What is the cause of needing to use an arm guard when shooting newer compound bows?A friend of mine gets whacked by the string when he shoots,thus leaving red welts.Is it mostly poor form or is it a poorly fit bow.Or is needing arm guards more common than i think.

Usually, poor form, too high poundage bow, too long draw or combo of all.

Tell your "friend", to hold out his hand/arm to his side in a natural relaxed position. Thumb not pointing up or down but dead natural. This is how the bow should fit in the hand. If he has to dig his heel of his hand, string can hit. If he has his elbow turned in, poor form.

Problem with too much wieght is usually a shooter has to dig the heel of his hand in to control the bow.

A proshop should be able to get him on track in no time.

It could be either, but it is most likely caused by using a bow with a draw length that is too long. He's probably being hit by the string because his bow arm is fully extended and locked causing the lower forearm to pivot into the string's path.

X2 well said, I can't add to that, excellent advice Mark.

ditto