It looked like everything was ready and everyone was in place with their new identities, but this was all about the hollow situation created like a game, not experiment. Making everyone literally realize their identity relied on an important process of asserting authority. At 2:30am when everyone was supposed to have a nice sleep, all prisoners were rudely awakened by blasting whistles for "counts" we have seen in many movies. Those counts usuallly serve the purpose in real life of making sure that every prisoner is still in custody, but here they were used to familiarize the prisoners with their numbers. More importantly, it allowed the guards to excise control over the prisoners. Undoubtedly, the prisoner were not completely into their roles and did not take the counts seriously. They were still getting tough to assert their independence. This was the beginning of a series of direct confrontations between them.
Push-ups were a common form of physical punishment imposed to punish infractions of the rules or displays of improper attitudes toward the guard or institution. It's noteworthy that one of the guards stepped on the prisoner's back while they did push-ups or made prisoners sit or step on the backs of fellow prisoners during their push-ups.
