The Relevance of the Scapulohumeral Rhythm

    The shoulder has one of the greatest mobility of any joints in our bodies. It helps us stabilize hand use, lift and push, elevate the body, and be able to bear weight. The shoulder itself is very critical when it comes to ADLs. The Scapulohumeral Rhythm is a ratio between the motion of the scapula and the motion of the humerus. The two joints that are involved in this rhythm are the scapulothoracic (ST) joint and the glenohumeral (GH) joint. To further explain, the ST joint is a joint that isn't what we consider a "real" joint. It is considered to be a functional joint instead. It is a bone-muscle-bone articulation that occurs between the scapula and thoracic wall. The GH joint is a ball and socket joint. It consists of a concave glenoid fossa of the scapula and a convex head of the humerus, which is twice the size of the glenoid fossa. These two joints create a 2:1 ratio within the Scapulohumeral Rhythm. Since every shoulder has 3 degrees of shoulder movement, there are 2 degrees of the GH joint and 1 degree of the ST joint. For a full 180 degrees of motion in the shoulder, the GH joint has 120 degrees (2/3) and the ST joint has 60 degrees (1/3).

    

    As I have mentioned ROM and MMT testing in a previous blog post, we know how clinically important it is to be able to palpate bony landmarks and be aware of where they are within the body. When it comes to scapulohumeral rhythm, it is important to palpate these landmarks of the scapula and be able to feel them as the client continues with shoulder movements. When palpating for these landmarks and something feels abnormal, that is usually a sign of injury or disease. This is something to be aware of when measuring clients to ensure their safety and not make their injuries any worse or cause more pain. The Scapulohumeral Rhythm allows for a large shoulder ROM and if the Scapulohumeral Rhythm is not normal, it can limit the client's shoulder function as well as their ROM since the ST and GH joints help out with that. If one of these joints is injured, then the other joint will not be able to work as efficiently as these two joints directly impact one another when it comes to movement. This leads to joint congruency resulting in decreased shear forces with the two bones. When it comes to shoulder abduction, we know the humerus must laterally rotate to achieve full ROM at the GH joint. This means that when we measure ROM for abduction, we should know that the position should be correct in order to have accurate results. Scapulohumeral Rhythm also has a good length-tension relationship within the muscles surrounding the joints and bones to help create an optimal function of the shoulder. This can affect ROM because if the length-tension relationship wasn't established, it would cause the client to use compensatory muscles and not use the muscles they need to execute their movements and leading to inaccurate measurements.