These muscles become tight and shortened for a variety of reasons.

Everyone has different underlying genetics, structural make up, biomechanics and so on. These can be quite good to begin with or they can be particularly defective for a variety of reasons. We then do a variety of unhelpful things to our bodies over our lifetimes which affect what we were born with. Typically, people will have poor posture - all that endless hunching and slumping creates shortening and tightness in these muscles. Then there are normally significant muscular imbalances in the pairs of muscles which should theoretically be evenly balanced to work efficiently. All our slumping and rounding means that the opposing muscles which should be pulling our shoulders back and our scapulae down and in are then weaker. This in turn makes it even harder to fight the pull of the short, tight, strong muscles that are responsible for rounding the shoulders forward and pulling the rib cage up.

When you add repetitive activity such as sports, work and just general day to day moving, this tends to exacerbate both underlying structural or genetic issues and the postural issues. It becomes a chronic, self perpetuating problem. The longer this imbalance goes on, the harder it becomes to fix. Stretching, trigger point release, exercises and physio can sometimes only do so much with muscles that have been shaped wrongly over years or decades. Sometimes surgical intervention then becomes desirable or necessary to try and undo the secondary consequences like the pressure on nerves or blood vessels causing pain, loss of function and so on.

Does that help explain a bit?

Sort of. I already understood things on that level.

But it doesn't explain why some people who have poor posture and biomechanics don't have TOS while others do. It doesn't explain why I get severe muscle spasms in my left neck, ribs and back, but not the right. These sides are not so uneven any more, at least not when looking in the mirror.

Sorry if I was insulting your intelligence with things you already knew. I just took your question at face value and assumed you were asking why muscle tightness occurred.

The answer to your question is that medical science doesn't know why some people end up with conditions like TOS and others don't. I have extremely tight shortened muscles with all the risk factors plus I am a manual wheelchair user so should be a prime candidate for TOS but I don't have it. I do however, have a rare incurable neurological condition, the cause and mechanism of which is even more of a mystery to medical science than TOS. The inability to determine why some people get a disease or medical condition and others don't is a universal problem throughout the whole of medicine.

If medical science actually knew what causes some people to develop illnesses, diseases and conditions and others not to, then it would know how to cure them. With a fairly small number of exceptions, it doesn't. All medicine can do for the vast majority of conditions is treat symptoms.

I don't think looking in the mirror is necessarily a very reliable way of assessing the detailed features of underlying structure, positioning and health of internal soft tissue and bone. You may look less uneven than you used to but there is clearly still a difference between your right and left side and still an underlying problem which you would be unlikely to see for yourself by looking in a mirror. Add to that the fact that nobody has a symmetrical body even if it 'looks' much the same on both sides. Sometimes tiny defects or differences make all the difference.

my left side is more messed up too

even though my right shoulder is winged



another worry is my awful tmj. i dont know how you deal with hypertrophied scm and masseter muscles.

My TMJ is 90% resolved using "The Quieting Reflex" by Charles Stroebel. Thank goodness!

wow, okay i will read into this! the palate expander has helped a lot but im still tight

i gotta weird/dumb question

do you guys know...if i can put my fingers behind my collarbone pretty easily, does that mean it cant be compressing between first rib/collarbone

if you press an area and it hurts/feels tender does that mean that's where the nerve compression is

I don't know the answer to those questions. Hopefully, others do.

i tried not doing any shoulder strengthening for a few days and now upper traps are full of triggerpoints

this is too much effing maintenance

I do not know the answer to your first question but I know that because ribs and such can move that just because you can put your fingers behind it doesn't necessary mean that it couldn't be pinching. Usually compression is not an all the time thing, and rather in certain positions, compression occurs and it causes damage over time.
As for the second question, if you press on an area and it is tender it does not mean that's where the compression is. There are many different reasons for pain. For example, my pec minors are very tight and hurt to press on BUT the compression was seen between my clavicle and first rib. The compression there causes other muscles to spasm which causes referred pain. Nerve pain is difficult because when you apply compression to any part of a pissy nerve it can radiate under the same principles as double or multiple crush syndrome. Thusly, trigger points.
Anyway, hope that helped at least a little?