Margaret Schenkman, Ph.D, PT, Victoria Rugo De Cartaya, PTThe upper extremity engages in diverse functions through a wide range of motion. The shoulder complex has multiple articulations, and upper extremity movement requires movement of all components of the shoulder complex. The combination of muscles acting during motion is dependent on biomechanical factors related to muscle size and length, joint angle, force of movement, gravity, etc. For these reasons analysis of shoulder movement is difficult. It is necessary to analyze both static and dynamic factors in posture and movement. In this paper, we summarize the structural and biomechanical information necessary for analysis of movement of the shoulder complex. We then review the literature describing shoulder complex movement and analyzing specific muscle functions. Finally, we describe four representative motions as examples of use of structural, biomechanical, and kinesiological information for the analysis of movement.
COMPONENTS OF THE SHOULDER COMPLEX
The bones of the shoulder complex are the humerus, scapula, and clavicle. The shoulder complex has three articulations: glenohumeral acromioclavicular, and sternoclavicular. In addition, there is one "functional" articulation between the scapula and the thorax. The sternoclavicular joint is unique because it represents the only bony connection between the entire shoulder complex and the thorax, and hence with the rest of the body. This implies that the shoulder complex depends on nonbony connections, to maintain its integrity with the body. The so-called scapulothoracic joint, comprised of muscular attachments of scapula to thorax, is of major importance in maintenance of that integrity.
The shape of articulating surfaces, ligamentous structures, and joint capsules are critical structural factors determining degrees of freedom, stability, and ultimate range of movement that can occur between two articulating surfaces. These factors, in combination with muscle length and strength, play a major role in determining individual differences in flexibility and mobility, predisposition to injury, and the distinction between normal and pathological movement. The clinician who is analyzing movement needs a good knowledge of these joint structures. The structural components of the shoulder complex have been well desribed. Some of the relevant information is summarized for easy accessibility (Tables 1 and 2). Arthrokinematics is important but is not discussed in this article.
Structure dictates function. The architecture of bony surfaces that articulate determines the degrees of freedom of available movement and sets outside limits to the available range of motion. Ligaments, joint capsular structures, and muscle length may further limit the available range. The relative degrees of stability and mobility are a reflection of the composite of these factors. The theory of evolution of the shoulder complex suggests that the functional changes occurred during the transition from quadriped to plantigrade. This transition necessitated alteration of the limb from one used primarily for stability in weightbearing to one used primarily as a mobile structure.
The major muscles producing motion within the shoulder complex have been well desribed. These muscles can be divided into three separate groups: muscles that originate on the shoulder complex and insert on the humerus or elbow, muscles that originate on the trunk and insert on the shoulder complex, and muscles that originate on the trunk and insert on the humerus (Tables 3 and 4). The shoulder complex functions as a kinematic chain. Although it is comprised of three distinct segments, movement of any one of those segments may produce movement in other segments. For example, movement of the humerus through action of the latissimus dorsi or pectoralis major (both of which originate on the trunk) will produce movement of the scapula and clavicle as well. Conversely, pathological shortening of latissimus dorsi or pectoralis major would limit available range of motion of the scapula although there is no direct connection to the scapula. One of the most important aspects of the shoulder complex is the intricate and delicately balanced interplay of all components of the kinematic chain.
Muscles almost always act in combinations to produce motion. In addition to those muscles that are primarily responsible for a given motion, there are muscles that play secondary and synergistic roles, or that provide stability elsewhere in the body. A prerequisite for normal movement is for all relevant muscles to act synchronously and appropriately. Different terminologies have been used to describe the various roles that muscles play. In this paper, we will use the following definitions. A prime mover is the main force that produces motion at a joint. If the prime mover is a muscle, it is also called an agonist. The term agonist may also be used to describe muscles primarily responsible for maintaining a position. A secondary mover, or assistant, is a muscle that produces only some of the combined actions of a prime mover or that is recruited only if the action becomes forceful. Synergists are muscles that contract at the same time as the prime mover to produce a desired motion. There are two distinct ways in which synergists can assist: helping synergists are pairs of muscles that have an action in common and an opposite action; they act together to produce the desired motion while the undesired motions cancel each other out. In contrast, a neutralizer is a synergist that acts indirectly to assist in a movement by canceling out undesired actions caused by the agonist. Frequently, muscles from many or all categories are required to produce a desired motion. The antagonists are muscles having an action opposite to the desired motion. Finally, stabilizers are forces that prevent unwanted motion at joints other than those joints where the prime mover exerts its action. For motion to occur normally, the relevant antagonists must appropriately lengthen and stabilizers must act coordinately to provide a stable base throughout the remainder of the body. Analysis of movement requires knowledge and accurate assessment of each of the forces that act with the prime movers to produce that movement. Excessive or diminished amount of any given force may alter the entire course of the movement. As the f-mvements of the shoulder complex are analyzed, all participaing forces will be considered.
COMBINED MOVEMENTS OF THE SHOULDER COMPLEX
Shoulder complex movements represent care fully orchestrated motion of all of its components. The humerus rotates around the scapula within the glenohumeral joint, the scapula rotates around the clavicle at the acromioclavicular joint, and the clavicle rotates around the sternum at the sternoclavicular joint. Movement of all of these components must occur for the arm to achieve 180° of humeral elevation.(The term elevation is frequently used in the literature without differentiation between abduction and flexion.)
If the humerus is held in internal rotation, only 60° of elevation is allowed. Furthermore, the humerus must externally rotate during elevation. Otherwise, by 90° the greater tubercle of the humerus will impinge on the coracoacromial arch. In normal movement, only 120° of glenohurneralelevation is permitted within the glenoid fossa. After 120°, motion is blocked by impingement of the surgical neck of the humerus on the acromion of the scapula and on the coracoacromial ligament. Humeral elevation beyond 120° is accomplished by rotation of the scapula in an upward direction. This rotation positions the glenoid fossa superiorly, passively carrying the humerus through the additional 60° of elevation.
Abbott and Lucas' have reviewed the consequences of excision of the clavicle and have pointed out that without clavicular participation the limb can still be actively elevated to 180°. However, strength and stability both are affected. This would be expected as the clavicle provides the only bony attachment of the shoulder complex to the skeleton. Furthermore, losk of the clavicle prevents a shift in the axis of scapular rotation which would in turn affect the participation of the lower trapezius as an upward rotator (see discussion below).
In summary, glenohumeral elevation to 180° must be accompanied by humeral external rotation and by scapular upward rotation. In turn, scapular upward rotation is accompanied first by clavicular elevation and then by clavicular backward rotation.
All components of these motions must occur in order for humeral elevation to occur smoothly and through full range of motion. These combined motions position the glenoid fossa superiorly to increase limb range of motion and also to prevent active insuffic(ency of muscles crossing the glenohumeral joint.
Vector representation of muscle action and resolution of force vectors into rotatory and translatory components of force are helpful in describing muscle actions. By this method it is evident that several important force couples act within the shoulder complex. A force couple is defined as two equal forces acting in opposite directions to rotate a part about its axis of motion. Two separate force couples are of particular importance in motion of the shoulder complex. The rotator cuff muscles act in concert with the deltoid to guide the head of the humerus during humeral elevation. The trapezius and serratus anterior act together to produce upward rotation of the scapula. Both force couples are depicted by vector representation (Figs. 3 and 4).
During elevation, the humeral head must be approximated in the glenoid fossa as the humerus is rotated. When the humerus is abducted, the rotatory component of the deltoid pulls the humerus outward and the translatory component of the deltoid pulls the humerus upward toward the acromion. The rotatory component of three of the rotator cuff muscles (subscapularis, teres minor, and infraspinatus) is exerted on the proximal side of the axis of humeral motion, pulling medially. Hence, it too pulls the humerus outward. The translatory component of the force of these rotator cuff muscles pulls the humerus downward (Fig. 3). Even though the two rotatory forces are exerted in opposite directions they combine to move the humerus in the same direction (abduction) because they are applied on opposite sides of the joint axis of motion, forming a force couple. In contrast, the translatory forces of the deltoid and the rotator cuff muscles cancel each other out, stabilizing the head of the humerus within the glenoid fossa." If the rotator cuff muscles are not adequately active, the translatory force of the deltoid would presumably pull the humerus irp ward into the acromion of the scapula. In summary, although the deltoid is considered a prime move for humeral abduction, it cannot work effectively in the absence of the rotator cuff muscles. In addition to guiding the head of the humerus within the glenoid fossa, infraspinatus and teres minor also provide the external rotatory force necessary to prevent impingement of the greater tuberosity of the humerus on the coracoacromial arch of the scapula.
Several muscles act together as a force couple that upwardly rotates the scapula. The upper portion of the force couple is comprised of the upper trapezius and upper digitations of serratus anterior. The lower portion of the force couple is comprised of the lower trapezius and lower digitations of serratus During approximately the first 90° of humeral elevation, the scapula rotates around an axis on the spine of the scapula near the vertebral border (Fig. 2). Most of the fibers of the lower trapezius exert a downward rotatory force on the scapula, hence they would not be expected to be active in this early stage. Once the humerus reaches 90° of elevation, and the scapula is about 30° upwardly rotated, the axis for scapular rotation shifts to a point on the spine of the scapula near the acromioclavicular joint. Then the lower trapezius would be effective as an upward rotatory force. Most authors do not differentiate between the role of the lower trapezius early in the range of humeral elevation and late in the range. However, Basmajian comments that the lower trapezius and lower fibers of the serratus anterior become more active as the scapula progresses through upward rotation. lnman et al. suggested that the trapezius, as a whole, plays a predominantly supportive role below 90° and actively participates in upward rotation above 90°. Data of lnman et al. also suggest that the lower trapezius is relatively inactive until 90° of abduction. Furthermore, their data indicate a distinction between the action of the lower trapezius in abduction and in forward flexion. They suggested that the lower trapezius is the more active component of the force couple in abduction but that the muscle must relax to allow forward flexion; the lower fibers of the serratus anterior then become the more active component. Differences in activity of the lower trapezius can be observed clinically by palpation.
ANALYSES OF SPECIFIC MOVEMENTS
In this section we have provided sample analyses of movement of the shoulder complex during selected motions. In these analyses (Tables 5-8) we have used a modification of the format of Riegger and Watkins. The four activities chosen for analysis are upper extremity abduction to 180°, unresisted return (adduction) of the upper extremity from 180° to neutral, resisted adduction of the humerus from 180° to neutral, and a sitting push-up. These activities were chosen to illustrate the diverse actions of the shoulder complex. Elevation of the upper extremity against gravity's pull is of major importance during many daily activities including reaching, dressing, eating, throwing, and grooming. The analysis of upper extremity abduction as an open kinematic chain is used to illustrate the intricate interplay of muscles of the shoulder complex during such activities. This analysis also illustrates .the shifting axis for scapular rotation and its consequence for action of scapular muscles. Adduction, or the return of the upper extremity to neutral from 180°, is used to illustrate the consequences of movement of the arm in the same direction as the pull of gravity. Under this circumstance, gravity acts as the prime mover and muscles of the shoulder complex act eccentrically to control the fall of the humerus, scapula, and clavicle under the force of gravity. In the third example, forceful shoulder adduction, the humeral adductors and extensors and the scapular downward rotators and depressors act together to bring the upper extremity down against an externally applied upward force. Finally, a sitting pushup is used to illustrate the use pf scapular and humeral musculature when the upper extremity is fixed and motion occurs through a closed kinematic chain. The sitting push-up is representative of functional activities such as crutch walking or vaulting.
During unresisted upper extremity abduction the humerus abducts a total of 120° and externally rotates, the scapula simultaneously upwardly rotates 60°, while the clavicle elevates and rotates backward around its long axis (Table 5). The prime movers for the humeral motion include the deltoids and suprapinatus. There has been some suggestion that the supraspinatus acts as an initiator of abduction. However, Basmajian and lnman et al. have found that this muscle acts coordinately with the deltoid, playing a quantitative but not a specialized role. While the middle deltoid can work during upper extremity abduction, if the anterior or posterior deltoid participate, they must work together as helping synergists to produce a pure abduction force. The biceps brachii may also contribute to abduction of the upper extremity if the humerus is externally rotated. This is commonly referred to as the biceps mechanism.
The muscles discussed above are considered the abductors of the humerus. They cannot, however, successfully accomplish their task unless they work in combination with other essential muscles. The infraspinatus, subscapularis, and teres minor of the rotator cuff play a critical role in humeral elevation. These are the muscles that work with the deltoid as a force couple, neutralizing the compression action of the deltoid at the glenohumeral joint (Fig. 3). Furthermore, the infraspinatus and the teres minor participate in the necessary external rotation of the humerus during abduction.
Scapular upward rotation during upper extremity abduction, is performed by the combined actions of the trapezius and the serratus anterior. From 0-90° of abduction of the upper extremity, the scapula rotates upwardly approximately 30°. This movement is produced by the upper trapezius and upper digitations of the serratus anterior. From 90-180° of abduction of the upper extremity, the scapula rotates a further 30°. During the latter part of the range, the lower trapezius and lower digitations of the serratus anterior join with the upper portions of these muscles to form a force couple with an upward rotatory force. It is important to recognize that there are a number of pairs of helping synergists active during these motions. The upper fibers of the serratus anterior and upper trapezius form a pair of helping synergists; the serratus anterior upwardly rotates and depresses the scapula while upper trapezius up wardly rotates and elevates the scapula. The upper and lower trapezius act as another pair of helping synergists; the upper trapezius upwardly rotates and elevates the scapula while the lower trapezius (acting from 30-60° scapular rotation) upwardly rotates and depresses the scapula. If all muscles do not work effectively, scapular motion will become unbalanced.
During abduction of the upper extremity as described, the clavicle elevates and then rotates backward along its long axis. Elevation occurs in part because of the force of the upper trapezius which inserts along the lateral one-third of the clavicle. Clavicular motion also results as a consequence of forces applied elsewhere in the kinematic chain (e.g., to the scapula and humerus). Scapular upward rotation with shoulder abduction causes the clavicle to elevate until the costoclavicular and coracoclavicular ligaments prevent further motion and produces rotation. Clavicular movement occurs without direct muscle action because it is connected with the rest of the shoulder complex kinematically and because of the forces produced by ligaments.
Finally, it is important to recognize that several muscles act at the scapula and humerus to provide stability necessary for the prime movers to work effectively. The teres major is not active during shoulder abduction but does become active during static positions of elevation; its activity increases with increasing load. lnman et al. also suggested that the middle trapezius and rhomboid serve to fix the scapula in the plane of motion during abduction but must relax somewhat to allow forward flexion.
In return of the upper extremity from 180° to neutral, all components of the shoulder complex reverse their movements: the humerus adducts and internally rotates; the scapula downwardly rotates (first around an axis of motion near the acromion and then around an axis of motion near the vertebral border); the clavicle rotates forward, then depresses. In this activity, gravity acts as a prime mover. Under the force of gravity, the limb, scapula, and clavicle would fall quickly and without control. Eccentric muscle action is therefore required, not to produce motion, but to control motion. The same muscles lengthen and act eccentrically in this motion as they did concentrically in upper extremity abduction. However, their action is eccentric rather than concentric. In this activity, the relevant muscles are designated controllers rather than movers (Table 6).
If adduction from 180° to neutral is performed against resistance, the shoulder adducts and scapular downward rotators become active (Fig. 5). In this instance, motion of the humerus from 180-90° results from the force of the latissimus dorsi, pectoralis major, and teres minor. From 90° to neutral, the latissimus dorsi and teres major will continue to exert adductor forces; the pectoralis major would predominantly exert a horizontal adduction force and hence is no longer a prime mover for this motion (Fig. 5). Because the latissimus dorsi, teres major, and pectoralis major (from 180-90°) also act as shoulder extensors, muscle action must be exerted to prevent extension as opposed to straight adduction. The pectoralis major (clavicular head) may act as a neutralizer for this extension after 90°. The action of the latissimus dorsi also produces scapular downward rotation and depression by its action on the scapula through the humerus. These motions may be enhanced by the direct actions of the rhomboids and levator scapulae on the scapula. Further assistance in downward rotation is provided by the lower trapezius during scapular rotation from 30° to neutral. It is important to notice the pairs of helping synergists that participate in scapular downward rotation: the rhomboids and lower trapezius retract and depress the scapula, the levator scapulae retracts and elevates the scapula. In summary, forceful downward movement of the humerus requires action of a different group of muscle acting both at the humerus and at the scapula than does simple return of the upper extremity to neutral from abduction (Table 7).
In a sitting push-up the upper extremity and entire shoulder complex is forcefully depressed. Because the limb cannot move downward, the actively contracting muscles cause the body to elevate relative to the humerus. The prime movers thus act in reverse action. This is an example of a closed kinematic chain. Prime movers are the latissimus dorsi, pectoralis major, and to a lesser extent the lower trapezius.lg These muscles act bilaterally and in reverse action; they move the trunk relative to the humerus in contrast to their normal action moving the humerus relative to the trunk (Fig. 6). Participation of other muscles will depend upon how the motion is carried out. If the humerus is not permitted to internally rotate, the external rotators must act in normal action. The rhomboids and lower trapezius assist in depression and downward rotation of the scapula in relation to the trunk (Table 8). The shoulder abductors and rotator cuff muscles act as stabilizers, controlling scapulohumeral alignment.
In this short paper we have not discussed postural muscles that provide stability of the body for upper extremity movement. The action of postural muscles, or stabilizers, varies depending on body position in space. The muscles that maintain stability will depend on the relationship of body alignment to the line of gravity at any given time. Hence stabilizers are variable and dynamic.
The movement of the shoulder complex is the sum of movement of all the joints. Too often it is attributed solely to the glenohumeral joint. Clinically, it is essential to evaluate and treat the entire shoulder complex in order to improve upper extremity function.
The authors express grateful appreciation to Cheryl Riegger, PhD, PT for critically reading the manuscript and to Christine Fiorelli for her assistance and patience in preparation of the figures.
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