Sitting on a chair, attach one end of a piece of elastic tubing to the net post, doorknob, or another stationary object. For pronation (shown here for the L arm) grasp the tubing in the left hand and wrap it around the hand several times. Rest your forearm on your thigh and position yourself so that there is tension on the band with your palm facing the floor. Start with the thumb pointed upward or back towards the net post. Without moving your body or upper arm, rotate the forearm so that the palm faces the ground in the end position.
Perform 2-3 sets of 15 repetitions of each exercise. If you have time constraints, make sure you perform the exercises with your dominant arm.