The 90-degree cable external rotation strengthens and stabilizes the entire shoulder region, more specifically, the rotator cuff. The exercise also improves mobility of the shoulder joint.
Stand with your feet shoulder-width apart in front of an adjustable cable machine. The pulley should be set low. Holding the handle with your right hand, lift your right elbow so that it's in line with your right shoulder and bent at 90-degree angle, palm facing the floor.
Rotate your right shoulder by bringing the handle up while keeping your elbow stable and maintaining that 90-degree angle. Reverse the motion to return to the starting position.
Avoid lowering the weight early. Be sure to pause when your forearm is completely vertical before reversing the motion to the starting position.