"Lat pulldowns have changed over the years. In the past, the lat pulldown was performed by pulling the bar behind the head to the shoulders. However, it was found that biomechanically, this was not a natural movement and could cause injury to the shoulder area as well as excess strain on the upper spine and neck. You can gain the same benefits by performing the lat pulldown in front of the head."
http://www.trinewbies.com/WeightTrai...atpulldown.asp