Chronic upper-extremity musculoskeletal pain associated with disability has been reported in 21% of the U.S. population. The complex anatomy of the shoulder plays an important role in positioning the entire upper extremity for hand function, thereby creating a vital kinetic chain for daily living. Because of its versatility in positioning and posture, the shoulder may be predisposed to muscle imbalance syndromes associated with common functional pathologies such as impingement, thoracic outlet syndrome (TOS), and shoulder instability. Other shoulder and neck pain conditions include: Upper Crossed Syndrome, Frozen Shoulder (Adhesive Capsulitis), Rotator Cuff Injuries, Supraspinatus Tendinosis, Infraspinatus and Teres Minor Tendinosis, Subscapular Tendinosis, Bicipital and Coricobrachilais Tendinosis, and Subacromial Bursitis.
The shoulder demonstrates an intricate balance of structure and function. Therefore, clinicians must understand both structural and functional approaches in managing upper extremity pain syndromes. To understand the functional pathology of shoulder dysfunction, clinicians must have a working knowledge of force couples, proprioception, and chain reactions. They must also be able to assess posture, balance and gait, movement patterns, and muscle strength and length. Performing appropriate assessment and initiating effective interventions is the only path to positive, successful outcomes.
Muscles of the Shoulder Girdle
Movement impairments are deviations from the ideal kinesiologic pattern of motion. A deviation is an alteration of the normal counterbalancing action of muscular synergists. Often, the assessment of the alignment of the shoulder girdle at rest indicates the presence of muscle impairments, which can be associated with movement impairments. Alterations in the counterbalancing actions are a major factor in the development of movement impairment syndromes.
The muscles of the shoulder girdle can be grouped under designations which are determined by the muscle attachments. These include the thoracoscapular, thoracohumeral, and scapulohumeral muscle groups.
Based on the kinesiology of shoulder motion, the thoracoscapular muscles must move the scapula correctly for the scapulohumeral muscles to provide optimal control of the humerus, and to maintain an optimal relationship of the glenoid and humeral head. Alteration in the actions of the thoracohumeral muscles can become the major source of movement impairment because of the large size of these muscles and their direct attachment to the humerus.
The thoracoscapular muscles include: the trapezius, levator scapula, rhomboids, serratus anterior, and pectoralis minor. They are responsible for the movement of the scapula, which must maintain an optimal relationship with the humerus to minimize abnormal stresses at the glenohumeral joint. Specifically, the head of the humerus must remain centered in relationship to the glenoid as motion occurs at the shoulder joint. The head of the humerus cannot remain centered if the scapula doesn’t rotate properly. The force couple action of the trapezius and serratus anterior is a key to the motion of the scapula.
Because shoulder motion does not involve true reciprocal activity, most of the thoracoscapular muscles are active with antagonistic motions during shoulder motion.
Alteration in the dominance or length of any one muscle can compromise the muscle balance. Understanding the synergistic and antagonistic actions of these muscles is essential for analysis of shoulder girdle motion. Impairments in alignment observed in the resting position are manifested as muscle impairments during movement.
Most patients with shoulder pain develop their condition as a result of movement impairments of the scapula, which have disrupted the relationship between the humerus and the glenoid. This disruption causes alterations in the accessory motions of the humerus, particularly anterior and superior glide.
Impairments of the pectoralis major and latissimus dorsi muscles can contribute to glenohumeral joint dysfunction. These muscles essentially bypass the scapula, attach directly to the humerus, and can disrupt the scapulohumeral rhythm. Both of these muscles are medial rotators of the humerus and depressors of the shoulder girdle, but not the head of the humerus. They are powerful movers (not stabilizers) with strong and extensive attachments to the axial skeleton. The range of shoulder lateral rotation can become limited during the last one third of the range of shoulder flexion if they become short or stiff. When they become dominant muscles, they can become a source of faulty control of the humerus in the glenoid. If one or both are short or dominant, they can restrict shoulder girdle (again, not the humeral head) elevation which is a motion that should accompany shoulder flexion.
The counterbalancing effects of the scapulohumeral muscles (rotator cuff, deltoid, teres major) are critical to optimal control of the humerus in its relation to the glenoid. The primary function of the rotator cuff is dynamic stabilization of the glenohumeral joint. Within the rotator cuff itself, a force couple between the subscapularis and infraspinatus/teres minor provides a (perpendicular) compressive force that draws the humeral head into the glenoid. This actually provides a depressor force vector that counteracts the elevation force of the deltoid. This rotator cuff-deltoid force couple is the key to shoulder abduction.
The most common impairements are:
- Shortness or stiffness of the lateral rotators (infraspinatus, teres minor).
- Insufficient activity of the lateral rotators and dominance of the teres major (a medial shoulder rotator) leading to inadequate lateral rotation of the humerus. Lateral rotation prevents the greater tuberosity from contacting the acromion.
- Insufficient activity of the subscapularis muscle, allowing the humeral head to glide anteriorly and superiorly.
- Dominance of the deltoid (anterior and middle heads) muscle, causing the humeral head to glide superiorly and medially.
- Shortness of the lateral rotators and the teres major impeding the maintenance of the correct axis of rotation for the humeral head.
- Shortness (limited space) in the joint capsule particularly inferiorly and posteriorly resulting from short/stiff lateral rotators.