That's my first thought too. Your lower leg is your FOUNDATION! You have no real balance or security without a good lower leg. Your heel should be directly under your hip. Your leg should have equal contact through your thigh/knee/calf (not more pressure in one of those three areas). Make sure the INSIDE of your leg is what has contact with your horse's side, not the back. Start with a good leg first, and other parts of your position/balance will come.