First, think of your bones. Floating in space. Supported and moved by muscles. With some joints in there. Muscles move the bones at the joints. To make it simple think of your elbow. The front is the biceps the back is the triceps. When you curl, you use your bicep. But your tricep works too, to support the elbow joint and act as a “break” or de-accelerator.
This goes the other way too. When you use your tricep to extend your arm, your bicep works “against” it. It pulls the other way, to support your elbow and keep you from over extending your arm.
The shoulder joint works like this too. Only it’s way, way more and move-able and complex. And there is a shoulder blade in there too.
There are a bunch of muscles that work together and against eachother to move and support all that stuff.
Sometimes, your shit gets wonky. Muscles that are supposed to support and “break” start to be used to “move.” Other little muscles are left out. Its a mess. Instead of the serratus being active and holding you upright, the lats, pecs and upper traps get all active and tight, pulling your shoulders forward. Making you rounded. With bad scap tilt.
Bad vs good. See the slump and the forward tilted shoulder blade?
It’s reversed here. Good vs bad. Can you see the “good” vertical scap and the bad tilted scap?
Good. Look at the vertical shoulder blade.
Bad, look at the tilt. The lower part of the shoulder blade is lifting away from the ribs. And the top is pulling forward, over the shoulder.
You need that stuff under your arm pit to work, to pull your shoulder blade into good tilt.
So, seriously READ THIS. Please follow this month long discussion, and the months worth of links in there.