What are things I can do using my fractured arm muscles to keep its strength up?

Nothing. Please try not to manipulate the arm where it is fractured. Continuous motion of the muscles on a fracture site, irrespective of anatomic location, puts stress and strain on the fractured ends trying to coalesce. It iwll take longer to heal and potentially cause delayed-union. Give it the time it needs to heal, then when your doctor clears you for activity, you can resume muscle strengthening.