The design of the glenohumeral joint makes it highly mobile. However, the trade-off for this mobility is stability, particularly from bone.  For this reason, the joint depends significantly on its stability from muscles, and in particular, the rotator cuff.

There are four rotator cuff muscles: the supraspinatus, infraspinatus teres minor, and subscapularis. The pneumonic SITS can help you remember the names of these.

They attach from the scapula to the humerus, and therefore can all act to move the shoulder in different directions. So supraspinatus is involved in shoulder ABduction. Infraspinatus and teres minor both externally rotate the shoulder, and subscapularis internally rotates it. 

The cuff muscles work as a team. The activity of one has to be supported by the others.  Let's add an important point here.  Muscles can destabilise a joint - any joint - not just the shoulder.  When they contract to create movment, the force they generate can cause the joint to misalign or even dislocate in extreme circumstances.  To prevent this, the contraction of other muscles is required to resist this negative force and maintain balance around the joint.  All of this is carefully orchestrated via the nervous system relaying information back and forth to the brain.

So, for example, during arm ABduction, supraspinatus will be more active as it creates that movement. However, subscapularis, infraspinatus and teres minor are still contracting to maintain joint stability. Without this contraction, supraspinatus would cause the humerus to slip up towards the acromion process.  This would compress the supraspinatus tendon between the humerus and the acromion process. A soft tissue compressed between hard bones leads to injury, inflammation and potentially rotator cuff tears.

There are lots of reasons why the rotator cuff become imbalanced in their activity and become injured.  These include falls, scapula winging, and poor control of movement, such as incorrectly performed chaturanga.