I just wanted to note ... that one does not have to be big or strong to swing a heavy hammer..... It is all about the technique. Without proper technique, swinging a four (plus ) pound hammer CAN damage you, especially if you do not have the size/muscle to over compensate the strain being put on you! ....... Case in point about proper technique vs. hammer weight ability.
I am 14 ( though only just ) years old, exactly 5 foot tall , i weigh 90 lbs and i am not overly strong. I can swing four or five pound hammers all day every day very efficiently, which proves that, doing it right lets anyone do it. Yes, it may sound pretentious of me to imply i am doing it the correct way... But surely i have not proven to you that it is possible i am considering the facts i have just told you.
The way i have found to be most effective is Brian Brazeal's method. Which i employ and which helps me forge things extreemly efficiently.
sorry about my teeny rant there just wanted to pitch in a little info on my thoughts about hammer stuff as a little addition to Jason's post
alec