I've simplified this account of shoulder muscles. These two work with a number of others, but all to the same effect, controlling upwards movement of the head of the Humerus.
Acting alone, the big lifting muscle of the shoulder (“Deltoid”,) would now also jam the humerus up against the underside of the coracoacromial arch.
Infraspinatus and subscapularis are the two next biggest shoulder blade muscles. They pass upwards from the lower part of the shoulder blade, to become the front and back of the rotator cuff.
As you raise your arm, they hold the head of the arm bone down to prevent excessive upwards movement of your humerus.
The lower fibers of pectoralis major, teres major and minor and latissimus dorsi are the others involved.
Exercises to strengthen these two muscles may be useful, but will need to wait until you have
stretched any soreness out of them. This link is to the shoulder treatment page with detailed instructions.
One such strengthening exercise is to sit on a kitchen chair and lift one’s bottom by pushing down with the heels of one’s hands on the edges of the chair.
As with any muscle strengthening exercise, you need to put some time into this.
You may need to improvise a bit, if you can't lift your bottom off the seat...
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