Scapulohumeral Rhythm

 Scapulohumeral rhythm may be a term that sounds fancy or like it would be too hard to understand, but it really just describes the relationship between the movement of the scapula (shoulder blade) and the humerus (arm proper). Essentially, as the humerus abducts, the scapula rotates upwardly as well. Clinically, scapulohumeral rhythm has many reasons that it is relevant. One reason is that it helps to ensure that the head of the humerus stays aligned with the glenoid fossa. This means that the head of the humerus has a surface to roll and glide on. Another reason that scapulohumeral rhythm is relevant is that it helps to maintain an optimal length-tension relationship. If the scapula was not able to move, the deltoid muscle would shorten to a point in which it could not shorten any more before the arm reached 180°.  Because the scapula contributes to some of the range of motion, the deltoid is able to maintain its ability to contract through a longer range of motion. Yet another reason scapulohumeral rhythm has clinical relevance is the fact that it can help prevent shoulder impingements. There are a few tissues that run through the subacromial space in between the head of the humerus and the coracoacromial arch: the supraspinatus tendon, the subacromial bursa, the long head of the biceps tendon, and part of the joint capsule. If the scapula does not move in sync with the head of the humerus, the tissues running through this space can become pinched resulting in pain.

Scapulohumeral rhythm is important to range of motion, as briefly mentioned previously. Since there are actually two joints contributing to the motion, the range of motion can be greater. The absence of this rhythm can significantly decrease total range of motion. A normal range of motion for abduction in the shoulder is 0-180°. This comes from the 2:1 ratio of glenohumeral to scapulothoracic motion at the shoulder. Essentially in a full range of motion, 120° comes from the glenohumeral joint while the other 60° comes from the scapulothoracic joint. Without scapulohumeral rhythm, abduction at the shoulder is unable to reach even half of its range of motion. You can actually demonstrate this on yourself. If you hold your clavicle down and keep it from moving, it also prevents the scapula from moving. When you go to lift your arm, it is most likely physically unable to reach 90° of abduction without the movement of the scapula.

What is the clinical relevance of the scapulohumeral rhythm? How can it affect your ROM measurements of the shoulder? Please give at least 5 reasons in your response.