Scapulohumeral Rhythm

In terms of shoulder complex kinematics, it is important to know that the clavicle, scapula, and the humerus function together as a kinetic chain. The purpose of this is to produce functional movement of the upper extremity, which provides much more range of motion than if a joint were to work alone. One important consideration related to shoulder functionality is called scapulohumeral rhythm, which is the ratio of movement between the scapula and the humerus. In this process, the scapula acts as a stable base for the glenohumeral joint.

Scapulohumeral rhythm is important clinically because it is a good indicator of a shoulder injury. Normal scapulohumeral rhythm is a relatively pain-free process that allows for a good deal of shoulder range of motion. A related issue that is seen clinically would be an impingement-- where the subacromial space was not maintained, leading to the compression of the tendons of the supraspinatus, long head of the biceps brachii, or the subacromial bursa.

The scapulohumeral rhythm can heavily affect ROM measurements of the shoulder. The shoulder itself has a range of 180 degrees, with 120 degrees provided by the glenohumeral joint, while 60 degrees would be provided by the scapulothoracic joint, as there is a 2:1 ratio between the two joints. A tool that really helps to measure the scapulohumeral rhythm is an inclinometer. If there were abnormalities in the rhythm of these two joints, it could skew the length/tension relationship that occurs between the muscles during active insufficiency, which would not produce an effective force.

In order to achieve full range of motion, such as when fully elevating the shoulder joint to reach the top shelf (flexion and abduction), the humeral head must compensate by laterally rotating so that the greater tubercle of the humerus passes by the acromion of the scapula. In this motion, the scapula also rotates upwardly and tilts posteriorly, leading to a greater range of motion. When the joints are working together, joint congruency is established, resulting in decreased shear forces and increased joint stability.