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

Impingement is caused by narrowing of the SAS (subacromial space) either due to boney growth (primary impingement) or superior migration of the humeral head caused by weakness or muscle imbalance (secondary impingement).  The result is inflammation or damage to the rotator cuff tendons; therefore, chronic impingement can lead to rotator cuff tendinosis.  As secondary impingement is related to glenohumeral instability, it is sometimes described as functional instability; it occurs mostly in athletes less than 35 years of age who use overhead throwing motions.

Pathomechanics of Impingement

The pathomechanics of secondary impingement may involve one or both of the shoulder force couples:  the deltoid and rotator cuff or the scapular rotators.  Weakness or damage of the rotator cuff leads to an inability to control the upward shear of the humeral head into the SAS after activation of the deltoid during shoulder abduction.

Imbalance in the scapular rotator force couple leads to weakness and altered activation of the middle and lower trapezius and serratus anterior in impingement.  These alterations are often seen bilaterally, a finding that suggests a central mechanism of chronic tendinosis pain, consistent with Janda’s theories.

Patients with impingement demonstrate altered kinematics, including less upward rotation and external rotation as well as increased anterior tilt.  The change in scapular kinematics changes the orientation of the glenoid and is thought to reduce the SAS, thus compressing the rotator cuff and biceps tendon.  These changes also progress with age.

Scapular dyskinesis can be describes as a loss in scapular retraction and external rotation with altered timing and magnitude of upward scapular rotation.  This leads to an anterior tilt of the glenoid and subsequent reduction of rotator cuff force.

Athletes with impingement have significantly more EMG activity in the upper trapezius and significantly less EMG activity in the lower trapezius.In addition to weakness and muscle imbalance, muscle fatigue alters both glenohumeral and scapulothoracic kinematics.  Rotator cuff fatigue allows the humerus to migrate superiorly, while scapular fatigue leads to less posterior tilt and external rotation of the scapula.

Muscle tightness has also been implicated in secondary impingement.  A tight pectoralis minor limits upward rotation, external rotation and posterior tilt, and reduces SAS.