Lie on a flat bench face down with one arm holding a dumbbell and the other hand on top of the bench folded so that you can rest your head on it.
Bend the elbows of the arm holding the dumbbell so that it creates a 90-degree angle between the upper arm and the forearm.
Now raise the upper arm so that the forearm is perpendicular to the floor and the upper arm is perpendicular to your torso. Tip: The upper arm should be parallel to the floor and also creating a 90-degree angle with your torso. This will be your starting position.
As you breathe out, externally rotate your forearm so that the dumbbell is lifted forward as you maintain the 90 degree angle bend between the upper arms and the forearm. You will continue this external rotation until the forearm is parallel to the floor. At this point you will hold the contraction for a second.
As you breathe in, slowly go back to the starting position.