The wrist is considered part of the hand at that time. Only the wrists can support a man's weight, through the middle of the "hand" it would tear out through the fingers ripping through the muscles and skin under the heavy weight. You should look up the medical aspecs of crucifixion. Here is a clip of an article from this page:
Medical Aspects of the Crucifixion of Jesus Christ by David Terasaka, M.D.
The procedure of crucifixion may be summarized as follows. The patibulum was put on the ground and the victim laid upon it. Nails, about 7 inches long and with a diameter of 1 cm ( roughly 3/8 of an inch) were driven in the wrists. The points would go into the vicinity of the median nerve, causing shocks of pain to radiate through the arms. It was possible to place the nails between the bones so that no fractures (or broken bones) occurred. Studies have shown that nails were probably driven through the small bones of the wrist, since nails in the palms of the hand would not support the weight of a body. In ancient terminology, the wrist was considered to be part of the hand. (Davis) Standing at the crucifixion sites would be upright posts, called stipes, standing about 7 feet high.(Edwards) In the center of the stipes was a crude seat, called a sedile or sedulum, which served a support for the victim. The patibulum was then lifted on to the stipes. The feet were then nailed to the stipes. To allow for this, the knees had to be bent and rotated laterally, being left in a very uncomfortable position. The titulus was hung above the victim's head.
The Romans made crucifixon the most painful death possible.... it was severe torture.
