The rotator cuff is a series of four muscles in the shoulder: the Supraspinatus, Infraspinatus, Subscapularis, and Teres Minor. It's four muscles in the shoulder region that insert onto your humeral head. They're very important muscles because not only do they allow us to move our shoulder, but they're actually also dynamic stabilizers of the shoulder. What that means is that these are actual muscles that help keep the shoulder within the socket to allow for stability of our shoulder because our shoulders a very special joint because it has so much mobility. If you think about your shoulder, you can move it quite extensively in comparison to say, your knee, which essentially goes like this *demonstrates* or your hip, which doesn't have much motion either, relative to the shoulder. What allows us to have that is if you think about the anatomy of the shoulder, it's essentially a dish which (we call the glenoid) - that's the socket and then you have your humeral head. There's not a whole lot of stability between a ball and a dish, so we need stabilizers of the socket. We have static stabilizers, which would be the ligaments, and then we have dynamic stabilizers, which are the muscles. The rotator cuff are four muscles within our shoulder that provide both stability for the shoulder and allow us to move our shoulder.