Structure and Function of the Shoulder Complex

• Identify the bones and primary bony features relevant to the shoulder complex.

• Describe the location and primary function of the ligaments that support the joints of the shoulder complex.

• Cite the normal ranges of motion for shoulder flexion and extension, abduction and adduction, and internal and external rotation.

• Describe the planes of motion and axes of rotation for the primary motions of the shoulder.

• Cite the proximal and distal attachments, actions, and innervation of the muscles of the shoulder complex.

• Describe the muscular interactions involved with active shoulder abduction.

• Describe the scapulohumeral rhythm.

• Explain the force-couple that occurs to produce upward rotation of the scapula.

• Identify the primary muscles involved with dynamic stabilization of the glenohumeral joint.

• Explain how the shoulder depressor muscles can be used to elevate the thorax.

• Describe the interaction between the internal and external rotators of the shoulder during a throwing motion.

muscular substitution

reverse action

scapulohumeral rhythm

Our study of the upper limb begins with the shoulder complex—a set of four articulations involving the sternum, clavicle, ribs, scapula, and humerus (Figure 4-1). This series of joints works together to provide large ranges of motion to the upper extremity in all three planes. Rarely does a single muscle act in isolation at the shoulder complex. Rather, muscles work in teams to produce highly coordinated movements that are expressed over multiple joints. The cooperative nature of the shoulder musculature increases the versatility, control, and range of active movements available to the upper extremity. Because of the nature of this functional relationship among the shoulder muscles, paralysis, weakness, or tightness of any single muscle can disrupt the natural kinematic sequencing of the entire shoulder complex. This chapter provides an overview of the kinesiology of the four joints of the shoulder complex and the important muscular synergies that support proper function of the shoulder (Figure 4-1).

Figure 4-1 The joints of the right shoulder complex. (From Neumann DA: Kinesiology of the musculoskeletal system: foundations for physical rehabilitation, ed 2, St Louis, 2010, Mosby, Figure 5-1.)

Osteology

Sternum

The sternum, often called the breast bone, is located at the midpoint of the anterior thorax and is composed of the manubrium, body, and xiphoid process (Figure 4-2). The manubrium is the most superior portion of the sternum that articulates with the clavicle—forming the sternoclavicular joint. The body or middle portion of the sternum serves as the anterior attachment for ribs 2 through 7. The inferior tip of the sternum is called the xiphoid process, meaning “sword shaped.”

Clavicle

The clavicle, commonly called the collarbone, is an S-shaped bone that acts like a mechanical rod that links the scapula to the sternum (Figure 4-3). The flattened lateral portion—called the acromial end—articulates with the acromion of the scapula, forming the acromioclavicular joint. The medial or sternal end of the clavicle articulates with the manubrium of the sternum, forming the sternoclavicular joint.

Scapula

Commonly called the shoulder blade, the scapula is a highly mobile, triangular bone that rests on the posterior side of the thorax (Figure 4-4). The slightly concave anterior aspect of the bone is called the subscapular fossa, which allows the scapula to glide smoothly along the convex posterior rib cage. The glenoid fossa is the slightly concave, oval-shaped surface that accepts the head of the humerus, composing the glenohumeral joint. The superior and inferior glenoid tubercles border the superior and inferior aspects of the glenoid fossa and serve as proximal attachments for the long head of the biceps and the long head of the triceps, respectively. The scapular spine divides the posterior aspect of the scapula into the supraspinatous fossa (above) and the infraspinatous fossa (below). The acromion process is a wide, flattened projection of bone from the most superior-lateral aspect of the scapula. The acromion forms a functional “roof” over the humeral head to help protect the delicate structures within that area. The coracoid process is the finger-like projection of bone from the anterior surface of the scapula, palpable about 1 inch below the most concave portion of the distal clavicle. The coracoid process is the site of attachment for several muscles and ligaments of the shoulder complex. The medial and lateral borders of the scapula meet at the inferior angle, or tip, of the scapula. Clinically, the inferior angle is important in helping track scapular motion.

Proximal-to-Mid Humerus

The proximal humerus (Figure 4-5) is the point of attachment for a multitude of ligaments and muscles. The distal humerus is discussed in the next chapter.

Figure 4-5 Anterior **(A)** and posterior **(B)** views of the right humerus. Proximal attachments of muscles are shown in red, distal attachments in gray. (From Neumann DA: Kinesiology of the musculoskeletal system: foundations for physical rehabilitation, ed 2, St Louis, 2010, Mosby, Figures 5-7, A, and 5-9.)

The humeral head is nearly one half of a full sphere that articulates with the glenoid fossa forming the glenohumeral joint. The lesser tubercle is a sharp, anterior projection of bone just below the humeral head. The larger, more rounded lateral projection of bone is the greater tubercle. The greater and lesser tubercles are divided by the intertubercular groove, often called the bicipital groove because it houses the tendon of the long head of the biceps. More distally, on the lateral aspect of the upper one third of the shaft of the humerus is the deltoid tuberosity—the distal insertion of all three heads of the deltoid muscle. The radial (spiral) groove runs obliquely across the posterior surface of the humerus. The radial nerve follows this groove and helps define the distal attachment for the lateral and medial heads of the triceps.

Arthrology

The shoulder complex functions through the interactions of four joints: (1) Sternoclavicular, (2) scapulothoracic, (3) acromioclavicular, and (4) glenohumeral joints. To fully understand how the shoulder functions as a whole, we must first examine the structure and kinematics of each individual joint.

Joints of the Shoulder Complex

Sternoclavicular Joint

General Features

The sternoclavicular (SC) joint is created by the articulation of the medial aspect of the clavicle with the sternum (Figure 4-6). This joint provides the only direct bony attachment of the upper extremity to the axial skeleton—accordingly, the joint must be stable while also allowing extensive mobility.

Figure 4-6 An anterior view of the sternoclavicular joints with the capsule and some of the ligaments removed on the left side. (From Neumann DA: Kinesiology of the musculoskeletal system: foundations for physical rehabilitation, ed 2, St Louis, 2010, Mosby, Figure 5-11.)

The SC joint allows motion in all three cardinal planes, and it is supported by a thick network of ligaments, an articular disc, and a joint capsule. The high degree of stability provided by this thick ligamentous network explains, in part, why fractures of the clavicle occur more frequently than dislocations of the SC joint.

Supporting Structures of the Sternoclavicular Joint

Figure 4-6 illustrates the supporting structures of the SC joint.

• Sternoclavicular Ligament: Contains anterior and posterior fibers that firmly join the clavicle to the manubrium

• Joint Capsule: Surrounds the entire SC joint; is reinforced by the anterior and posterior SC joint ligaments

• Interclavicular Ligament: Spans the jugular notch, connecting the superior medial aspects of the clavicles

• Costoclavicular Ligament: Firmly attaches the clavicle to the costal cartilage of the first rib and limits the extremes of all clavicular motion except depression

• Articular Disc: Acts as a shock absorber between the clavicle and the sternum; helps improve joint congruency

Kinematics

The SC joint structure is a saddle joint with concave and convex surfaces on each of the joint’s articular surfaces (Figure 4-7). This conformation allows the clavicle to move in all three planes. Motions include elevation and depression, protraction and retraction, and axial rotation (Figure 4-8).

Figure 4-7 The right sternoclavicular joint has been opened up to expose matching surfaces of the saddle joint. (From Neumann DA: Kinesiology of the musculoskeletal system: foundations for physical rehabilitation, ed 2, St Louis, 2010, Mosby, Figure 5-12.)

In essence, all movements of the shoulder girdle (i.e., the scapula and clavicle) originate at the SC joint. A fused SC joint would therefore significantly limit movement of the clavicle and scapula and hence would limit movement of the entire shoulder.

Scapulothoracic Joint

General Features

The scapulothoracic joint is not a “true” joint in the traditional sense. It refers to the junction created by the anterior aspect of the scapula on the posterior thorax. Scapulothoracic joint motion typically describes the motion of the scapula relative to the posterior rib cage.

Normal movement and posture of the scapulothoracic joint are essential to the normal function of the shoulder. Clinicians therefore focus a great deal on evaluating and treating the quality and amount of motion between the scapula and the thorax.

Kinematics

Motions at the scapulothoracic joint include elevation and depression, protraction and retraction, and upward and downward rotation (Figure 4-9). All motions are functionally linked to the motions that occur at the other three joints of the shoulder complex; these functional relationships are discussed in depth later.

Elevation and Depression

Scapular elevation involves the scapula sliding superiorly on the thorax (e.g., shrugging the shoulders). Depression occurs when the scapula slides inferiorly on the thorax (Figure 4-9, A; e.g., returning shrugged shoulders to a resting position; depressing the entire shoulder, as occurs when pushing up from a sitting position).

Protraction and Retraction

Protraction describes the motion of the scapula sliding laterally on the thorax, away from midline, whereas retraction describes movement of the scapula toward the midline (Figure 4-9, B).

Upward and Downward Rotation

Upward rotation occurs as the glenoid fossa of the scapula rotates upwardly, as a natural component of raising the arm overhead (Figure 4-9, C). Downward rotation occurs as the scapula returns from an upwardly rotated position to its resting position. This motion naturally occurs as an elevated upper extremity is lowered to one’s side.

Acromioclavicular Joint

General Features

The acromioclavicular (AC) joint is considered a gliding or plane joint, created by the articulation between the lateral aspect of the clavicle and the acromion process of the scapula (Figure 4-10). In essence, this joint links the motion of the scapula (and attached humerus) to the lateral end of the clavicle. Because strong forces are frequently transferred across the AC joint, several important stabilizing structures are required to maintain its structural integrity.

Figure 4-10 Anterior view of the right acromioclavicular joint, including many of the surrounding ligaments. (From Neumann DA: Kinesiology of the musculoskeletal system: foundations for physical rehabilitation, ed 2, St Louis, 2010, Mosby, Figure 5-17.)

Supporting Structures of the Acromioclavicular Joint

Figure 4-10 illustrates the supporting structures of the AC joint.

• Acromioclavicular Ligament: Joins the clavicle to the acromion; helps to prevent dislocations of the scapula and links motion of the scapula to the clavicle

• Coracoclavicular Ligament: Composed of the conoid and trapezoid ligaments. Together, these ligaments help suspend the scapula from the clavicle and prevent dislocation.

• Coracoacromial Ligament: Attaches the coracoid process to the acromion process; one of the few ligaments of the body that attaches proximally and distally to the same bone. Along with the acromion, the coracoacromial ligament completes the coracoacromial arch—a functional “roof” that protects the head of the humerus.

Kinematics

The AC joint allows motion in all three planes: Upward and downward rotation, rotation in the horizontal plane (internal and external rotation), and rotation in the sagittal plane (anterior and posterior tilting) (Figure 4-11). These relatively slight but important adjustment motions help to fine-tune the movements between the scapula and the humerus. Equally important, these motions allow the scapula to maintain firm contact with the posterior thorax.

Glenohumeral Joint

General Features

The glenohumeral (GH) joint is created by the articulation of the humeral head with the glenoid fossa of the scapula (Figure 4-12). Recall that the head of the humerus is a large, rounded hemisphere, and that the glenoid fossa is relatively flat. This bony conformation, in conjunction with the highly mobile scapula, allows for abundant motion in all three planes but does not promote a high degree of stability. It is interesting to note that the ligaments and capsule of the GH joint are relatively thin and provide only secondary stability to the joint. The primary stabilizing force of this joint is garnered from the surrounding musculature, particularly the rotator cuff muscles.

Figure 4-12 Anterior view of the right glenohumeral joint showing many of the surrounding ligaments. (From Neumann DA: Kinesiology of the musculoskeletal system: foundations for physical rehabilitation, ed 2, St Louis, 2010, Mosby, Figure 5-25.)

Supporting Structures of the Glenohumeral Joint

• Rotator Cuff: A group of four muscles including the supraspinatus, infraspinatus, subscapularis, and teres minor. These muscles surround the humeral head and actively hold the humeral head against the glenoid fossa. These muscles are discussed at length in a subsequent section.

• Capsular Ligaments: A thin fibrous capsule that includes the superior, middle, and inferior glenohumeral ligaments. This relatively loose capsule attaches between the rim of the glenoid fossa and the anatomic neck of the humerus (see Figure 4-12).

• Coracohumeral Ligament: Attaches between the coracoid process and the anterior side of the greater tubercle. It helps limit the extremes of external rotation, flexion, and extension, as well as inferior displacement of the humeral head (see Figure 4-12).

• Glenoid Labrum: A fibrocartilaginous ring that encircles the rim of the glenoid fossa. The labrum serves to deepen the socket of the GH joint, nearly doubling the functional depth of the glenoid fossa. The labrum also helps seal the joint, thereby contributing to stability by maintaining a suction effect between the humerus and the glenoid fossa.

• Long Head of the Biceps: The proximal portion of the tendon wraps around the superior aspect of the humeral head, attaching to the superior glenoid tubercle. This tendon helps provide anterior stability because it acts as a partial extension of the glenoid labrum.

Consider this…

Injuries to the Glenoid Labrum

The glenoid labrum is a fibrocartilaginous ring of connective tissue that increases the stability of the glenohumeral joint. The labrum performs this important function in two ways. First, it deepens the socket of the shallow glenoid fossa, improving the “fit” of the joint. Second, the labrum creates a “suction cup effect” between the head of the humerus and the glenoid fossa. Even small tears of the labrum can cause instability and excessive micro-motions at the glenohumeral joint.

Numerous structural and functional reasons explain why the labrum is so often involved with shoulder pathology. First, the superior portion of the labrum is only loosely attached to the adjacent glenoid rim. Second, approximately 50% of the fibers of the long head of the biceps tendon are direct extensions of the superior glenoid labrum. Large forces that tax the biceps tendon can partially detach or tear the loosely attached superior labrum. Most often, this type of injury results in a SLAP lesion (Superior Labrum from Anterior to Posterior), which involves the superior aspect of the labrum. This is a relatively common occurrence in throwing athletes such as baseball pitchers. Symptoms of SLAP lesions often involve pain with overhead activities and “clicking” or “popping” of the shoulder. Bankart lesions, on the other hand, involve tears to the anterior-inferior portion of the glenoid labrum. This type of injury often results from a traumatic anterior dislocation of the humerus. Patients with Bankart lesions typically complain of significant shoulder instability, or feel as if the shoulder could “pop out” during various activities.

Regardless of the type of lesion, surgery may be indicated if the tear of the labrum is large—or if conservative methods of treatment are unsuccessful. Physical therapy for these conditions usually involves regaining strength and range of motion and participating in a muscle stabilization program that fits the needs of the patient.

Kinematics

The GH joint is a ball-and-socket joint that allows 3 degrees of freedom. The primary motions of this joint are abduction and adduction, flexion and extension, and internal and external rotation (Figure 4-13). Horizontal abduction and horizontal adduction are commonly used terms to describe special motions of the shoulder and are described in the following section.

Figure 4-13 The right glenohumeral joint showing the conventional osteokinematic motions of the humerus. The axes of rotation are color coded with the associated planes of motion. (From Neumann DA: Kinesiology of the musculoskeletal system: foundations for physical rehabilitation, ed 2, St Louis, 2010, Mosby, Figure 5-30.)

Abduction and Adduction

Abduction and adduction of the GH joint occur in the frontal plane about an anterior-posterior axis of rotation, which courses through the humeral head. Normally, the GH joint allows approximately 120 degrees of abduction; the full 180 degrees of shoulder abduction normally occurs by combining 60 degrees of scapular upward rotation with the abduction of the GH joint. This important concept is discussed further in a subsequent section.

The arthrokinematics of abduction involves the convex head of the humerus rolling superiorly while simultaneously sliding inferiorly (Figure 4-14, A). Without an inferior slide, the upward roll of the humerus will result in the humeral head jamming into the acromion. This is known as impingement and often results in damage to the supraspinatus muscle or the subacromial bursa, which becomes pinched between these two bony structures (Figure 4-14, B). The arthrokinematics of GH joint adduction is the same as that of shoulder abduction but in the reverse direction.

Figure 4-14 A, Proper arthrokinematics of the glenohumeral (GH) joint during abduction involving a superior roll and inferior slide of the humeral head. B, A superior roll without an inferior slide, resulting in impingement of the subacromial bursa and supraspinatus. *ICL*, Inferior capsular ligament; *SCL*, superior capsular ligament. (From Neumann DA: Kinesiology of the musculoskeletal system: foundations for physical rehabilitation, ed 2, St Louis, 2010, Mosby, Figures 5-31 and 5-32, B.)

Flexion and Extension

Flexion and extension of the GH joint occur in the sagittal plane about a medial-lateral axis of rotation. During these actions, the humeral head spins on the glenoid fossa about a relatively fixed axis—an arthrokinematic roll and slide is not necessary.

Approximately 120 degrees of flexion and 45 degrees of extension are available to the GH joint. Similar to abduction, the full 180 degrees of shoulder flexion is obtained by incorporating approximately 60 degrees of scapular upward rotation.

Internal and External Rotation

Internal and external rotation of the GH joint occurs in the horizontal plane about a vertical (longitudinal) axis of rotation (see Figure 4-13). Internal rotation results in the anterior surface of the humerus rotating medially, toward the midline, whereas external rotation results in the anterior surface of the humerus rotating laterally, away from the midline.

Horizontal Abduction and Horizontal Adduction

With the shoulder in roughly 90 degrees of abduction, movement of the humerus toward the midline in the horizontal plane is considered horizontal adduction. Movement away from the midline in the horizontal plane is considered horizontal abduction. Examples of these actions include a rowing motion or a push-up.

Interaction Among the Joints of the Shoulder Complex

Up to this point, we have discussed the arthrology and kinematics of each joint of the shoulder complex. It must be understood, however, that movement of the entire shoulder is the result of movement in each of its four joints. All four joints must properly interact for normal shoulder motion to occur. An excellent example of this interaction is the scapulohumeral rhythm.

Scapulohumeral Rhythm

During normal shoulder abduction (or flexion), a natural 2 : 1 ratio or rhythm exists between the GH joint and the scapulothoracic joint. This means that for every 2 degrees of GH abduction, the scapula must simultaneously upwardly rotate roughly 1 degree. For example, if the shoulder is abducted to 90 degrees, only about 60 degrees of that motion occurs from GH abduction; the additional 30 degrees or so is achieved through upward rotation of the scapula. The full 180 degrees of abduction normally attained at the shoulder is the summation of 120 degrees of GH joint abduction and 60 degrees of scapular upward rotation (Figure 4-15).

Figure 4-15 Posterior view of the right shoulder complex after the arm has abducted 180 degrees. The 60 degrees of scapular upward rotation and 120 degrees of glenohumeral (GH) joint abduction are shaded in purple. The scapular upward rotation is depicted as a summation of 25 degrees of elevation at the sternoclavicular (SC) joint and 35 degrees of upward rotation at the acromioclavicular (AC) joint. (From Neumann DA: Kinesiology of the musculoskeletal system: foundations for physical rehabilitation, ed 2, St Louis, 2010, Mosby, Figure 5-35.)

120 degrees of glenohumeral joint abduction

+ 60 degrees of scapulothoracic joint upward rotation

= 180 degrees of shoulder abduction

Acromioclavicular and Sternoclavicular Joint Interaction Within the Scapulohumeral Rhythm

Scapulothoracic motion is an integral part of nearly every shoulder movement. Furthermore, motion at the scapulothoracic joint is dependent on the combined movements of the AC and SC joints. The full 60 degrees of scapulothoracic upward rotation is achieved by combining about 30 degrees of clavicular elevation with 30 degrees of AC joint upward rotation (see Figure 4-15).

30 degrees of sternoclavicular joint elevation

+ 30 degrees of acromioclavicular joint upward rotation

= 60 degrees of scapulothoracic joint upward rotation

In treatment of a patient with a shoulder dysfunction, it is important to remember the integrated relationship of the joints within the shoulder complex, because a problem in one joint will likely affect the other three.

Box 4-1 summarizes the interactions among the joints during common shoulder motions.

Box 4-1 Summary of Bony Movements During Common Shoulder Motions

The following provides a summary of normal kinematic interactions among the humerus, the scapula, and the clavicle during common shoulder motions.

Horizontal Abduction

• Horizontal abduction of the humerus

• Retraction of the scapula

• Retraction of the clavicle

Horizontal Adduction

• Horizontal adduction of the humerus

• Protraction of the scapula

• Protraction of the clavicle

Shoulder Flexion

This motion involves the typical scapulohumeral rhythm: a 2 : 1 ratio of glenohumeral flexion and scapulothoracic upward rotation.

• Flexion of the humerus

• Upward rotation of the scapula

• Elevation and posterior rotation of the clavicle

Shoulder Extension

The exact kinematics of this joint varies, depending on the range of motion through which the shoulder is being extended. The following movements occur during a pulling motion, beginning at 90 degrees of shoulder flexion and moving to 10 degrees of extension.

• Extension of the humerus

• Downward rotation and retraction of the scapula

• Depression and retraction of the clavicle

Shoulder Abduction

Abduction involves the 2 : 1 ratio of glenohumeral abduction to scapular upward rotation—the scapulohumeral rhythm.

• Abduction of the humerus

• Upward rotation of the scapula

• Clavicular elevation and posterior rotation

Clinical insight

Two Ways to Help Prevent Shoulder Impingement

To achieve full range of motion during abduction, the prominent greater tuberosity must be positioned to clear the undersurface of the acromion; this can be accomplished by externally rotating the shoulder or performing abduction in the scapular plane.

To illustrate this, first try to perform frontal plane abduction with your arm in full internal rotation (thumb pointing down), then in a neutral position (palm facing down), and finally in full external rotation (thumb pointing up). The limited range of motion experienced in a neutral or internally rotated position is caused by the greater tuberosity impinging against the acromion process. However, if the shoulder is externally rotated, the greater tuberosity is positioned posterior to the coracoacromial arch, thereby avoiding full impact with the acromion.

Even with the humerus in full external rotation, complete abduction of the shoulder may result in impingement if performed in the true frontal plane (Figure 4-16, A). Therapists often request that their patients perform shoulder exercises in the scapular plane as a way to prevent recurring impingement. The scapular plane is about 35 degrees anterior to the frontal plane (Figure 4-16, B). Shoulder abduction in the scapular plane, often referred to as scaption, positions the greater tuberosity of the humerus under the highest point of the acromion and helps to prevent bony impingement, regardless of the amount of rotation of the glenohumeral joint. This can be verified by performing abduction in the scapular plane, with the upper extremity positioned in internal rotation, in neutral, or in external rotation.

Figure 4-16 A side view of the right glenohumeral joint comparing abduction of the humerus in the **(A)** true frontal plane and in the **(B)** scapular plane. (From Neumann DA: Kinesiology of the musculoskeletal system: foundations for physical rehabilitation, ed 2, St Louis, 2010, Mosby, Figure 5-38.)

Scapular plane abduction is more natural than abduction in the pure frontal plane. The humeral head fits better against the glenoid fossa, and the ligaments and muscles (in particular, the supraspinatus) are more optimally aligned to promote proper shoulder mechanics.

Consider this…

Static Passive Locking Mechanism of the Glenohumeral Joint

When the arm is at rest, near the side of the body, the head of the humerus is held flush against the glenoid fossa, in part by the static locking mechanism of the glenohumeral (GH) joint. It is interesting to note that with optimal posture of the scapula, little GH joint muscle activity is required for stability at rest. Recall that the glenoid fossa is relatively flat and shallow, whereas the humeral head is large and round, making the anatomy of this joint more like a golf ball sitting on a quarter than like a ball-and-socket joint. The static locking mechanism helps provide stability to this loose-fitting joint.

Ideal posture of the scapula positions the glenoid fossa so that it is tilted about 5 degrees upward (Figure 4-17, A). This position not only improves the contact of the articulation but allows the surrounding soft tissues to help support this joint. The superior capsular ligaments provide an upward force vector to counteract the downward force of gravity. When these forces are combined, the resultant vector is a compressive force directed through the middle of the glenoid fossa, enhancing the static stability of the GH joint.

Figure 4-17 The static locking mechanism of the glenohumeral joint. A, The rope indicates a muscular force that holds the glenoid fossa slightly upward. B, Loss of the upward force—indicated by the cut rope—allows the glenoid fossa to downwardly rotate with a resultant inferior slide of the humerus. CF, Compression force; G, gravity; *SCS*, superior capsular structure. (From Neumann DA: Kinesiology of the musculoskeletal system: foundations for physical rehabilitation, ed 2, St Louis, 2010, Mosby, Figure 5-28.)

As illustrated in Figure 4-17, B, when the scapula becomes downwardly rotated, as commonly occurs after a stroke involving weakness or paralysis of the trapezius muscles, the static locking mechanism becomes ineffective. Not only does the humeral head lose its ledge on which to rest, but the direction of the upward forces created by the superior capsular ligaments is changed, reducing the overall potential of these structures to produce a passive compression force (CF).

The relatively large amount of GH joint instability produced by relatively small alterations in the posture of the scapula is good evidence that proper posture of the scapula contributes significantly to the stability of the GH joint.

Muscle and Joint Interaction

As discussed, all four joints of the shoulder must cooperate to produce normal shoulder motion. The muscles of the shoulder complex, therefore, must work in a highly coordinated fashion. For organizational purposes, this text divides these muscles into two categories: (1) Muscles of the shoulder girdle, and (2) muscles of the GH joint. A brief summary of the innervation scheme of the entire upper extremity is provided in the next section.

Innervation of the Shoulder Complex

The entire upper extremity receives innervation primarily through the brachial plexus (Figure 4-18). The brachial plexus is formed by a network of nerve roots from the spinal nerves C5-T1. Nerve roots C5 and C6 form the upper trunk, C7 forms the middle trunk, and C8 and T1 form the lower trunk. The trunks travel a short distance before forming the anterior or posterior division. The divisions then reorganize into lateral, medial, and posterior cords, named by their position relative to the axillary artery. The cords eventually branch into nerves that primarily innervate muscles of the upper extremity.