The Biomechanics of Shoulder Impingement

The glenohumeral joint (shoulder joint) is the most moveable joint in the body. This is partially due to its shape. The head of the humerus (upper arm bone) is shaped like a ball that fits into a socket on the end of the scapula called the glenoid fossa. This joint works similar to the way a pestle glides in a mortar or a golf ball fits on a tee.

The glenohumeral joint does not have a deep socket like the hip so it relies on ligaments, tendons, and muscles keep it centered in the glenoid fossa. This allows the joint surfaces to align properly and move smoothly through the full range of motion. The muscles of the rotator cuff are the main muscles responsible for glenohumeral joint stability.  If rotator cuff muscles are weak they may no longer keep the humeral head centered. The stronger deltoid muscle will pull the humeral head upward thus reducing the space between it and the acromion process of the scapula. The rotator cuff tendons pass through the space between the acromion process and the head of the humerus. Narrowing of this space can impingement of rotator cuff tendons.

Another factor in impingement is scapulo-humeral rhythm. Scapulo-humeral rhythm is the coordination of movement between the glenohumeral joint and the other joints that make up the shoulder complex, the sternoclavicular joint, the acromioclavicular joint and the scapulothoracic joints. Proper scapulo-humeral rhythm permits the head of the humerus to be centered within the glenoid fossa through its full range of motion. If the scapulo-humeral rhythm is disrupted the head of the humerus will not remain centered which can cause rotator cuff impingement.

Sub-acromial bone spurs, variations in the shape of the acromion, poor posture and ligament injury can also contribute to narrowing of the sub-acromion space and rotator cuff impingement. Repetitive movement across the impinged structures can result in serious damage and inflammation. Inflammation of the sub-acromial bursa or rotator cuff tendons further reduces the sub-acromion space and worsens shoulder impingement.

Performing repetitive or overhead arm movements such as house painting, weightlifting, or pitching, increases the risk for developing shoulder impingement. The usual symptoms of shoulder impingement are pain which increases with activity and decreases with rest, a dull, localized aching deep within the shoulder, a painful arc of motion as the arm is lifted sideways, and crepitation or a catching sensation during motion.

An untreated shoulder impingement can damage rotator cuff tendons and lead to a rotator cuff tear. It is important to get proper treatment for shoulder impingement as soon as it occurs.


Ludewig, P. M., & Braman, J. P. (2011). Shoulder Impingement: Biomechanical Considerations in Rehabilitation. Manual Therapy, 16(1), 33–39.

Page, P. (2011). Shoulder muscle imbalance and subacromial impingement syndrome in overhead athletes. International Journal of Sports Physical Therapy, 6(1), 51–58.

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