When therapist and medical professionals talk about shoulder stability, they tend to stress the importance of proper rotator cuff function. What exactly is a rotator cuff? What is its role in shoulder stability?
What is the rotator cuff?
The rotator cuff is a layman term given to a group of 4 muscles and their tendons that connects the humerus and scapula. They essentially work to pull the humerus head into the glenoid cavity, providing integrity to the shoulder joint through its entire movement range.
The four muscles are the Supraspinatus muscle
, Infraspinatus muscle
, Teres minor muscle
and Subscapularis muscle
The rotator cuff muscles are relatively smaller than the 'big' shoulder muscles such as the deltoids, trapezius. While their role in the large movements of the arm are smaller in terms of 'force' exerted, they play a crucial role in enabling those movements. They keep the glenohumeral joint stable allowing the bigger muscles to work more effectively. As you can see from the diagram above, the four muscles are arrange to almost around all the point of the humeral head. This allows at least one of the four muscles to be able to 'pull-in' the humeral head into the glenoid cavity.
Why is stability of the joint important in terms of effective movement?
A key principle that the body employs is the lever and fulcrum principle. This allows us to move large objects with a smaller amount of force. And one of the key efficiency factor is the stability of the fulcrum. The lever principle depends on a firm and stable fulcrum to rest the lever of one through the movement. So how does the rotator cuff contribute towards the shoulder joint stability?
Take for a example when we raise our arm up from the side. This movement is primarily driven by our bigger deltoid shoulder muscle. However, given the shape and the angle of the glenoid cavity, this movement pulls the humeral head upwards out of the gleniod cavity. So without the rotator cuff muscles, the shoulder joint would 'slip' upwards and outwards a little from the glenoid cavity, changing the fulcrum position, resulting a poor performance of the deltoid muscles in raising the arm.
Take a look at our previous article to discover how to best strengthen your rotator cuff muscles: The rotator cuff stability