I’ve seen that video like a thousand times. What is the exact cause of that injury cause he didnt really lower himself before breaking his leg..
His tibia broke under the load because it was too great. I suspect he either had a pre-existing fracture or deformity of the tibia combined with the fact that he probably stepped JUST right on the leg to put enough sheer force on it that it broke. The compressive force capacity of the tibia is unreal. If the forces are lined up down the long axis of the tibia it can easily withstand well in excess of 685 lbs on one leg. As I said this is EXTREMELY rare.