Excerpted from:  (Page, Frank, Lardner.  Assessment and Treatment of Muscle Imbalance, The Janda Approach.  2010, Human Kinetics, Champagne IL.)

Shoulder instability can result from several factors including: altered glenoid position (hypoplasia), humeral retroversion (the normal humeral head has 30º of retroversion to the frontal axis of the elbow joint), and rotator cuff weakness.  Glenohumeral instability is classified by the direction of instability.  The most common directions are anterior and inferior.  Instability in these directions is often due to capsular deficiency in the inferior glenohumeral ligament.  Multidirectional instability describes a more global instability of the glenohumeralcapsule, one that involves multiple planes.

Instability is classified as traumatic or atraumatic in origin.  Traumatic instability involves unilateral dislocation in one direction (usually anterior and inferior) and usually requires reconstructive surgery.  Atraumatic instability is often multidirectional, evident in both shoulders and treated with rehabilitation.

Impingement is related to instability.  The term functional instability, which is defined as activity–related symptoms with or without clinically detectable laxity, is often used to describe the phenomenon of instability leading to impingement.  Mild instability increases the demands on the rotator cuff for stabilization, causing fatigue, anterior subluxation, and functional impingement.  Functional instability is related to sensorimotor dysfunction and often exhibits altered muscle activation patterns and muscle imbalances in strength and flexibility.

The glenohumeral joint provides important proprioceptive information to the surrounding muscles that provide dynamic stability.  Damage to the glenohumeral ligaments disrupts the capsular mechanoreceptors, thus reducing feedback to the dynamic stabilizing muscles.

The rotator cuff provides primary dynamic stabilization, while the biceps and deltoid provide secondary stabilization.  Any imbalance in strength or activation of the dynamic stabilizers can contribute to functional instability.  For instance, weakness of the infraspinatus decreases the compressive forces of the rotator cuff, while tightness of the pectoralis major increases anterior shear forces, promoting anterior instability.

Patients with shoulder instability often demonstrate altered muscle activation patterns.  Typically, activation of the serratus anterior, deltoid, and supraspinatus is decreased, while biceps activation is sometimes increased.  Scapular kinematics may also be altered (similar to impingement) by decreased posterior tilt and decreased upward rotation of the scapula.  The important role dynamic scapular stabilization plays in instability is supported by the high correlation between scapular position and centering of the humeral head on the glenoid.

Athletes who perform overhead movement patterns are particularly vulnerable to functional instability.  Swimmers with instability often have impingement, a condition otherwise known as ‘swimmers shoulder’.  Throwing athletes with shoulder instability demonstrate altered EMG patterns during throwing , including increased activity in the biceps and supraspinatus and decreased activity in the internal rotators and serratus anterior in order to avoid anterior instability.

Glenohumeral instability has been associatedwith imbalances in ROM, most notably an increase in external rotation and a decrease in internal rotation.  Excessive external rotation or a tight posterior capsule , commonly seen in athletes performing overhead movement patterns, is thought to increase inferior and anterior translation of the humerus, thus leading to instability.  Recently, some experts have suggested that capsular length is not associated with the characteristic imbalance of increased external rotation and decreased internal rotation.  They discovered significantly more posterior translation of the glenohumeral joint in both shoulders of baseball pitchers when compared to internal rotation.  This finding suggests laxity rather than tightness of the posterior capsule.  So, it is possible that the lack of internal rotation is related to muscular tightness rather than capsular tightness.