I recently caught the beginning of this video and one of the things that I like about it is Doctor Richard Hawkins use of words like "the roof of the shoulder" in referring to the accromion process. It makes understanding shoulder impingement a lot easier for non-medical people.
He talks about the roof of the shoulder developing bony spurs which in turn impinge on the ligaments of the rotator cuff. These ligaments, particularly that of supraspinatus, are impinged upon when the arm is raised to the side or overhead.
("Impinge" means to "contact" or press against. I first heard the term while in the army learning how to fix guns. In describing the internal operation of a weapon, the firing pin would impinge on the percussion cap causing fulminate of mercury to explode….)
I'm guessing that the spurs might be caused by the humerous banging into the root of the shoulder.
And that possibly happens because the upper and mid fibers of the trapezius aren't working properly.
The upper trapezius attaches at one end to the rear of the base of the skull and at the other end to the outer end of the collar bone, near where the collar bone attaches to the roof of the shoulder joint.
The middle fibers of the trapezius originate at the back of the upper neck and attach to the roof of the shoulder.
For these muscles to be able to pull up on the outer edge of the shoulder blade effectively it helps if they have a stable foundation. That foundation can be created by lifting the chest and pulling the head rearwards and upwards with the chin down.
It also helps if they have room to move. That room to move is created by the same above two actions, opening the chest and lengthening the neck (by pulling the head up and back.)
The neck and chest work together and so for alot of movements of the neck it helps to start with the ribcage, in this case lifting the chest and in particular bending the upper portion of the thoracic spine backwards. While the direction of movement is a backwards bend, since the upper thoracic spine tends to curve forwards, this movement acts to straigthen the thoracic spine (or make it straighter.)
The cervical spine, the part of the spine that joints ribcage to head, tends to straighten also as a result.
Straightening the cervical spine increases the distance between the head and ribcage. This creates some length in the trapesius muscle, assuming the shoulder blade is down. Then when the arms are lifted, the trapezius can act to pull up on the outer edge of the shoulder blade, lifting and angling the roof of the shoulder so that the upper arm bone doesn't impinge as it is lifted.
Now is having a "lengthened neck" and open chest a guarantee that the outer fibers of the trapezius will activate when the arm is lifted?
I don't know.
And that's why in my classes I teach a exercise specifically designed to activate these fibers. It isn't so much the exercise as where the awareness is focused while doing this exercise.
I've included it in FAT Exercises for Shoulder Blade Awareness and Control.
Just remember, if you want to avoid shoulder impingement syndrome then use the upper fibers of the trapezius muscle to move the outer edge of the shoulder blade upwards (more so than the inner edge.)