T Nation

Box Squat

Do any of you/has anyone seen people performing box squats in which athletes rock off of the heels when seated on the box and exploding up on the toes when performing box squats? I have seen multiple doing them this way and they said they were instructed to do so.
I have never been taught to ever have your feet leave the floor in any aspect as it is any easy way to turn a squat into a good morning type movement or lead to injury.
Am I correct that the feet should be at all times gripping the floor while the move is performed?