Scapular Disconnection

This disconnection permits scapular mobility in many directions, such as retraction, protraction, elevation, depression, anterior/posterior tilt, external/internal and upward/downward rotation. During humeral motion, the scapula provides a stable base and helps with the transfer of energy. The scapula is not the only device that supports upper extremity movement, but movement is also assisted by the acromioclavicular, sternoclavicular, and scapulothoracic joints that are within the shoulder. The acromioclavicular joint is a synovial joint located superior to the GH joint and consists of the junction between the acromion and the clavicle. The sternoclavicular joint is a synovial saddle joint between the manubrium of the sternum and the clavicle
bone. The scapulothoracic joint is the muscular interaction between the scapula and the thorax and is not a true anatomic joint. A function of the scapulothoracic joint is to increase the range of motion for the humerus and to add a large lever for the muscles that attach to the scapula. These joints and bony structures work in unity in order to maintain proper scapular kinematics during daily tasks.
The head of the humerus fits into the glenoid fossa and forms the glenohumeral (GH) joint. The GH joint is a multiaxial ball-and-socket-synovial joint that offers the greatest range of motion and movement potential of any other joint in the body, but in exchange stability is sacrificed. The static and dynamic forces of the muscles and ligaments holding the GH joint in place affect the stability. When the humerus and scapula move in unison there is a need for GH alignment in order to maintain maximum joint stability; this is called scapulohumeral rhythm
The humerus has a large range of motion due to the lack of bony articulations between the trunk and scapula.
With a large range of motion there is a dependence on active control to stabilize the scapula. The main stabilizers of the scapula are the serratus anterior, rhomboid major and minor, levator scapulae, and upper and lower trapezii. Stability of the scapulothoracic joint depends on the coordination of the surrounding musculature. Dysfunction or weakness in the scapular musculature alters mechanics and positioning of the scapula and may lead to alterations in the biomechanics of the GH joint. An unstable scapular base may affect the center of rotation of the GH joint and alter the length-tension relationship of the muscles involved in humeral movement. There is a possibility that any inefficiency in scapular stabilization could cause a decrease in neuromuscular performance and possibly predispose the individual to injury of the GH
joint
The rotator cuff is a grouping of muscles on the scapula that consists of the supraspinatus, infraspinatus, teres minor, and subscapularis muscles (Jobe & Pink, 1993; Kamkar, Irgang, & Whitney, 1993).
The rotator cuff has the ability to rotate, depress, and stabilize humeral head within the GH joint. The persistent force pulls the humeral head into the glenoid fossa during arm movement in order to stabilize it into position . The infraspinatus and the subscapularis are the external rotators and humeral stabilizers. The infraspinatus is a key external rotator of the humerus and, because it is the only muscle of the rotator cuff accessible with surface EMG, offers a convenient entry point for studying the role of fatigue in rotator cuff function The infraspinatus also has a role in keeping the humeral head stabilized and separated from the glenoid within the GH joint. Consequently, a weak or damaged infraspinatus could lead to instability of the humeral head and further lead to shoulder impingement syndrome
.