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The shoulders, neck, and thoracic spine are all very interconnected. Your thoracic spine is the foundation on which your scapulae and shoulders move, so if it doesn’t work properly neither will they. Most significantly, if your t-spine is not able to extend (straighten) properly, it will limit your ability to move your arms overhead because it affects their starting position.
Imagine your scapula as a raft on a lake. If you were standing on it, you could shift your weight to move the raft back and forth, side to side. It would have a lot of freedom. Now imagine that the lake was frozen, and how rigid the raft would become as a result. It’s the same with your shoulders: a stiff t-spine can’t move to accommodate the scapulae, and if the scapulae can’t move properly your shoulder range of motion becomes greatly reduced.
The FMS approach to t-spine mobility seems to favour rotation, I suppose as means of attending to asymmetry in the region. I’ve found that performing more general mobility in t-spine extension (as is typically done on the MWOD) does a lot to open up the t-spine, which may make your rotation mobilization more effective when performed afterwards.
Now, once you’ve made some improvements in your t-spine, a big part of correcting your screen is going to be restore mobility and stability to the scapulae. Your t-spine is the foundation for your scaps, but the scaps are the foundation for your arms. Once you learn how to stabilize your scapulae, you’ll understand the true scope of your mobility challenges, and have a more reliable and quality way to test/retest relevant movements.
The MWOD does a pretty good job of explaining shoulder mechanics, so you should start to get the hang of it after a while, especially if you’re seeing some FMS practitioners. Just be uncompromising in the quality you demand of the movements you’re trying to improve. Remember that quality is the primary focus of corrective exercise, so make the movements you perform as basic as possible to move well.
For example: I’m currently working to balance out the behind-the-back part of the shoulder mobility screen. Right now, after doing soft-tissue work to open up the stiff muscles on the back of my shoulder, my first stability exercise is nothing more than lying on my back with my elbow out to the side of my shoulder, carefully performing rep after rep of reaching my hand towards the ground, taking exquisite care to keep my shoulder from rolling forward. This has helped me understand where I run out of shoulder range of motion before my scap gets pulled out of place, and has helped me pattern that stability into my rotation at a very basic level.
I have personally found improving internal rotation to be a huge challenge, one perpetuated by previously half-hearted attempts to correct it. Make sure you understand that long-term improvements to the screen can come very slowly, just like any improvements in training. Keep plugging away, and you’ll be way stronger for it in the end.