The former wrestler has filmed a documentary for U.S. network HBO called Rock & A Hard Place which takes a look inside a program run by the Miami-Dade County Corrections and Rehabilitation Department in Florida.

The bootcamp gives youngsters the chance to cut their prison sentences by completing a course of rigorous military-style training, and Johnson documented their graduation day on his Instagram feed, writing, "I told these young men six months ago when they first arrived that the life they once knew, was over. That they would get broken down to their core - physically and mentally in ways they never imagined - but I also told them they would never break. Big difference.

"They never broke and today they graduate and become free men. Better men. Very proud of this program, its staff and the impact it has on our prison system and more importantly, caring about young lives that matter. Congratulations men. Just like we talked about... Be a leader, be disciplined and earn that respect. It's all up to you."