The infraspinatus is one of the four muscles of the rotator cuff. It gets injured most commonly from poor posture or repetitive motions. When it is not working at its best it will affect the whole shoulder.
Some symptoms:
shoulder pain
pain radiating to neck, upper back or down the arm
Feelings of weakness and stiffness with shoulder movements
Discomfort when sleeping or laying on your side
Release the infraspinatus with a lacrosse ball to help relieve pain