The shoulder is inherently the most unstable joint in the body, well at least for the major joints. This is because you sacrifice stability for mobility. In other words, we have to do a lot of things with our shoulders like reach overhead and behind the back, feed ourselves, dress, groom, etc. If something is very stable, it doesn’t move much. The anatomy of the shoulder is such that it lends to much greater mobility, and much less stability.

Just look at the ball and socket of the hip, which is a much more stable joint:

Compare it to this picture of the shoulder. Notice how small the socket is compared to the hip: 

I once heard an instructor of mine compare the shoulder to a car. The scapular muscles are like the body of the car. The rotator cuff is like a steering wheel. And the deltoid is like the gas pedal. I think this is an accurate description. Now, if you have a powerful deltoid, but the rotator cuff and scapular muscles are able to keep up…well you have a less than ideal situation. Like a ferrari with a loose axle and no power steering. 

This is typically where most people run into shoulder injury. They focus on what one might call the ‘vanity muscles’. The deeper layer of muscles that control the position of the shoulder may be under trained compared to the powerful deltoid. That may be all well and good under light loads. But as soon as you challenge yourself and try to press something heavy, the deltoid may be able to handle it, but if the strength of the rotator cuff and scapular muscles isn’t quite there, you can lose position. This is where problems arise. 

It reminds me of a weight lifting belt. The point of the belt is to keep the spine stable on a heavy lift such as a deadlift. You may be able to deadlift a PVC pipe with perfect spinal stability, but under heavy load, you will begin to lose position. Unless, of course, you’ve trained the spinal stabilizers to handle that load. Most people have not, and this is where the weight belt comes in handy. 

Back to the shoulder. If shoulder stability is a problem for you, strengthening the scapular muscles and rotator cuff are paramount in creating a stable environment for movements such as an overhead lift. 

I would generally start off with basic scapular exercises and rotator cuff exercises such as external rotation. As the athlete progresses, I like to add in exercises that challenge stability from multiple angles such as this: