This is a quick little rant / moan / educational piece on a very common misconception and myth that drives me a little crazy every time I hear it, be it from therapists, doctors, trainers or coaches and need to get this message out for thr sake of my sanity…
THE SUPRASPINATUS DOES NOT INITIATE MOVEMENT OF THE SHOULDER
Now I can sort of understand how this myth has become established, as indeed when the Supraspinatus isn't functioning effectively due to pain or pathology, the arm does appear paralysed, it just can't move much anywhere, this is called pseudo paralysis
But… it is NOT due to the fact that Supraspinatus initiates movement, it is simply due to it being unable to counter act the superior translation forces of the deltoid acting on the humeral head, causing it to shift upwards and cause more pain, and this has been known for a long, long time, back in 1963 Linge & Mulder explained it here, so why this myth continues today is mind boggling!!!
However to try and quash this myth once and for all, Prof Karen Ginn and her team did a nice little study here looking at the EMG readings of all the shoulder muscles and found that Supraspinatus does indeed fire before movement occurs, but NOT before other muscles such as the Deltoid, Infraspinatus and Serratus Anterior so again providing further evidence that the Supraspinatus does NOT initiate movement.
EMG readings from Reed et al 2013
As mentioned, what does cause the pseudo paraylsis is the humeral head superior migration, due to the Supraspinatus being unable to counter act the forces of the Deltoid acting on it, however this doesn't automatically mean the Supraspinatus is very badly damaged, it can just be weakened or inhibited due to pain.
Also a further little known effect of Supraspinatus inactivty is that Deltoid muscle activity actually increases, further compounding the superior humeral head migration effect.
McCully et al did a rather nice study looking at this by temporarily knocking out Supraspinatus in subjects with a nerve block, they found that even with very small levels of Supraspinatus inactivity as the muscle recoved after the nerve block resolved the Deltoid activity was still significantly increased.
This is an important consideration and also why pseudo paraylsis it's NOT a reliable sign of severe rotator cuff damage or tears, the lag and drop arm tests however are better (but not infallible) (source)
Its important to remember that the Supraspinatus can sometimes, in fact often, just be 'tweaked' or mildly irritated for pseudo paralysis to occur, and although it can be alarming, worrying and disconcerting for the individual, if reassured all is ok and explained that once the pain reduces and the supraspinatus function returns, so the paralysis reverses and arm movement returns!
So in summary, please, for the sake of my sanity, stop telling patients with pseudo paralysis that it's due to Supraspinatus starts movement of the shoulder, it just doesn't, and please don't tell those with pseudo paralysis that they have a tear or need surgery, it's not often the case!
Thanks for reading