Scapulohumeral Rhythm

Scapulohumeral rhythm is the interconnected systems of scapula movement at the scapulothoracic joint and the glenohumeral joint, and can be described as a ratio of movement at these two joints. The general rule of thumb is that for every 3˚ of shoulder movement performed 1˚ comes from the scapulothoracic joint and 2˚ come from the glenohumeral joint. This is important because without the contributions from both joints many shoulder movements would be inadequate for the tasks we do in our daily lives. There are anatomical limitations to the glenohumeral joint, keeping us from being able to accomplish what we would consider full range of motion of the shoulder with this joint alone. For instance, we have the glenoid fossa that serves to keep our humerus in place, however this fossa creates "walls" that the humerus cannot pass to achieve full upward rotation or abduction, and this is where the scapulothoracic joint comes into play since it can manipulate the positioning of the scapula, and more specifically the glenoid fossa, to allow the humerus a greater range of motion. Without full upward rotation, damage can occur in the subacromial space. With the joints moving together you can also reduce the shear forces that occur when pressure is being applied in opposite directions, which helps with joint preservation. The relationship between the glenohumeral and scupulothoracic joint also helps promote a more optimal length-tension relationship of the muscles. The adjustments to the position of the scapula at the various positions of the glenohumeral joint helps to keep the proper tension for muscle strength. ROM measurements can be affected due to one of these joints not properly working, since the norms we follow for the measurements are based on the combination of these joints to acquire the full range of motion expected in the movements of the shoulder.