The supraspinatus is part of the rotator cuff muscle in the back of the upper arm. Many people injure this muscle because they overuse the front deltoids. The shoulder is a muscle that goes inane directions and is made up of several muscles. In essence there is no joint there its a false joint. The best thing to do is make sure you work out the whole shoulder. I would recommend shrugs, military press, lateral raises and also side raises and rear raises. Working the whole area is better then isolating these muscles to avoid stronger front or rear deltoids that can cause serious shoulder problems over time. Also the cable cross over on your side are great to hit those often neglected muscles.