Strengthening the rotator cuff muscles is integral to keeping your shoulder joint strong and injury free. For a detailed description of what causes shoulder impingement, please visit my blog post on that topic here.
The rotator cuff consists of four small muscles. The supraspinatus is the topmost muscle. It lays across the top of the scapula (shoulder blade). The muscles (laying posteriorly - the closest to the skin) are the teres minor and the infraspinatus. The muscle under the scapula (laying anteriorly) is the subscapularis.
The supraspinatus, along with the deltoid, abducts (lifts up from the side) the arm. The teres minor and infraspinatus externally rotates the arm. The subscapularis internally rotates the arm.
The rotator cuff muscle that gets most injured is the supraspinatus. The muscle I strained was my left supraspinatus.
Rounded shoulders = winged scapula
Our rounded shoulders are the main reason we injure our rotator cuff muscles. Most of us have desk jobs in which we sit in a rounded forward posture. This posture causes tight and shortened pectorals (chest) muscles and lengthened upper back muscles. When muscles stay in an abnormally lengthened position, they become inactivated and weak, often leading to an injury when worked. Also, when the muscles around our scapula (the rotator cuff muscles) do not hold our scapula hugging close to our back, this causes them to wing out, which puts stress on the entire scapulothoracic joint, as well as the shoulder joint (glenohumeral joint), which also can lead to an injury.
Retract and depress = winging disappears
So to combat this default posture the exercise I suggest is to retract and depress your shoulders, also called "packing your shoulders". To do this contract your muscles in your upper back to bring your shoulder blades together towards your spine. Along with that movement press your shoulders down. This will slide your shoulder blades down your back. Hold this position for 10 seconds and repeat. I recommend doing this exercise 3 times, holding 10 seconds each. Work up to holding for longer time frames and repeating this sequence often during the day. Every time you carry anything, use this movement to keep your scapulothoracic joint stable.
To strengthen your external and internal rotators of the arm these next two exercises are very important. To work the teres minor and infraspinatus we need to perform external rotation: fasten tubing to a doorknob or other stationary object, lock your 90 degree flexed elbow into your side, making sure your shoulders are "packed", slowly rotate your arm outward, pause for 1 second, and then slowly come back to starting position. Repeat 10-12 reps for 3 sets.
To work the subscapularis perform internal rotation. The movement is reversed, as the force on the arm is coming from the opposite side. Start with your 90 degree flexed elbow hugging into your side, making sure your shoulders are "packed". Slowly internally rotate your arm into the midline of your body, hold for 1 second, and slowly release back to starting position. Repeat 10-12 reps for 3 sets.
I recommend two lateral raise exercises that will help strengthen the supraspinatus, which is the muscle out of the four that gets injured the most. Performing a lateral raise that is 30 degrees forward of the side works the supraspinatus the best and keeps your scapula hugging on your back. Start with your arms at your side, making sure your shoulders are "packed". Slowly raise your hands, slightly in front of you, thumbs up. Hold 1 second in the up position, and then slowly lower hands. Repeat 10-12 reps, 3 sets.
The next lateral raise is called scaption. With hands starting in front of you and making sure your shoulders are "packed", raise your hands up and back, squeezing your shoulder blades together and depressing your shoulders even more when at the up position. Hold for 1 second and slowly lower your arms.
Another scaption exercise is called Y to W exercise. Lay your torso over a stability ball, with hands by the floor. With your shoulders packed, lift your arms into a Y position, slowly move to the W position, squeezing your shoulder blades together and down your back. Hold for 1 second, and slowly lower your arms. Repeat 10-12 reps, 3 sets.
The last exercise I suggest for the rotator cuff is the rocket. This exercise also works your erector spinae muscles (your back extensors, which is part of your core). Lie prone on a mat. Make sure your legs stay in a neutral active position, meaning toes and knees are pointing straight down and the legs are neither internally or externally rotating. Pack your shoulders and lift your torso off the floor, making sure you keep your blades squeezing together and down. Tuck your chin in slightly. Hold for 10 seconds and then slowly lower. Repeat 10 reps, 3 sets.