The acromioclavicular joint, also known as the AC joint, is where the acromion meets the clavicle. The muscles that surround the AC joint are the: deltoids, supraspinatus, infraspinatus, teres minor and subscapularis. The last four muscles are also known as the rotator cuff and can easily be remembered by the acronym S.I.T.S. AC joint exercises are going to be those that work these muscles.
External Rotation
External rotation of the shoulder joint is one of the most important exercises for the upper body because the rotator cuff muscles are involved with virtually every movement of the shoulder. Increasing strength of the rotators greatly reduces the risk of injury. To perform the external rotation: using either resistant tubing or a weighted cable located about waist height coming across the body, bend the elbow to 90 degrees and grip the handle. Keeping the elbow next to the body, rotate the arm with the humerus (upper arm bone) serving as the axis of rotation. Slowly control the resistance back to the starting position.
Internal Rotation
Internal rotation, while a stronger movement than external rotation due to the increased muscles involved in the movement, is equally important to train to keep strength balanced in the shoulder musculature. Instead of having the resistance coming across your body, position yourself far enough away that you have to reach out to your side to grip the handle of the tubing or cable. Bring the elbow in to the side, the same as it is with external rotation, still using the humerus as the axis of rotation, pull the resistance inward across the body, controlling the weight on the way back.
Lateral Raise
The lateral raise is a great exercise for the deltoids and the rotator cuff group. With a weight in each hand at your sides, raise your hands to shoulder level and control back down again. Be careful not to shrug the shoulders as this does not work the deltoids or the rotator cuff. This exercise can also be adjusted to a front raise by lifting the weights straight in front rather than to the sides. This variation changes the weight load on the muscles, and while all the same muscles are involved, they work in a slightly different manner.
