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The upper limb is not often involved in weight bearing, therefore its stability has been sacrificed to gain mobility.145 The glenohumeral joint is distinct because it

maintains stability despite having few restraints.158 These restraints consist of the static and dynamic components. Bony, cartilaginous, capsular, and ligamentous structures all function to provide static stability of the glenohumeral joint.159 The musculature surrounding the shoulder composes what is known as the dynamic stabilizers.158

Although the static stabilizers of the upper limb have an important role, it is

imperative to understand the influence of the dynamic stability provided to the shoulder in order to fully appreciate mobility and function of this region. The dynamic stability is primarily the result of neuromuscular control between the scapulothoracic musculature

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and the rotator cuff (RC) muscles.158 This dynamic stability can be described further as scapulohumeral balance. Scapulohumeral balance refers to the theory that the humeral head is balanced in the glenoid if the net joint reaction force passes through the fossa.160 Therefore, the glenohumeral joint will remain stable, as long as the scapula is positioned so that the glenoid fossa encloses the net forces acting on the humeral head.160 It is essential that the RC and scapulothoracic musculature work in a synchronous manner so the resultant compressive force across the joint falls within a stable arc provided by the glenoid concavity.160 If these muscles do not work together efficiently, deficits may arise which could lead to shoulder dysfunction such as impingement syndrome.

When assessing the upper extremity, it is essential to understand the function of the surrounding musculature. The rotator cuff’s primary function is to guide and stabilize the GH joint.161, 162 The RC muscles help to strengthen the GH joint in every direction except inferiorly and are well positioned to resist GH shear stresses in order to prevent pathologic translation.145, 158, 161 The RC consists of the supraspinatus, infraspinatus, teres minor, and subscapularis. Each of the individual muscles of the rotator cuff along with the biceps brachii originate on the scapular body and insert onto the humeral head, thus pulling the humerus closer to the glenoid cavity upon activation. Simultaneous contraction of these 5 muscles creates a compression effect of the humeral head into the glenoid cavity. As the rotator cuff and biceps brachii musculature contract to pull the humeral head downward and inward, this humeral head compression is coupled by the upward and outward pull of the anterior, middle, and posterior deltoid musculature. This mechanism is commonly referred to as the glenohumeral force couple, and, when in balance, functions to center the humeral head in the glenoid cavity.

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The interaction of the RC muscles works in combination with other muscles in the shoulder girdle to provide stability. Another classification of dynamic stabilizing

structures in the upper extremity is known as the scapulothoracic muscles. Several research studies have established the importance of a coordinated, synchronous action of the glenohumeral and scapular muscles.17, 163-165 The muscles included in the

scapulothoracic region that provide this dynamic stability are the: latissimus dorsi,

serratus anterior, pectoralis major, rhomboid major and minor, and the upper, middle, and lower trapezius. These muscles are capable of producing large torques about the shoulder joint because of their cross-sectional anatomy and distance from the joint center of

rotation.158

When discussing the dynamic stability of the shoulder joint provided by the scapulothoracic region, it is important to discuss the scapula force couple. The scapula force couple refers specifically to the stability and balance provided to the

scapulothoracic region by the upper trapezius, lower trapezius, rhomboid major,

rhomboid minor, levator scapulae, and serratus anterior.166 It is important to understand how contraction of each of these muscles effect movement of the scapula. Contraction of the upper trapezius creates scapular upward rotation, elevation, and retraction.145 When the lower trapezius is contracted, it causes scapular upward rotation, depression, and retraction.145 When the rhomboid major and minor are fired, both muscles produce scapular retraction, downward rotation, and depression.145 Activation of the serratus anterior creates scapular protraction and upward rotation.145

There is a need to understand the intricate relationships between the static and dynamic stabilizing structures at the shoulder when evaluating and treating this region.

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Research has shown that even the smallest alteration in performance and coordination of the muscles surrounding the shoulder has the potential to lead to dysfunctions and compensations that could compromise normal joint function and lead to disabilities,17 resulting in inactivity,167and lower quality of life.168 As mentioned previously in the literature review, muscle weakness in the upper extremity has been found among 18-23% of breast cancer survivors and this weakness may be present even up to 2 years after their surgery.76, 81, 84 Breast cancer treatments appear to effect a percentage of these survivors upper extremity strength, specifically around the scapulothoracic region. Therefore, weakness of the upper extremity must be evaluated when a breast cancer survivor has complaints of either pain and/or loss of function when using their upper extremity. It is important that clinicians understand the function of the scapulothoracic muscles, effects breast cancer treatments may have on this region, and are able to assess weakness appropriately. In summary, when clinicians develop rehabilitation programs for breast cancer survivors to correct dysfunction at the shoulder, it is necessary to break down this complex region into its various components in order to discover the fundamental

principles that may be the underlying cause of the problem.