A Commonsense Approach to Addressing Shoulder Instability

By Todd Galati, M.A.−

Shoulder instability is very common and has many causes. Before you can design an exercise program to help your clients with shoulder instability, you must first understand shoulder structure and function, common causes of instability and potential injuries. Armed with this knowledge, you can make better decisions about the functional issues that can be addressed through a fitness program, appropriate movement assessments and resultant exercises, and when to refer.

Shoulder Structure and Function

At the root of shoulder instability is its structure. The shoulder joint is a shallow ball-and-socket joint that is formed by the attachment of the head of the humerus with the glenoid fossa of the scapula. The glenoid labrum provides a bit more depth to the glenoid fossa’s “socket,” while the ligaments, joint capsule and rotator cuff muscles collectively hold the shoulder joint together. The scapula is then attached to the torso via a two-joint linkage formed by the scapula’s attachment to the clavicle at the acromioclavicular joint and the clavicle’s attachment to the sternum at the sternoclavicular joint. These connections are given dynamic support from the scapulothoracic joint formed by the muscles that originate on the torso and insert on the scapula and humerus (e.g., trapezius, latissimus dorsi). These combined structures form the shoulder complex, a highly mobile unit that allows greater range of motion and increased velocity when throwing or striking.

When the shoulder complex is functioning properly, it has pain-free mobility built upon a stable scapula. Scapular stability is essential for programs focused on improved shoulder function, mobility and strength. The scapula is mobile, adding to the full range of motion of the shoulder complex; however, its primary design is to serve as a stable platform from which the shoulder moves the upper limb. When the muscles that act on the scapula are weak, they compromise scapular stability. In her book, Therapeutic Exercise for Musculoskeletal Injuries, Peggy Houglum describes the difference between a stable and an unstable scapula for the shoulder as being “similar to the difference between running on firm ground and running on a suspended wood-and-rope footbridge.” In this analogy, the firm ground provides a stable base for propulsion, while the unstable footbridge “places high energy demands on the individual’s leg muscles, causes incoordination and inefficiency of movement, and increases risk of injury” (Houglum, 2005). As with this footbridge example, an unstable scapula will transfer its instability to the shoulder joint, causing movement issues and increased risk of injury.

The bones, articulations and ligamentous structures provide static stability for the shoulder complex, while the muscles and nervous system provide dynamic stability. Static stability of the shoulder can be compromised by injuries to the ligaments (e.g., shoulder separation), bones (e.g., fractured clavicle) or tightness in the joint capsules. Injuries to the ligaments or joint capsule can impair the normal proprioceptive function of the neural receptors in the injured ligaments, which can inhibit the response of the muscles. Dynamic stability can also be compromised by injuries (e.g., biceps tendinitis), shoulder impingement or an imbalance in the muscle force couples acting on the scapula.
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