It seems like you are looking for a more complicated answer...I think just a lot of leg is the best answer you can get right now. Also, why do you think you need a lot more muscle in your arms? If it's to help you stay on when he swerves, then that's a big no no, don't use the reins as a way to anchor you to the horse. If it's to be able to redirect your horse back to the outside with the reins, you shouldn't need much strength for that because the horse should respond to very light pressure. I think you need a decent amount of strength to keep rein contact, but other than that I'm not sure.