A grenade is the same thing you might see football players do before a game or during practice. Essentially it is a updown, you stand upright and keep your feet moving, then on command fall to your face in the down pushup position and after your chest has hit the ground you get back up as fast as possible and continue chopping your feet, therefore restarting the process.

What makes/made this hard is that you would usually have a "rubber duckie", fake m-16 in your hands while doing the "grenade/grenades"..

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