The Shoulder Complex, Part II
Fine Anatomy: Studying arm movement, kinesiology and exercises.
TThe multiarticular complex of the shoulder gives rise to the dynamic movement potential of the arm at the glenohumeral joint. If it were not for the physiological necessity of the scapulo-thoracic “joint” (discussed in the previous Fine Anatomy column, “The Shoulder Girdle,” IDEA Personal Trainer, October 2003, p.36) and its role during abduction or flexion of the upper limb to elevate, rotate, tilt and swivel, the elementary movements of the arm would be greatly limited.
The glenohumeral joint is the primary joint of the shoulder and most mobile joint in the human body. This ball and socket joint is often compromised due to its shallow socket and dynamic movement potential. There are four primary movements created at this joint: flexion, extension, abduction and adduction. There are also degrees of horizontal flexion, extension, abduction and adduction. Circumduction and medial and lateral rotation are also performed at the glenohumeral joint. This article focuses on the fine anatomy of this joint and its four basic movements.
The muscles that create the “rotator cuff” also assist in motions of abduction and adduction of the humerus. These muscles and their medial- and lateral-rotational movements will be covered in the next segment of this series. The fourth and final Fine Anatomy article on the shoulder will cover shoulder movements of circumduction and horizontal motion.
In this article the word “arm” will be used to describe movement and action of the humerus; “forearm” will refer to the radius and ulna. The muscles described in this section are located near the scapula or upper thorax, however, they insert and act on the humerus and arm motion. The degree of range of motion (ROM) within this joint is created over three main axes and three planes of movement in space.
- The Transverse Axis. When lying in a frontal plane, controls the arm movements of flexion and extension (performed in a sagittal plane).
- The Antero-Posterior Axis. When lying in a sagittal plane, controls the arm movements of abduction and adduction (performed in a frontal plane).
- The Vertical Axis. A more complex axis, the vertical axis runs through the intersection of the frontal and sagittal planes and corresponds to the third axis in space. It controls the arm movements of flexion and extension (performed in a horizontal plane) when the arm is abducted to 90 degrees.
Lateral and medial rotation of the arm occur about the long axis of the humerus. Rotation of the arm about its long axis can occur in any shoulder position due to its three axes and three degrees of freedom. Descriptions of this rotation usually assume that the arm hangs vertically along the body as a base reference point. To measure the range of rotation, the elbow must be flexed to 90 degrees with the forearm in a sagittal plane.
The glenohumeral joint has two very different functions that can often be contradictory: 1) it must allow a great amount of movement potential for arm motion and therefore be very flexible; and 2) it must provide a strong, stable socket for actions such as lifting or pushing resistance.
The shoulder comprises five joints—two physiological joints (the subdeltoid and scapulo-thoracic “joints”)—and three “true” joints (glenoid fossa, acromioclavicular and sternoclavicular). The glenohumeral joint—formed by the articulation of the head of the humerus and the glenoid fossa of the scapula—is the connection of the thorax to the upper humerus. The two other true joints creating upper-limb ROM are the acromioclavicular joint (between the distal clavicle and acromion process of the scapula) and the sternoclavicular joint (between the medial clavicle and manubrium of the sternum). In-depth discussion about these three “true” joints can be found in the October 2003 Fine Anatomy column.
The synergy and complexity of these joints that correlate to the body’s dynamic motion make the shoulder unique. The proximal end of the humerus (upper arm bone) articulates with the glenoid fossa of the scapula at the medial head. Landmarks to note are the lateral greater tubercle and anterior lesser tubercle (for muscle attachment) and the bicipital (intertubercular) groove running between the two tubercles and the anatomical neck just below the head of the humerus.
One of the strongest ligaments reinforcing the capsule is the superior corachohumeral ligament. This ligament runs from the border of the coracoid process to the greater tubercle. Anteriorly, there are three glenohumeral ligaments—the superior, middle and inferior bands. Both the superior and middle band run from the upper margin of the glenoid. The superior band tracks over the humeral head; the middle band runs in front of the humerus; and the inferior band runs across the anterior edge of the glenoid and below the humeral head.
The capsule of the glenohumeral joint is also reinforced by blending with tendons of the rotator cuff muscles. The capsule is weakest anteroinferiorly, where it neither has support from these muscles nor ligament support. A standard resting position for the glenohumeral joint is described as the arm in slight flexion, abduction and internal rotation.
In assigning muscle function, assume that the arm starts out in this standard anatomical position. In different positions, functional changes occur and may even be reversed. For example, pectoralis major is a flexor of the arm up to 60 degrees. Beyond 90 degrees, it can no longer move the arm forward or upward. In fact it begins to function as an extensor, bringing the arm back toward anatomical position. The same idea applies to latissimus dorsi in extension. In cases like these, deltoid tends to “take over” at the extremes of movement as other muscles become ineffective and even restrict further range to exist.
Flexion is movement in the anterior direction and may begin from a position of 45-degree extension. It describes an arc forward through the zero anatomical position to the 180-degree overhead position. There are three phases of flexion:
- From 0 degrees to 60 degrees the anterior fibers of the deltoid, coracobrachialis and the clavicular fibers of pectoralis major are involved. If the tension of the coraco-humeral ligament, teres minor, teres major or infraspinatus is too resistant, the range will be limited.
- From 60 to 120 degrees, trapezius and serratus anterior are involved and the range can be limited by the resistance of latissimus dorsi and the costo-sternal fibers of pectoralis major.
- The 180-degree position is attainable by the combined movements of the shoulder joint and the shoulder girdle (described in the previous article.) In this third phase of flexion (120 to 180 degrees), movement of the spinal column is necessary. The glenohumeral joint can be flexed only to approximately 120 degrees. The remaining 60 degrees is attained as a result of the abduction and lateral or upward rotation of scapula at the scapulothoracic joint, allowing the glenoid fossa to face more anteriorly and the humerus to flex to a fully vertical position.
Extension is movement in the posterior direction and technically refers to the arc of motion from 180 degrees flexion to 45 degrees extension. If the elbow joint is flexed, the range of shoulder joint extension will be increased because the tension of biceps is released. The degree of range compared to flexion is smaller.
Abduction is movement in a lateral direction through a range of 180 degrees to a vertical overhead position, (120 degrees of abduction and 60 degrees of upward scapular rotation). This movement is achieved in three phases with the combined coordination of the shoulder girdle and glenohumeral joint range of motion. Abduction from 0 to 60 degrees takes place at the glenohumeral joint; from 60 to 120 degrees requires recruitment of the scapulo-thoracic joint; and from 120 to 180 degrees involves both joints as well as flexion of the trunk to the opposite side.
Adduction is movement toward the mid-sagittal plane in a medial direction and technically refers to the arc of motion from full elevation overhead through the zero anatomical position to a position obliquely upward and across the front of the body. Adduction combined with extension moves the arm behind the body; combined with flexion it moves the arm in a 30 to 45 degree range in front of the body. True adduction in the frontal plane is mechanically impossible due to the presence of the trunk.
- Coracobrachialis arises from the coracoid process and inserts on the medial surface of the humeral shaft. It acts on flexion and adduction of the humerus.
- Biceps brachii has two origins: 1. Short head—from the coracoid process and interdigitates with coracobrachialis. 2. Long head—from a tubercle above the glenoid fossa. Both insert on the radial tuberosity and the bicipital aponeurosis, (a broad fibrous sheath that blends with the deep fascia of the medial forearm). Biceps is primarily a flexor and supinator of the forearm but is also involved in abduction, adduction and flexion of the arm.
- Triceps brachii is primarily a forearm extensor but its long head originates from the scapula and assists in adduction of the arm.
- Pectoralis major has two heads. The clavicular head arises from the anterior, medial clavicle and the sternocostal head arises from the sternum and costal cartilages 1 through 6. Pectoralis major inserts on the lateral aspect of the bicipital groove. The tendon is situated such that the fibers from the clavicular head insert below those from the sternocostal head. Contraction of both heads produces adduction and medial rotation of the arm. The clavicular head independently flexes the extended shoulder, and the sternocostal head independently extends the flexed shoulder. In some instances, such as a pull-up, pectoralis major pulls the thorax toward the “fixed” arm.
- Latissimus dorsi is the widest back muscle and originates from the sacral and iliac crests, thoracolumbar fascia, spinous processes of T7 to T12, and posterior surface of the lower ribs. The tendon wraps around the medial side of the humerus, and twists and inserts on the bicipital groove. It assists pectoralis major in raising the thorax toward the arm during pull-ups. It acts upon extension, adduction and medial rotation of the arm.
- Teres major originates from the posterior surface of the inferior angle of the scapula and inserts on the medial aspect of the bicipital groove, aside latissimus dorsi. Teres major and teres minor (which will be discussed in the next segment of this series) have distinct functions and different innervation. It acts upon extension, adduction and medial rotation of the arm.
- The deltoid group originates from the spine and acromion of the scapula and lateral one-third of the clavicle, and inserts on the deltoid tuberosity on the lateral side of the humeral shaft. There are medial, anterior and posterior fibers that create an array of movement for the arm. The main action—abduction of the arm—is created by contraction of the middle fibers. The anterior fibers act upon flexion and medial rotation. The posterior fibers act in extension and lateral rotation of the humerus.
Note: The rotator cuff muscles (subscapularis, supraspinatus, infraspinatus and teres minor) will be discussed in the next segment.
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