. An obvious physical impossibility. What would hold the body up?
The whole thing is to make the individual strain to hold himself up else he will suffocate. Crucifixion was not designed to do anything more than to try to get the maximum suffering by suffocation. The nails just ensured that the person didn't get away easily, and also in rather painful fashion. And the feet were nailed in place as well.
The feet were obviously nailed after he was placed on the post, else there would be no suffocation.