There are many known causes of shoulder pain but the most commonly misdiagnosed cause is the serratus anterior muscle tension.
The serratus anterior is a muscle that originates on the surface of the 1st to 8th ribs at the side of the chest and inserts along the entire anterior length of the medial border of the shoulder blade. The serratus anterior acts to pull the shoulder blade forward around the thorax and also pulls the lower end of the shoulder blade laterally and forward and thus rotates the shoulder blade up to make elevation of the arm possible.
A tensed serratus anterior muscle limits shoulder blade upward rotation, during upward movement of the arm thus narrowing the space (sub acromion space) which the shoulder muscles pass through to attach to the arm bone. This leads to muscles (cuff) or tendons impingement, the symptoms presents as follows:-
A physiotherapist will perform a series of test to rule out tendonitis, rotator cuff tear, joint arthritis and bursitis.
An MRI or ultrasound can be done to rule out the same.