Scapulohumeral Rhythm

     The shoulder joint has the greatest range of motion in the human body. It is described as a ball and socket synovial joint. Due to its wide range of mobility, this joint lacks in stability. Both the scapula and humerus work to provide motion at this joint. The rule for scapulohumeral rhythm is as follows: for every 3 degrees of shoulder motion, approximately 1 degree occurs at the scapulothoracic (ST) joint and two degrees occurs at the glenohumeral (GH) joint. Scapulohumeral rhythm is important because it decreases the amount of sheer forces at the shoulder joint, and it allows for healthy length-tension relationships. Damage to either one of these areas greatly affects range of motion at the shoulder. 

    Understanding scapulohumeral rhythm increases our understanding of range of motion. For example, during shoulder flexion, the GH joint contributes approximately 100-120 degrees (2/3 of a normal 0-180 range). Additionally, the ST joint contributes approximately 50-60 degrees of elevation. This elevation of the ST joint allows the humerus to move more freely in the glenoid fossa during shoulder flexion. The ratios of scapulohumeral rhythm also apply to movements such as shoulder abduction, internal and external rotation and more. 

    As previously mentioned, scapulohumeral rhythm decreases sheer forces and allows for proper length-tension relationships. Due to the shoulder's wide range of motion, multiple muscles are able to withstand strong forces and provide adequate contractions. Simultaneous motion at both the ST and GH joints prevent shoulder impingement between the acromion process of the scapula and the humerus. 


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