As someone mentioned above, your thumb and its base do all the work at the bows grip. Nothing from your life line to the pinky side of your hand should be on the back side of the grip.Theknuckles of your four fingers on your grip hand should be at a 45 degree angle to the bow itsself. Hold your hand out like your holding the bow with your knuckles at a 90 degree angle (straight up and down) and then rotate them to the proper 45 degrees and watch as your fore arm rotates out of the strings way. This hand position should be obtained before you ever draw the bow, not afterwards. My wife was also having this problem with the correct draw length and this solved it.
So if your draw length is correct and hand position is correct ther is no need for an arm guard. Arm guards were made for heavy clothing.