pecs, and lats to internally (medially) rotate the humerus. The anterior fibres assist the pectoralis major to flex the shoulder.
The deltoid muscle is the prime mover of arm abduction along the frontal plane when all its fibres contract simultaneously. The arm must be medially rotated for the deltoid to have maximum effect. This makes the deltoid an antagonist muscle of the pectoralis major and latissimus dorsi during arm adduction.
The anterior deltoid is also involved in preventing the dislocation of the humeral head when a person carries heavy loads. This function helps to keep carried objects at a safer distance from the thighs, as during a farmer's walk. The deltoid compensates for the loss of strength in the rotator cuff, and its fatigue or injury may result in a decline in abduction.
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Frequently asked questions
The internal rotators of the arm/humerus at the shoulder include the anterior part of the deltoid muscle, subscapularis, teres major, latissimus dorsi, and pectoralis major.
The subscapularis is the main internal rotator of the shoulder. It is the largest and strongest cuff muscle, providing 53% of total cuff strength.
The internal rotators of the thigh/femur at the hip include the tensor fasciae latae, gluteus generalis, anterior fibers of gluteus meralis, and adductor longus and adductor brevis.
