LET’S START WITH THE OBVIOUS: WHY SO DEEP? – Well, there’s no rule that you have to stop Push-Ups with your chest level with your hands, except that the floor kinda gets in the way! (Ignoring the fact that most folk don’t even go that low anyway).
However, if we train properly, ie, choose the exercises to fit our bodies and a particular stimulus we want, we might decide that we want to use more range of motion than the floor normally allows us. There is NO BIOMECHANICAL RULE THAT WE SHOULD STOP LEVEL WITH THE FLOOR.
Thus, we use Deep Push-Ups, and set ourselves up so that we can go as low as we need.
MORE RANGE OF MOTION = BETTER MUSCLE ACTIVATION – this is a very good (general) principle in strength training and bodybuilding. All things being equal, want better muscle activation? >>>Use more range of motion. This is how you do it without the floor getting in the way (don’t you just hate that!).
HARDER THAN PUSH-UPS – If you’re strong enough to go all the way to the floor, then going deeper is a logical progression. As long as your feet are elevated level with your hands, then deep Push-Ups take more strength than regular Push-Ups and therefore are very much a progression. Don’t be fooled by my use of these with elevated hands for beginners–yes it helps to teach them like this, but feet level with hands and we are talking serious strength.
GREAT FOR LEARNING PUSH-UPS – If you get your set-up right, you’re likely better using a Deep Push-Up because they give you a better sense of how to use your muscles well in the Push-Up. Plus, this way, when you attempt real Push-Ups on the floor, you may just be pleasantly surprised how it feels, all most like you’ve trained in a greater range of motion than you need on the real thing.
AND OF COURSE, IT’S STILL A PUSH-UP – so the usual benefits apply, you’re strengthening through your chest and triceps (and the front of your shoulders–particularly with narrowing hand placement).