I cannot help but wonder why the heel bone didn't break -- forces generated by the moving body must've been significant,
@jedakk did
some calculations years ago.
Unless the criminal was stretched on the cross in such a way that immobilization was almost complete.
This is what I wrote about this subject thirteen years ago:
I emailed Joe Zias, the archeologist who came up with the most recent version of the nails-through-the-heels scenario, and asked him how he arrived at that. His reply to me was that a lot of people were crucified about that time in Judea, and maybe this was one of them. Seems like pretty slim logic to me, but many people have embraced this theory as the absolute truth when it's nothing but a "rectal extraction" in reality.
First of all, the physics of this scenario bother me. With the feet nailed flat against the stipes, there is only a vertical component pressing downward on the shanks of the nails, but with the feet nailed through the heelbones to the sides of the cross, there is a large torsional, or twisting, force on the nail. When I calculate an estimate of that based on the weight of an average size man, it looks to be enough to actually twist a wrought-iron nail of the size that was found. But it would certainly crack or shatter the heelbone before that happened. Obviously, the heelbone is intact, so my belief is that the heels could never have been nailed to the sides of the cross as theorized by Zias. Perhaps this was a victim who was crucified upside down? Seems more likely to me, but there are other scenarios.
Second, there is the business about wooden plaques being used under the nailheads so that they would not pull through the wrists or feet. That seems impossible simply based on a look at the bone structure the large heads of those Roman nails would have had to be pulled through - especially so if we are talking about pulling a one-inch diameter nailhead through a person's heel bone. If wooden plaques were ever actually used, perhaps it was to keep the rough edges of the nail heads from cutting into a wrist or foot and opening a vein, causing the victim to bleed to death. This would be even more likely if the nails bent under the victim's weight, forcing the lower edge of the nail head against the flesh.
So we've got one more calcaneus bone with a nail through it. Did they nail these men's heels to a cross or something else? Were they alive when they were hung up on display? If you were going to hang a man upside down on a stake, this would be a way that might work, and we know the Romans did that on occasion.
As far as a common head-up crucifixion, I think it much more likely that nails were driven through the top of each foot, two nails, one for each foot. Nails placed up high on the foot have plenty of bone to bear against, and even though the feet would be behind the center of mass of the body, making it tend to arch outward when rising, nailing the heels to the sides of the cross would make raising one's self so difficult and tiring that a victim would exhaust himself quickly.