A number of significant muscles control motion at the shoulder and provide dynamic stabilization. Rarely does a single muscle act in isolation at the shoulder. For simplicity, the muscles acting at the shoulder may be described in terms of their functional roles: scapular pivoters, humeral propellers, humeral positioners, and shoulder protectors (
The scapular pivoters comprise the trapezius, serratus anterior, levator scapulae, rhomboid major, and rhomboid minor.
98As a group, these muscles are involved with motions at the scapulothoracic articulation, and their proper function is vital to the normal biomechanics of the whole shoulder complex. The scapular muscles can contract isometrically, concentric, or eccentrically, depending on the desired action and whether the action involves stabilization, acceleration, or deceleration. To varying degrees, the serratus anterior and all parts of the trapezius cooperate during the upward rotation of the scapula.
The trapezius muscle (
Fig. 16-10) originates from the medial third of the superior nuchal line, the external occipital protuberance, the ligamentum nuchae, the apices of the seventh cervical vertebra, all the thoracic spinous processes, and the supraspinous ligaments of the cervical and thoracic vertebrae. The upper fibers descend to attach to the lateral third of the posterior border of the clavicle. The middle fibers of the trapezius run horizontally to the medial acromial margin and superior lip of the spine of the scapula. The inferior fibers ascend to attach to an aponeurosis gliding over a smooth triangular surface at the medial end of the spine of the scapula to a tubercle at the scapular lateral apex.
It has been suggested that the upper fibers of this muscle have a different motor supply than that to the middle and lower portions.
100Recent clinical and anatomical evidence seems to suggest that the spinal accessory nerve provides the most important and consistent motor supply to all portions of the trapezius muscle, and although the C2–4 branches of the cervical plexus are present, no particular elements of innervation within the trapezius have been determined.
One of the functions of the trapezius is to produce shoulder girdle elevation on a fixed cervical spine. For the trapezius to perform its actions, the cervical spine must first be stabilized by the anterior neck flexors to prevent simultaneous occipital extension from occurring. Failure to prevent this occipital extension would allow the head to translate anteriorly, resulting in a decrease in the length, and therefore the efficiency, of the trapezius
102and an increase in the cervical lordosis.
Complete paralysis of the trapezius usually causes moderate-to-marked difficulty in elevating the arm over the head. The task, however, can usually be completed through full range as long as the serratus anterior remains totally innervated.
The muscular digitations of the serratus anterior (see
Fig. 16-5) originate from the upper eight to ten ribs and fascia over the intercostals. The muscle is composed of three functional components
- The upper component originates from the first and second ribs and inserts into the superior angle of the scapula.
- The middle component arises from the second, third, and fourth ribs and inserts into the anterior aspect of the medial scapular border.
- The lower component is the largest and most powerful, originating from the fifth through ninth ribs. It runs anterior to the scapula and inserts into the medial border of the scapula.
The serratus anterior is activated with all shoulder movements, but especially during shoulder flexion and abduction.
105Working in synergy with the trapezius, as part of a force couple (see later), the main function of the serratus anterior is to protract and upwardly rotate the scapula,
107while providing a strong, mobile base of support to position the glenoid optimally for maximum efficiency of the upper extremity.
108Its lower fibers draw the lower angle of the scapula forward to rotate the scapula upward while maintaining the scapula on the thorax during arm elevation.
109This moves the coracoacromial arch out of the path of the advancing greater tuberosity and opposes the excessive elevation of the scapula by the levator scapulae and trapezius muscles.
110Without upward rotation and protraction of the scapula by the serratus anterior, full G-H elevation is not possible. In fact, in patients with complete paralysis of the serratus anterior, Gregg et al.
108reported that abduction is limited to 110 degrees.
Dysfunction of the serratus anterior muscle causes winging of the scapula as the patient attempts to elevate the arm.
111Scapulothoracic dysfunction can also contribute to G-H instability, as the normal stable base of the scapula is destabilized during abduction or flexion.
The serratus anterior muscle is innervated by the long thoracic nerve (C5–7).
Paralysis or weakness of the serratus anterior muscle results in disruption of normal shoulder kinesiology. The disability may be slight with partial paralysis or profound with complete paralysis. As a rule, persons with complete or marked paralysis of the serratus anterior cannot elevate the arms above 90 degrees of abduction.
The levator scapulae muscle (see
Fig. 16-10) originates by tendonous strips from the transverse processes of the atlas, axis, and C3 and C4 vertebrae, and descends diagonally to insert into the medial superior angle of the scapula.
The levator scapulae can act on the cervical spine (see
Chap. 23) and on the scapula. If it acts on the cervical spine, it can produce extension, side flexion, and rotation of the cervical spine to the same side.
114When acting on the scapula during upper extremity flexion or abduction, the levator scapula muscle acts as an antagonist to the trapezius muscle, and provides eccentric control of scapular upward rotation in the higher ranges of motion.
Both the trapezius and levator scapulae muscles are activated with increased upper extremity loads.
The levator scapulae muscle is innervated by the posterior (dorsal) scapular nerve (C3–5).
The rhomboid major muscle (
Fig. 16-10) originates from the second to fifth thoracic spinous processes and the overlying supraspinous ligaments. The fibers descend to insert into the medial scapular border between the root of the scapular spine and the inferior angle of the scapula.
The rhomboid minor muscle (see
Fig. 16-10) originates from the lower ligamentum nuchae, and the seventh cervical and first thoracic spinous processes, and attaches to the medial border of the scapula at the root of the spine of the scapula.
The rhomboid muscles help control scapular positioning, particularly with horizontal flexion and extension of the shoulder complex.
The rhomboid muscles are innervated by the posterior (dorsal) scapular nerve (C4–5).
The total muscle mass of the shoulder's internal rotators (subscapularis, anterior deltoid, pectoralis major, latissimus dorsi, and teres major) is much greater than that of the external rotators (infraspinatus, teres minor, and posterior deltoid).
103This fact explains why the shoulder internal rotators produce approximately 1.75 times greater isometric torque than the external rotators.
117Peak torques of the internal rotators also exceed the external rotators when measured isokinetically, under both concentric and eccentric conditions.
The latissimus dorsi muscle (see
Fig. 16-12) originates from the spinous processes of the last six thoracic vertebrae, the lower three or four ribs, the lumbar and sacral spinous processes through the thoracolumbar fascia, the posterior third of the external lip of the iliac crest, and a slip from the inferior scapular angle. The scapular slip allows the latissimus dorsi to act at the scapulothoracic articulation. The latissimus dorsi inserts into the intertubercular sulcus of the humerus. The muscle functions as an extensor, adductor, and powerful internal rotator of the shoulder, and also assists in scapular depression, retraction, and downward rotation.
119It is innervated by the thoracodorsal nerve (C6–8).
Figure 16-12Graphic Jump Location
Latissimus dorsi. (Reproduced with permission from Morton
DA, Foreman KB, Albertine KH:
The Big Picture: Gross Anatomy.McGraw-Hill, 2011)
The teres major (see
Fig. 16-12) originates from the inferior third of the lateral border of the scapula and just superior to the inferior angle. The teres major tendon inserts into the medial lip of the intertubercular groove of the humerus. The teres major functions to complement the actions of the latissimus dorsi in that it extends, adducts, and internally rotates the G-H joint. It is innervated by the lower subscapular nerve (C5, C6).
The pectoralis major (see
Fig. 16-13) originates from the sternal half of the clavicle, half of the anterior surface of the sternum to the level of the sixth or seventh costal cartilage, the sternal end of the sixth rib, and the aponeurosis of the obliquus externus abdominis. The fibers of the pectoralis major converge to form a tendon that inserts into the lateral lip of the intertubercular sulcus of the humerus. Although this muscle does not insert into the scapula, it does act upon the scapulothoracic articulation through its insertion on the humerus. The function of the pectoralis muscle depends on which fibers are activated:
- Upper fibers (clavicular head)—IR, horizontal adduction, flexion, abduction (once the humerus is abducted 90 degrees, the upper fibers assist in further abduction), and adduction (with the humerus below 90 degrees of abduction) of the G-H joint.
- Lower fibers (sternal head)—IR, horizontal adduction, extension, and adduction of the G-H joint.
Figure 16-13Graphic Jump Location
Pectoralis major muscle. (Reproduced with permission from Morton
DA, Foreman KB, Albertine KH:
The Big Picture: Gross Anatomy.McGraw-Hill, 2011)
The pectoralis major is innervated by the medial (lower fibers) and lateral (upper fibers) pectoral nerves (C8–T1 and C5–7, respectively).
The pectoralis major and latissimus dorsi muscles are referred to as
humeral propellermuscles as they have been shown to be the only muscles in the upper extremity to have a positive correlation between peak torque and velocity during the propulsive phase of the swim stroke.
The pectoralis minor (see
Fig. 16-9) originates from the outer surface of the upper margins of the third to fifth ribs near their cartilage. The fibers of the pectoralis minor ascend laterally, converging to a tendon that inserts into the coracoid process of the scapula.
The pectoralis minor muscle is innervated by the medial pectoral nerve (C6–8).
The deltoid muscle originates from the lateral third of the clavicle, the superior surface of the acromion, and the spine of the scapula (
Fig. 16-13). It inserts into the deltoid tuberosity of the humerus. The deltoid can be described as three separate muscles—anterior, middle, and posterior—all of which function as humeral positioners, positioning the humerus in space.
The deltoid muscle is innervated by the axillary nerve (C5–6).
The rotator cuff muscles (
Fig. 16-4), which consist of the supraspinatus, infraspinatus, teres minor, and subscapularis, are commonly involved with shoulder pathology. The anatomy of these muscles was described previously (see
“Glenohumeral Joint”section). These muscles are referred to as the protectors of the shoulder since, in addition to actively moving the humerus, they fine-tune the humeral head position during arm elevation.
98Compared with most joints that have a single axis on which torques are generated, the shoulder is very different, because it has no fixed axis. As a result, each muscle activation creates a unique set of rotational moments, which necessitates precise coordination in the timing and magnitude of muscle contractions.
73Jenp et al.
120used electromyography (EMG) to detect the most specific positions of highest activation for the individual rotator cuff muscles. The greatest activation of the subscapularis was with the arm in the scapular plane at 90 degrees of elevation and neutral humeral rotation. The subscapularis has also been shown to be an effective humeral head depressor in ER, whereas it produces almost no A-P translation in abduction and ER. The infraspinatus–teres minor muscles are very effective humeral head depressors with the arm in the sagittal plane and the humerus elevated to 90 degrees in the midrange of external/IR (the so-called “hornblower's” position). The supraspinatus could not be effectively isolated.
The rotator cuff muscles have an important role in the function of the shoulder and serve the following:
- Assist in the rotation of the shoulder and arm.At the G-H joint, elevation through abduction of the arm (
Table 16-4) requires that the greater tuberosity of the humerus pass under the coracoacromial arch. For this to occur, the humerus must externally rotate, and the acromion must elevate.
121ER of the humerus is produced actively by a contraction of the infraspinatus and teres minor, and by a twisting of the joint capsule. A force couple exists in the transverse plane between the subscapularis anteriorly and the infraspinatus and teres minor posteriorly in which cocontraction of the infraspinatus, teres minor, and subscapularis muscles both depresses and compresses the humeral head during overhead movements.
++ Table Graphic Jump Location Table 16-4 Contributors to Glenohumeral Abduction ||Download (.pdf)
Table 16-4 Contributors to Glenohumeral Abduction
Degree of Abduction
The concerted action of the active stabilizers (deltoid, biceps, and rotator cuff muscles) and the passive restraints (articular surfaces, osseous structures, and ligaments) is necessary for purposeful function. The supraspinatus contracts to initiate abduction of the glenohumeral joint.
aThe remaining rotator cuff muscles also contract to pull the humeral head into the glenoid fossa. At approximately 20 degrees of humeral abduction, scapular upward rotation begins with concurrent clavicular elevation and axial rotation.
b,cAt approximately 90 degrees, or a little more in females, the upper extreme of G-H abduction is reached, and clavicular elevation ceases due to tension of the costoclavicular ligament.
dContinued abduction of the humerus requires continued upward rotation of the scapula, which by this point has rotated through a range of approximately 30 degrees.
As the scapula continues to upwardly rotate, the glenoid fossa faces superiorly and laterally, and its inferior angle moves laterally through approximately 60 degrees. The scapular contribution peaks between 90 and 140 degrees:
fThe scapular upward rotation is accommodated at both the S-C and A-C joints by a posterior axial rotation of the clavicle of 30–40 degrees and a clavicular elevation of approximately 30–36 degrees.
cThe muscles producing this movement are the serratus anterior and trapezius, acting as a force couple on the scapulothoracic joint. The movement is limited by the acromion and S-C joint, and by the scapular and humeral adductors (notably the latissimus dorsi and pectoralis major).
Abduction beyond 150 degrees requires adequate motion at the vertebral joints of the upper thorax and cervical spine.
gBilateral abduction demands that the thoracic spine extends and the lumbar lordosis increases.
The importance of the ER during humeral elevation (
Table 16-5) can be demonstrated clinically. If the humerus is held in full IR, only approximately 60 degrees of G-H abduction is passively possible before the greater tuberosity impinges against the coracoacromial arch and blocks further abduction. This helps explain why individuals with marked IR contractures cannot abduct fully, but can elevate the arm in the forward plane.
++ Table Graphic Jump Location Table 16-5 Contributors to Glenohumeral Elevation ||Download (.pdf)
Table 16-5 Contributors to Glenohumeral Elevation
Degree of Elevation and Main Contributor
0–60 degrees: glenohumeral elevation
A combined motion of flexion, abduction, and external rotation occurs at the glenohumeral joint, produced by the anterior deltoid, coracobrachialis, and the clavicular fibers of pectoralis major. Motion is limited by the increasing tension in the posterior coracohumeral ligament and the stretching of the shoulder extensors, adductors, and external rotators.
60–120 degrees: sternoclavicular and acromioclavicular elevation
The scapula depresses, protracts, and abducts on the posterior thoracic wall, such that the glenoid fossa faces anteriorly and superiorly and its inferior angle faces laterally and anteriorly. This motion is accommodated by the S-C and A-C joints. The scapulothoracic motion is produced in the same manner as with abduction, by the serratus anterior and trapezius, and is limited by the ligaments of the two joints, and the tension in the shoulder extensor and adductor musculature.
120–180 degrees—costospinal elevation
bstates that the extreme of flexion is the same as the extreme of abduction. That is, during unilateral elevation, the lateral displacement is produced by the contralateral spinal muscles while bilateral abduction requires an exaggeration of the lumbar lordosis to bring the arms vertical. In addition, the medial attachments of the first and second ribs descend while those of the fourth to sixth ascend and the third acts as the axis. Bilateral abduction demands that the thoracic spine extends and the lumbar lordosis increases.
However, in clinical practice, shoulders having large rotator cuff tears and good function are frequently encountered. In the coronal plane, there is another force couple between the deltoid and the inferior rotator cuff muscles (infraspinatus, subscapularis, and teres minor). With the arm fully adducted, contraction of the deltoid produces a vertical force in a superior direction, resulting in an upward translation of the humeral head relative to the glenoid. Cocontraction of the inferior rotator cuff muscles produces both a compressive force and a downward translation of the humerus that counterbalances the force of the deltoid, thereby stabilizing the humeral head.
EMG studies have shown that during casual elevation of the arm in normal shoulders, the deltoid and the rotator cuff act continuously throughout the motion of abduction, each reaching a peak of activity between 120 and 140 degrees of abduction.
122However, during more rapid and precise movements, such as those involved with throwing, a more selective pattern emerges with specific periods of great intensity.
123Weakening of the rotator cuff appears to allow the deltoid to elevate the proximal part of the humerus in the absence of an adequate depressor effect from the rotator cuff, resulting in a decrease in the subacromial space and impingement of the rotator cuff on the anterior aspect of the acromion.
- Reinforce the G-H capsule.The rotator cuff muscles, together with the coracohumeral ligament, and the LHB (often referred to as the fifth rotator cuff muscle) function as contractile ligaments. For example, firing of the rotator cuff muscles increases the tension of the middle G-H ligament when the arm is abducted to 45 degrees and externally rotated.
- Control much of the active arthrokinematics of the GH joint.Contraction of the horizontally oriented supraspinatus produces a compression force directly into the glenoid fossa.
124This compression force holds the humeral head securely in the glenoid cavity during its superior roll, which provides stability to the joint and also maintains a mechanically efficient fulcrum for elevation of the arm.
124In the shoulder midrange position, when all of the passive restraints are lax, joint stability is achieved almost entirely by the rotator cuff. In addition, as previously mentioned, without adequate supraspinatus force, the near vertical line of force of a contracting deltoid tends to jam or impinge the humeral head superiorly against the coracoacomial arch.
Long Head of the Biceps Brachii
The biceps brachii muscle is a large fusiform muscle in the anterior compartment of the upper extremity, which has two tendinous origins from the scapula (
Fig. 16-7). The medial head and LHB normally originate from the coracoid process and supraglenoid tubercle of the scapula, respectively. However, a great deal of research has noted that the origin of the biceps tendon varies, not only in the type of insertion (single, bifurcated, or trifurcated), but also in the specific anatomical location where it inserts.
73The proximal LHB tendon receives an arterial supply from labral branches of the suprascapular artery.
126As it leaves its origin, the LHB tendon is surrounded by a synovial sheath, which ends at the distal end of the bicipital groove, making the tendon an intra-articular but extrasynovial structure.
73As the LHB tendon moves between the greater and lesser tuberosities, it is stabilized in position by the tendoligamentous sling comprising the coracohumeral ligament, superior G-H ligament, and fibers from the supraspinatus and the subscapularis.
127Once in the bicipital groove, the LHB tendon passes under the transverse humeral ligament, which bridges the groove.
128After coursing through the groove, the two heads join to form the biceps muscle belly at the level of the deltoid insertion.
129The medial tendon is interarticular, lying inside the G-H capsule.
41This tendon is not as common a source of shoulder pain as the long tendon, and it rarely ruptures.
The function of the biceps as a forearm supinator and secondarily as an elbow flexor is well known.
130At the shoulder joint, however, the function of the LHB tendon is less clear, with most references regarding it as a week flexor of the shoulder.
131Cadaveric studies have suggested that the LHB tendon functions as a humeral head depressor (in full ER), an anterior stabilizer, a posterior stabilizer, a limiter of ER, a lifter of the glenoid labrum, and a humeral head compressor of the shoulder.
135The muscle has also been described as having an important role in decelerating the rapidly moving arm during activities such as forceful overhand throwing.
73In the anatomical position, the biceps has no ability to elevate the humerus. If the arm is rotated 90 degrees externally, the tendon of the long head lines up with the muscle belly to form a straight line across the humeral head. As the biceps contracts in this position, the humeral head rotates beneath the tendon, resisting ER of the humeral head and increasing the anterior stability of the G-H joint.
137Contraction of the LHB in this position fixes the humeral head snugly against the glenoid cavity, as the resultant force passes obliquely through the center of rotation of the humeral head and at right angles to the glenoid.
136The humeral head is prevented from moving upwards by the hood-like action of the biceps tendon (
Fig. 16-7), which exerts a downward force and assists the depressor function of the cuff.
140Interestingly, the biceps tendon was found to be wider in cuff-deficient shoulders in one study.
The biceps brachii muscle is innervated by the musculocutaneous nerve.
A number of pathological conditions have been associated with the LHB tendon including LHB tendon degeneration, superior labrum anterior and posterior (SLAP) lesions, LHB tendon anchor abnormalities, and LHB tendon instability.
The G-H joint has three DOF: flexion/extension, abduction/adduction, and internal/ER. Available ranges of motion
at the G-H jointare approximately as follows:
- Flexion and abduction.Approximately 100–120 degrees are available, with females demonstrating slightly more motion than males.
- External rotation.Approximately 60–80 degrees are available, with females demonstrating slightly more motion than males.
- Internal rotation.Approximately 80–90 degrees are available, with females demonstrating slightly more motion than males.
- Extension.Great variability exists with extension, with ranges existing from 10 to 90 degrees.
G-H motions (
Table 16-11) consist of a combination of glides and rolls based on the concave–convex rule (see
Chap. 10). At the G-H joint, the concave–convex rule dictates that the articulating surface moves in the opposite direction of the shoulder motion (
Table 16-6). Motions at this joint do not occur in isolation, but rather as coupled motions.
142For example, ER and abduction occur with flexion, and ER and adduction accompany extension.
++ Table Graphic Jump Location Table 16-6 Glenohumeral Joint Motions and Their Appropriate Axis and Accessory Motions ||Download (.pdf)
Table 16-6 Glenohumeral Joint Motions and Their Appropriate Axis and Accessory Motions
Plane/Axis of Motion
Complete movement at the shoulder girdle involves a complex interaction between the G-H, A-C, S-C, scapulothoracic, upper thoracic, costal, and sternomanubrial joints, the upper thorax, and the lower cervical spine. Within the joints of the shoulder complex, there appear to be no well-defined points within the range where one joint's motion ends and another begins. Rather, they all blend into a smooth harmonious movement during arm raising (see
During shoulder rotation and arm activities, the scapula invariably acts as a platform upon which the activities are based. It is worth noting that the supporting structures of the G-H joint are only effective if the scapula can maintain its ROM with the humerus (see
“The Dynamic Scapula”section).
The G-H joint has been described as being similar to a golf ball on a tee due to the size relationships. A more accurate biomechanical description is that the G-H joint is like a ball on a seal's nose.
143As the ball or humeral socket moves, the seal's nose, or the scapula, needs to move to maintain the position of the ball on the glenoid. The orientation of the G-H joint causes motions at this joint to occur in the scapular plane. The shoulder has the greatest ROM of any joint, with a vast array of muscles producing those motions.
1The correct function of these muscles is dependent on length–tension relationships and coordinated activation.
48Over 1,600 different positions in three-dimensional space can be assumed by the shoulder.
145Due to this wide ROM, the G-H joint is faced with the task of maintaining equilibrium between functional mobility and adequate stability during normal activities of daily living.
146When sport is added to the equation, extremely high forces can be generated at the shoulder. For example, the angular velocity of an overhead throw reaches over 7,000 degrees per second, which is the fastest recorded human movement.
It has been estimated that the anterior translation forces generated with pitching are equal to one-half body weight during the late cocking phase, and there is a distraction force equal to body weight during the deceleration phase.
The complex kinematics of this region probably account for the fact that strains and sprains may remain symptomatic for much longer than in other joints.
Full elevation of the arm occurs through an arc of approximately 180 degrees and can occur in an infinite number of body planes.
149Locally, this motion is a result of abduction of the G-H joint and upward rotation of the scapulothoracic joint. During abduction of the shoulder, the G-H joint is reported to contribute up to 120 degrees of the total arc of motion, with the remaining 60 degrees occurring at the scapulothoracic joint (see
“The Dynamic Scapula”section). Arm elevation beyond 90 degrees requires motion in other, more distal joints such as the A-C and S-C joints (see
“The Dynamic Scapula”section) and the vertebral joints of the upper thorax and lower cervical spine (
Glenohumeral Joint Arthrokinematics
Flexion at the G-H joint involves a pure spin if it occurs strictly in the sagittal plane; no roll or slide is necessary. Tension within the surrounding capsular structures, particularly the posterior structures, may cause a slight anterior translation of the humerus at the extremes of flexion.
46However, although flexion in the sagittal plane involves a pure spin, elevation of the arm in the scapular plane involves a combination of flexion, abduction, and ER. Thus, at the joint surface of the G-H joint during arm elevation in the scapular plane, the head of the humerus spins (flexion component), glides inferiorly (abduction component), and glides anteriorly (ER component) (
Depending on the type of function performed, motions of the shoulder complex involve both local motions and motions at other joints.
150labels the motions that occur in other joints of the body during an activity such as throwing (i.e., trunk and hip rotation) as
distant functions. In contrast, those motions occurring at the shoulder during the same activity (i.e., G-H ER) are termed
The majority of shoulder motions involve a series of sequentially activated links in a kinetic chain of body segments.
150For those motions requiring more force, the number of links in the kinetic chain increases—the sequence of activation starts as a ground reaction force (GRF) and moves up through the knees and hips to the trunk, and into the shoulder. Approximately 50% of the total kinetic energy and force occurring at the G-H joint originate from a combination of the GRF and the forces from the legs and hips.
151At the shoulder, G-H, A-C, S-C, and scapulothoracic joints, motion occurs simultaneously as a result of muscle action and ligamentous tension in these joints. The specific sequence of muscle activation in the upper extremity depends on the activity, although the direction of activation is usually from proximal to distal as this is the most efficient method for producing large forces and accelerations to the arm. As part of this activation sequence, specific muscle activation patterns and joint positions are developed depending on the activity. Any changes to this sequence of activation can produce an abnormal movement pattern, involving substitution or compensation from the more distal links.
152For example, a throwing athlete with decreased trunk rotation due to stiffness has to rely more on the shoulder to provide the force for the throw. These adaptive patterns eventually result in either decreased performance or increased injury risk.
The G-H joint accounts for approximately two-thirds of all shoulder motions, with the remainder provided by the scapulothoracic joint.
87For full motion to occur, a complex interaction between the deltoid, rotator cuff, LHB, G-H capsule, glenoid articulating cartilage, and scapular pivoters (trapezius, serratus anterior, levator scapulae, and rhomboids) is required.
Stabilization of the Static Shoulder
The dependent shoulder requires very little muscular support. Its vertical stability is a result of the inferior lateral projection and upward inclination of the glenoid fossa, which is maintained by a mild contraction of the fibers of the trapezius. It was traditionally theorized that the humeral head was prevented from rolling off of this lateral projection by a moderate contraction of the supraspinatus and the deltoid.
154More recent studies have demonstrated that the muscle tone of the rotator cuff is not a significant contributor to the static inferior stability of the dependent shoulder with light loads, but that maintenance of the intra-articular pressure and the adhesion and cohesion properties of the articular surfaces are far more significant.
156However, the rotator cuff does provide a passive restraint to translation, especially to posterior translation, during the early-to-midranges of elevation.
During the mid and end ranges of motion, a combination of several different static restraints create a vector that keeps the humeral head securely seated in the glenoid, through concavity compression—negative articular pressure (see
160Static restraint is also provided by the anatomic curvature of the humerus and glenoid, the extra depth of the labrum, and ligamentous restraints (see
161The ligamentous restraints contribute especially at the end ranges of motion
162and are assisted with concomitant muscle activity (see
++ Table Graphic Jump Location Table 16-7 Static Restraints to Inferior Translation (Dependent on the Position of the Arm) ||Download (.pdf)
Table 16-7 Static Restraints to Inferior Translation (Dependent on the Position of the Arm)
Degrees of Glenohumeral Abduction
Superior G-H and coracohumeral ligaments
Inferior G-H ligament (posterior band in external rotation, anterior band in internal rotation)
++ Table Graphic Jump Location Table 16-8 Static Restraints to Internal Rotation (Dependent on the Position of the Arm) ||Download (.pdf)
Table 16-8 Static Restraints to Internal Rotation (Dependent on the Position of the Arm)
Degrees of Glenohumeral Abduction
Posterior band of inferior G-H ligament, teres minor, posterior capsule (superior)
Anterior and posterior bands of the inferior G-H ligament
Posterior band of the inferior G-H ligament, posterior capsule (inferior)
Table Graphic Jump Location Table 16-9 Dynamic and Static Restraints to External Rotation (Dependent on the Position of the Arm) ||Download (.pdf)
Table 16-9 Dynamic and Static Restraints to External Rotation (Dependent on the Position of the Arm)
Degrees of Glenohumeral Abduction
Subscapularis, superior G-H, and coracohumeral ligaments
Subscapularis, middle G-H ligament, superior fibers of the inferior G-H ligament
Inferior G-H ligament
Stabilization of the Dynamic Shoulder
Dynamic stability of the shoulder complex is dependent on a variety of mechanisms including the optimal alignment of the scapula, correct G-H orientation, and the quality of the length–tension relationship of the involved muscles, and correct functioning of the static restraints (
The deltoid, pectoralis major, latissimus dorsi, and teres major muscles are prime movers of the G-H joint. EMG studies have shown that these muscles function along the line of pull, creating the potential for infinite lines of pull that may allow an almost 360 degrees arc of motion.
163Indeed, like the temporomandibular joint, the G-H joint enjoys the benefit that all of its prime movers compress the joint surface, thus optimizing joint stability. The secondary movers of the G-H joint are the LHB, and the triceps.
The LHB and the triceps muscles are major dynamic stabilizers of the G-H joint, predominately functioning as “shunt” muscles (muscles that produce a compression at the joint surfaces of the joints they cross) during high-velocity activities.
As motion occurs at the G-H joint, the glenoid cavity of the scapular adopts a diverse number of reciprocal positions. It is likely that these scapular positions are based on both the functional task and the placement of the hand. The scapular stabilizers include the serratus anterior, latissimus dorsi, trapezius, rhomboids, levator scapulae, and the pectoralis minor. Dysfunction or inhibition of any of these muscles can alter the position of the glenoid significantly, resulting in abnormal centering of the humeral head within the glenoid.
The function of the rotator cuff in normal and pathologic conditions has been the subject of several studies.
141Until recently, EMG or cadaver studies have been the primary method of evaluating the contribution of each rotator cuff and shoulder muscle to a particular motion or exercise.
145For example, using cadavers, Keating et al.
146determined that the subscapularis contributes 53% of the cuff moment and believed it to be the most important muscle in humeral head stabilization.
Magnetic resonance imaging (MRI) is now being used to show increases in muscle signal intensity detected immediately following exercise.
149On the basis of the level of signal intensity, this so-called exercise-induced enhancement seen on MRI can determine which muscles are used for a given exercise.
152For example, one study of the shoulder
149demonstrated that side-lying abduction produced the greatest signal intensity in the supraspinatus, infraspinatus, and subscapularis muscles. Surprisingly, scaption with internal rotation (SIR), previously associated with isolation of the supraspinatus muscle, did not provide the highest increase in any muscle of the rotator cuff.
149However, caution must be used in drawing conclusions from single studies, and further research is certainly warranted in this area.
As previously mentioned, the role of the capsuloligamentous complex in stabilizing the G-H joint during dynamic activities is complex and varies with both shoulder position and the direction of the translation force.
166The posterior capsule is the main restraint against posterior translation of the humerus on the glenoid fossa with the arm below 90 degrees of abduction.
167With the arm at 90 degrees of abduction, the inferior G-H ligament and the posteroinferior capsule become the main restraint.
167The posterior band of the inferior G-H ligament resists inferior translation when the arm is at 90 degrees of abduction.
The synchronized motions that occur between the scapula and the humerus during elevation are a combination of scapulothoracic motion and scapulohumeral motion.
The angle between the glenoid and the moving humeral head has to be maintained within a safe zone of 30 degrees of angulation during activities to decrease shear and translatory forces.
168For this to occur, the scapula must be positioned muscularly in relation to the moving humerus and must also act as a stable base of muscle origin for the rotator cuff muscles. If the scapula cannot be controlled, the glenoid cannot be positioned correctly to allow for the optimal length–tension relationships within the shoulder complex.
169This synchronized motion between the glenoid cavity and the humerus is referred to as
Fig. 16-14). Proper rhythm involves a rotation of the scapula during arm elevation. By allowing the glenoid to stay centered under the humeral head, the strong tendency for a downward dislocation of the humerus is resisted and the glenoid is maintained within a physiologically tolerable range (
Fig. 16-14). At full abduction, the glenoid completely supports the humerus.
Several studies have examined the scapulohumeral rhythm three dimensionally.
171An early study by Inman
87determined that a 2:1 ratio existed between the motion occurring at the G-H joint and scapula, respectively. However, more recent studies have shown that this ratio is not consistent throughout the ROM.
153As the humerus elevates to 30 degrees (setting phase), there is minimal movement of the scapula. In this initial portion of abduction, G-H motion predominates and the ratio has been found to be 4.4 degrees of G-H motion for every degree of scapular motion (4.4:1 ratio). From 30 to 90 degrees, the scapula abducts and upwardly rotates, and the ratio becomes 5 degrees of G-H motion to 4 degrees of scapular motion (5:4 ratio). As the shoulder moves above 90 degrees of abduction to full abduction, the scapula abducts and upwardly rotates 1 degree for every 1 degree of humeral elevation (1:1 ratio).
170However, it must be kept in mind that these ratios are based on two-dimensional radiographic projections of angular rotations taken at discrete positions of elevation, whereas in reality, the arm moves in three dimensions, and that the scapulohumeral rhythm has also been found to change with external loading of the arm.
A reverse scapulohumeral rhythm, in which the scapula moves more than the humerus, occurs in conditions such as adhesive capsulitis.
Scapulothoracic motion is a vital component of shoulder function and consists of rotation and translation around approximately three axes of motion. These axes are considered to be embedded in the scapula.
- Anterior and posterior tipping occurs around an axis parallel to the scapula.
- Protraction and retraction of the scapula occur through protraction and retraction of the clavicle at the S-C joint.
- IR and ER occur around an axis running through the scapula from superior to inferior.
Elevation and depression of the scapula occurs through elevation and depression of the clavicle at the S-C joint. Throughout the 180 degrees of arm elevation, a total of 30–35 degrees of clavicle elevation occurs, in addition to rotation of the clavicle (30–35 degrees) around its longitudinal axis.
173This clavicular elevation and rotation occur in two main phases of shoulder abduction. Assuming a 2:1 scapulohumeral rhythm, shoulder abduction up to 90 degrees occurs as a summation of 60 degrees of G-H abduction, and 30 degrees of scapulothoracic upward rotation. The 30 degrees of upward rotation occurs predominantly through a synchronous 20–25 degrees of clavicular elevation at the S-C joint and 5–10 degrees of upward rotation at the A-C joint.
103The elevation of the clavicle raises the acromion during arm elevation allowing for the subacromial structures to pass under the coracoacromial arch.
24Shoulder abduction from 90 to 180 degrees occurs as a summation of an additional 60 degrees of G-H joint abduction and an additional 30 degrees of scapulothoracic upward rotation.
103During this late phase, the clavicle elevates only an additional 5 degrees at the S-C joint, whereas, at the A-C joint, the scapula upwardly rotates 20–25 degrees. Thus, by the end of 180 degrees of abduction, the 60 degrees of scapulothoracic upward rotation can be accounted for by 30 degrees of elevation at the S-C joint and 30 degrees of upward rotation at the A-C joint.
103The motion at the A-C joint is controlled by tension in the coracoclavicular ligaments. Finally, the clavicle has been demonstrated in vivo to posteriorly rotate around its long axis during the late phase of shoulder abduction.
24It is, as yet, unclear whether this posterior rotation occurs at the S-C or at the A-C joint. The rotation of the clavicle, though, is controlled by tension in the coracoclavicular ligaments and the clavipectoral fascia.
Elevation of the arm in the scapular plane in healthy subjects is accompanied by posterior tipping and upward rotation of the scapula.
172The upward rotation of the scapula occurs about an axis that passes through the base of the spine of the scapula and occurs in various phases (see Tables
16-5). Rotation of the scapula about the vertical axis shows a somewhat more variable pattern,
172with some studies showing ER occurring predominantly at higher elevation angles,
175whereas others demonstrate IR.
- Upward and downward rotation occurs around an axis perpendicular to the plane of the scapula that travels through the A-C joint and S-C joint.
71The upward rotation of the scapula during shoulder abduction helps to maintain an effective length–tension relationship between the three groups (force couples) of muscles that attach to the scapula. A
force coupleis defined as two forces that act in opposite directions to rotate a segment around its axis of motion.
During the first 30 degrees of upward rotation of the scapula, the serratus anterior muscle and the upper and lower divisions of the trapezius muscle are considered the principal upward rotators of the scapula. Together these muscles form two force couples; one formed by the upper trapezius and the upper serratus anterior muscles (
Fig. 16-14) and the other by the lower trapezius and lower serratus anterior muscles.
The prime muscles that abduct the G-H joint are the middle deltoid and the supraspinatus muscles.
103Elevation of the arm through flexion is performed primarily by the anterior deltoid, coracobrachialis, and LHB brachii.
103The trapezius appears to be more critical for controlling the scapula during the initial phases of abduction, whereas the serratus has been found to be the most effective upward rotator of the scapula.
181The lower trapezius contributes during the later phase of shoulder abduction by preventing tipping of the scapula and assisting in the stabilization of the scapula through eccentric control of the scapula during scapular upward rotation.
During approximately the first 150 degrees of arm elevation through flexion
- the upper and lower fibers of the trapezius contract concentrically,
- the fibers of the lower serratus anterior contract concentrically,
- the levator scapulae contracts eccentrically,
- the rhomboids contract eccentrically.
Approximately from 150 to 180 degrees
- the lower fibers of the serratus anterior contract isometrically,
- the lower fibers of the trapezius contract concentrically,
- the pectoralis minor contracts eccentrically,
- the upper fibers of the serratus anterior contract eccentrically.
The middle trapezius and rhomboids may also contribute to the scapular motions involved during arm elevation.
182The antagonists are the pectoralis major, teres major, latissimus dorsi, and coracobrachialis, all working eccentrically. Normal motion of the scapula on the thorax is believed to include consistent contact between the thoracic wall, the medial border, and inferior angle of the scapula.
182Loss of this contact has been clinically implicated as evidence of abnormal scapular kinematics. These abnormal scapular kinematics may result in additional stress on the anterior shoulder stabilizers.
The appropriate force couples for the acromial elevation that occurs during G-H abduction are the lower trapezius and serratus muscles working together, paired with the upper trapezius and rhomboid muscles (
The EMG activity of the levator scapula muscle, upper and lower trapezius muscles, and serratus anterior muscle during arm elevation increases progressively as the humeral angle increases.
68Activities that maintain an upwardly rotated scapula while accentuating scapular protraction, such as a push-up plus, elicit the greatest serratus anterior EMG activity.
The last few degrees of shoulder elevation consist of upper thoracic movement once full G-H joint and shoulder girdle motion have been completed.
186As the arm continues to elevate beyond the 150-degree mark, the thorax begins to extend and ipsilaterally rotate and side flex.
The scapula also functions during retraction and protraction along the thoracic wall (see
24Protraction occurs as the serratus anterior at the scapula and the pectoralis major at the humerus contract simultaneously. Retraction is produced by the combined action of the trapezius and rhomboids.
187A 15–18-cm translation of the scapula around the thoracic wall occurs during retraction and protraction, depending on the size of the individual and the vigorousness of the activity.
151This retraction and protraction is used during activities such as reaching and pulling, respectively.
Lastly, the scapula functions to transfer the large forces and energy from the legs, hips, back, and trunk to the actual delivery mechanism, the arm and hand.