First, check with your doctor to make sure that you are cleared for that kind of activity when the cast comes off. You could ask for a few appointments with a physical therapist to teach you exercises to do to get your muscles back in shape. If your cast does not go above your elbow, the main muscles you need to worry about are the forearm ones that flex and extend your wrist. You can work on those by resting the forearm on a table with your hand over the edge, then using a small weight and lifting it with the motion at your wrist. Flip your arm over to get both sides. You will probably also have to work on stretching it. Be gentle but firm, don't bounce the stretch, and hold it for at least 15-20 seconds. But definitely talk to your doctor about your plans to make sure that it won't cause any harm. You could also look into getting a doctor's note to waive that part of the test until you are back to normal. Good luck!