Hey. I actually have trapezius injury on my right side and my shoulder is a little pushed forward. the situation is quite bad that I cant elevate the shoulder up as it is stuck. but I do not have T.O.S on that side. Roo's test is false.
anatomically speaking, only an extra rib/medial scalene/anterior scalene
can cause compression in the brachial plexus area...
shoulder related injuries are different syndromes.
on my left side though - I have T.O.S(A. Scalene is deformed) but my shoulder is perfectly well, I can easily move it around/elevate it whatever.
so; bad shoulder muscles like traps and scapula in my opinion can aggravate T.O.S - not cause it.
if you are serious about studying the neck muscles and diseases...
1) study neck anatomy such as muscles and blood vessels
2) study thier function(google it)
3) perform tests that are available on youtube to see if they are injured

that's what I did and that's how I know exactly whats wrong with me.