In Part 1 of Shoulder Injuries we looked at the structure and function of the shoulder. In this article we’re looking at the way various muscles change the way the shoulder moves.
Why do muscles change the way the shoulder moves?
The most basic and obvious place to start is the Deltoid. The big teardrop shaped muscle over the shoulder that makes the shoulder shape. One of its main functions is to abduct the arm. If you stand with your arm at your side and then raise it up in an arc away from your waist so that eventually it is stopped in its tracks by your ear, that is abduction.
The basic premise is that the Deltoid provides the force to bring the arm up in an arc. In doing so, the muscle also pulls the humerus upwards into the socket. This action is of course assisted by the rotator cuff (specifically the supraspinatus in the first 30 degrees of motion).
The problem comes when there is no counter-pull from other muscles.
If the Deltoid is particularly strong, or other muscles are weak, the top of the arm (head of the humerus) hits the underside of the shoulder (coracoacromial arch) and creates pain which can manifest itself in lots of ways. You might even hear some crunching or clicking.
There are 4 main muscles which should theoretically prevent this from happening, and they are collectively called the rotator cuff muscles.
They are Supraspinatus, Infraspinatus, Subscapularis and Teres Minor. They all start on the Scapula and end on the top half of the arm. As a group, their main job is to compress the humeral head (top of the arm) into the space under the shoulder. Together, they counter the pull of the deltoid and enable the top of the humerus to move in space within the shoulder without impinging on anything.
As mentioned in Part 1, if one or more of these muscles is damaged or tired, or locked long or short from any number of reasons, the efficiency of the rotator cuff as a unit is compromised, and shoulder pain is likely.
However, it’s not quite as simple as that.
Although we have identified the Deltoid as the main muscle involved in the shoulder, and the rotator cuff – 4 more muscles being majorly involved, there are other accessory muscles which directly affect the shoulder as well.
Rhomboid major and minor, levator scapula, teres major, serratus anterior, trapezius, pectoralis minor, biceps brachii, triceps brachii, latissimus dorsi, and then there are all the muscles which are fascially connected to the shoulder as well, at this point, you could include almost any muscle in the body.
So, if any of these muscles is short, tight, fibrotic, inefficient, fascially adhered to another muscle in the area, that can affect how the shoulder moves.
So now we know how many muscles there are, what the heck is force coupling?
The scapula moves in 3 dimensions. It rotates clockwise and anticlockwise, it shifts left, right, up and down, and it also tilts forward and backward. Each muscle attached to the shoulder has a number of specific jobs and they work with and against other muscles in pairs and 3’s in order to achieve specific movements. The muscles which work together to create these movements may not necessarily be attached to the same part of the scapula, and may, in other movements actually work antagonistically.
For example – in upward rotation, the upper and lower trapezius work together with the serratus anterior – however, in adduction of the scapula, the pull of the trapezius is counterbalanced by the abduction of the serratus anterior.
That’s obviously a bit of a simplistic version, but you can see that these three muscles have a bit of a paradoxical relationship, alternately pulling with and then against each other depending on which movement the scapula needs to do.
Only one part of this relationship needs to give out to cause issues in the movement quality of the scapula, which will have the effect of creating movement impairments, and therefore pain, in the shoulder.