Biomechanics Blog 3 - Scapulohumeral Rhythm

The shoulder is a very complex and beneficial joint system to have. The scapulohumeral rhythm is what helps us to have such a large ROM in our shoulder in the 3 planes of motion. Without it, we would not have efficient arm movement to carry out daily tasks. For every 3 degrees of motion in shoulder, the scapulothoracic portion of the joint is responsible for 1 degree of motion and the glenohumeral joint for 2 degrees of motion. If either part of this joint is functioning improperly, it will result in a significant decrease in ROM. Raising the humerus up to flex the shoulder causes the scapula to upwardly rotate at the same time. This allows the glenoid fossa to stay in contact with the humeral head to avoid dislocation and a decrease of shear forces that can result in pain and friction. The rotation also allows the shoulder abductor muscles the optimal length-tension relationship to function properly. The subacromial space also benefits from the rotation because it helps maintain the space and prevent impingement of structures within it. So if this joint is out of rhythm it can cause various problems with our shoulder function.