Scapulohumeral Rhythm- Biomechanics Blog 3

Scapulohumeral Rhythm is the interaction between the movements of the scapula and the humerus, via the glenhumeral and scapulothoracic joints, as the shoulder moves along its planes of axis. This allows for smooth, functional motion at the shoulder during movement and stabilizes the shoulder when in anatomical position. Typically, the glenohumeral joint will move 2 degrees and the scapulothoracic joint will move 1 degree for every 3 degrees of total motion. We can see this during shoulder abduction, if full range of motion is 180 degrees, the glenohumeral joint will move 120 degrees and the scapulothoracic joint will move 60 degrees. By distributing the motion onto two surfaces, the body is able to acheive an optimal length to tension relationship; during abduction of the shoulder, the movement of the scapula allows for the deltoid to continue to lengthen in order to create more force. If the scapula were immobile, muscles such as the deltoid would be shortened, decreasing range of motion and force at the shoulder. Additionally, decreases in mobility of the scapula can cause impingement, pain, and development of scar tissue due to damage of the subacromial space. Scapulohumeral rhythm can affect range of motion measurements because of how the joints are dependent on one another for motion. This may result in a decrease in reliability of measurement if one joint ROM is outside of normal range or increase possibility of compensatory movements during testing.