Simply put.

Rotator Cuff Impingement occurs when your rotator cuff tendon, typically your supraspinatus tendon is pinched between your acromion(part of your shoulder blade) and humerus head (arm bone).

So why on earth would this occur?

A few reasons actually:

1.Weak stabilizing rotator cuff muscles.

Your rotator cuff muscles prevents your humerus head from gliding superiorly (up) when you raise your arm, when they are weak impingement occurs.

2. Tight chest muscles.

Overdevolped pectoralis will cause protraction and internal rotation of your shoulder. This decreases the space in your gleno-humeral joint which in turns cause your rotator cuff to be impinged.

Here is picture of the mechanism of a rotator cuff impingement.

What you need to do with this condition is stop it immediately because over time it can lead to a rotator cuff tear.

First step is to restore muscle balance in your shoulder, and lastly improve your rotator cuff strength with pain free motion.