It feels good, or it hurts.
Those are the 2 main reasons why we do what we do. If an action elicits a pleasure response, most organisms gravitate towards repeating that action. You can think of this as the "Club Med" response. Contrarily, if an action elicits pain, then most organisms tend to avoid reproducing that action. This is the "Yipes!" response.
Now, let's say that something you might do (whether in sport, in work, in daily life, in whatever) causes you some sort of injury or discomfort. Guess what's going to happen? Your neurological system trains you to avoid it like the plague. You either mentally associate some sort of distaste with the action, and/or find some way around it if you have to continue to move the affected joint(s).
In the case of the shoulder, I've heard it described by former professors as the body's loosest joint when compared to the loads that are placed on it. If you look closely at the skeletal anatomy of the shoulder, there's not a lot of support for it. The bones that make up the shoulder interact with thin, relatively small surfaces that are tied together with thin ligaments and relatively narrow muscles (with the exception of the lats... This is sorta giving away the ending already, but I have to mention it here). That relative looseness allows the shoulder to achieve incredible ranges of motion compared to other joints, but the construction sometimes gets pushed beyond its limits thanks to either excessive forces or faulty movement patterns.
In fact, most of the time, you see patients whose injuries result from forces that would NOT have been excessive if the proper biomechanics were being followed. Let's look at a few examples...
- A martial artist or boxer throws a hard punch on a heavy bag that jars his shoulder because his shoulder was too protracted at the point of impact. Instead of keeping his elbow in FRONT of the plane of his body, he wound up for the punch by drawing his arm way behind it. Additionally the shoulder elevation further weakened the stabilizing ability of the shoulder girdle muscles, most likely straining both his rotator cuff and his pec. The next time he hits the bag, the strike will be more painful and probably less committed. If the round continues, there will be more guarding, reticence, and compensation.
- A young mother is holding her young toddler for extended periods, but she leaves most of her shoulder stabilizers too relaxed. The shoulder can't handle the load of her child's weight for such long stints, and the muscles become congested and fatigued.
- While training with his trainer, an athlete is playing catch with his trainer using the medicine ball. The trainer throws the ball short once and the athlete bends forward kyphotically to reach for the ball. His shoulders are loose, slack, and unbraced, and the medicine ball jerks his arms forward as it drops.
That's how the injury happens. The compensation mechanisms for those same 3 individuals might look like this...
- The martial artist's punches start to become more grazing rather than heavy, penetrating blows. Instead of reaching forward with that injured side or attacking with it, he chooses to block or shield with it instead. Additionally, while training or sparring, the injured shoulder is drawn in tighter and tighter as a protective response. That, in turn, manifests as pain and limited range of motion during the rest of the day.
- The young mother starts exhibiting neck & shoulder pain. Her neck becomes her core, and she tries to lift the sagging child by shrugging her shoulder up and also by kicking her hip out underneath the load. Eventually, if the problem is left to go on long enough, she'll feel pain, numbness/tingling, and/or weakness down the affected arm and into her hands, not to mention lower back pain (which sorta takes me towards my next installment).
- The athlete will likely start getting closer to try to catch the medicine ball more conveniently. However, when throwing the ball back, he'll be trying to generate the forward momentum in ways that allow him to protect that injured shoulder. He'll likely recruit muscles that shouldn't have a role in stabilizing the shoulder for a throwing motion. As Gray Cook would say, "The prime movers become stabilizers, and the stabilizers become movers." No good can come of that.
As you train yourself and watch those around you, keep your eye on the shoulder mechanics you see. You might find that you start noticing connections between certain movement patterns and certain aches and pains that people complain about. And then you'll hopefully start looking at your own movements with a keener, sharper, more disciplined eye.