Adam Clarke on 1 Samuel 17:39

I cannot go with these - In ancient times it required considerable exercise and training to make a man expert in the use of such heavy armor; armor which in the present day scarcely a man is to be found who is able to carry; and so it must have been then, until that practice which arises from frequent use had made the proprietor perfect. I have not proved them says David: I am wholly unaccustomed to such armor and it would be an encumbrance to me.