It is important to make yourself aware of the anatomy of the shoulder complex before you can understand scapulohumeral rhythm. The shoulder complex consists of the clavicle, the scapula, and the humerus and they all function together. The collaboration between the scapula and the humerus is called the scapulohumeral rhythm. This is relevant in terms of a clinical bases when range of motion (ROM) comes into play. The rhythm is a ratio of each members movements, a two to one ratio. What that means is that for every three degrees of shoulder movement, there is one degree of movement at the sternothoracic joint (ST joint) and two degrees of motion at the glenohumeral joint (GH joint). So in clinical practice, when you're measuring shoulder abduction range of motion, the client is supposed to be able to have a full one-hundred and eighty degrees of motion, sixty from the ST joint and one-hundred and twenty for the GH joint. If either the scapula or the humerus isn't able to do their part in the movement, then the rhythm is completely thrown off and ROM won't be accurately measured. The scapulohumeral rhythm is important because it helps align the glenoid fossa with the head of the humerus. The joint congruency of this mechanism results in a decreased shear forces. This allows for the optimal length/tension relationship for the muscles involved. Again, if this relationship is helter-skelter, the ROM measurement of the shoulder won't meet its ROM norm. It also helps maintain the subacromical space so there isn't any impingement. If the client didn't have a good rhythm, they would get an impingement. So if the client had issues with his or her muscles that attach to the scapula and the scapula didn't move, there would be friction in their subacromial space which is bad. A good rhythm is needed for the client to have efficient arm movement.
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