The shoulder is certainly the most mobile and perhaps one of the most complex joints in the human body. Its great mobility and range of motion are balanced by a lot of stabilising structures that support it. One such anatomical unit is the rotator cuff.
The individual muscles
There are 30 muscles that comprise the shoulder complex. Some of them primarily move the shoulders while others mainly stabilise it.
The rotator cuff is a group of four muscles and their tendons that primarily stabilise the glenohumeral joint. These four are supraspinatus, infraspinatus, teres minor, and subscapularis.
Supraspinatus - abduction of humerus
Young Lae Moon [CC BY 3.0 (http://creativecommons.org/licenses/by/3.0)], via Wikimedia Commons
Subscapularis’s origin is directly over the ribs on the anterior surface of the scapula. It inserts on the lesser tuberosity of the humerus. It is the main internal rotator of the humerus.
Subscapularis provided 53% of the total cuff strength and is the strongest in the group. The lower 40% of the insertions is composed of muscle and the upper 60% is tendinous. In neutral position, it provides passive restraint.
What these 4 muscles have in common is that they all originate from the scapula and insert in the humerus. They are grouped together for two main reasons. They all can rotate the humerus and their tendons coalesce into a capsule that forms a musculotendinous cuff around the shoulder. The muscles become inseparable at that level. The only exception is subscapularis, which joins the cuff separately via the rotator interval.
Together as a group, the rotator cuff muscles hold the head of the humerus against the glenoid fossa to stabilise the joint. This enlarges the range of motion in the glenohumeral joint while avoiding mechanical obstructions.
During abduction and flexion, they contract together with the deltoid. This compression of the glenohumeral joint, known as concavity compression, enables further elevation of the arm by the deltoid. Without this action, the humerus would move upward, which will decrease the efficiency of the deltoid. Additionally, the muscles prevent pinching during shoulder movements by tightening the joint capsule.
Almost every shoulder movement relies heavily on the rotator cuff muscles. Therefore, maintaining a balance between flexibility and strength in these muscles is essential for the health and optimal functioning of the shoulder.
People who perform repetitive movement with the arm above the head are more susceptible to rotator cuff damage and injuries. Injuries usually manifest as a weakness when lifting the arm. On the other hand, some people with MRI diagnosed tears may have no compromised functionality, loss of strength or pain. Perhaps this is due to the fact that there is some redundancy among the 30 muscles working around the shoulder.
Generally, pathologies of the rotator cuff can be divided into two types. Degenerative is where there is progressive loss of function over time. Rotator cuff tendonitis and shoulder bursitis usually happen over time and may cause pain or deterioration in function.
Traumatic is where the damage is done instantaneously. Rotator cuff tears, shoulder labrum tear, and shoulder separation are examples of traumatic injuries.
A shoulder impingement is somewhat in-between. It can either happen as a trauma due to poor mechanics and improper loading or over time due to degeneration of the surrounding structures.
These four muscles play an essential role in the mobility and stability of the shoulder. Because they are located deeply in the shoulder they don’t get as much attention as the deltoid. It is important to take good care of the rotator cuff muscles in order to enjoy pain-free shoulder movements and have the supporting structure for developing shoulder strength.