Ah, the humble pushup. It doesn't receive nearly as much credit as it deserves when it comes to muscle building. Shall I sing its praises?
Relative Upper Body Strength
The most obvious of the benefits. Pushups, primarily, challenge the triceps, delts, pecs (sort of, depending on the angle of your feet). I would even argue that your biceps receive a piece of the action as antagonist muscles (aka: slowing down the decent and counteracting the triceps). It's relative strength since the ability to perform a repetition(s) is based on your bodyweight as opposed to an external load, such as a dumbbell. Possession of a high relative strength is a key quality not only for athletes, but for anyone. It's a fantastic thing to be able to control and move your own bodyweight.
Core Strength
Pushups are essentially moving planks. When I say "core" I mean all of it, front and back: rectus abdominus (the coveted "six pack"), obliques, transverse abdominus, as well as the glutes, erector spinae, multifidi... I could go on. Pushups engage all the muscles surrounding your spine and pelvis to maintain a neutral position of both (or, ahem, they should).
Pushups connect the core and the upper body into one solid movement. Oftentimes trainees have pretty decent upper body strength (thanks to our collective love-affair with benching, curls, and tricep push downs) but fairly poor core strength. Having the former without the latter is like trying to drive a car on flat tires: you can have the most powerful engine in the world, but if you can't transfer that power to the ground, that car stays in the driveway.
Poor pushup form includes saggy hips or a banana-shaped back (here's our car with flat tires).
Classic Banana-Back from greatist.com
Often in this situation, a person's hips will drop more quickly than the chest during the descent in a pushup. This is indicative of both a lack of strength and spine and pelvic stability. Now the lower back is in a compromised position and is ripe, like a banana left in a paper bag for too long, for irritation or injury. Core strength and spinal stability are essential for both athletic success and injury prevention. You would do well to improve both!
Remember! Practice doesn't make perfect, it makes permanent. Therefore, training perfect pushup form is the antidote to poor pushup form.
Shoulder Stability and Health
Two major players are involved in shoulder health: the rotator cuff and the scapula (shoulder blade). The rotator cuff keeps the humerus (the upper arm bone) in its proper place in the socket, like a suction cup. The pushup is one of the best exercises to challenge and strength the RC, both statically and dynamically. The shoulder blades should glide along the rib cage (and not stuck up by our ears) and pushups encourage and reinforce this movement- when done properly of course. Muscles surrounding the scaps (mid- and lower-traps, serratus, ect.) act as both movers and stabilizers of the scapulae to corral them into the correct positions throughout the movement. Pushups require these bad boys to activate too in the form of antagonists to the "pushing" muscles in the front (mentioned above).
Ok, ok, enough praising. Assuming that you have mastered the pushup, let's get to the challenge. Ready to blow up your upper body?
I stole this pushup ladder from Dan John, as I do most thing since he's pretty much Yoda of the strength world.
from strengthnet.com
The Ladder:
1 --> 10 --> 1 pushups. Start at 1 rep, stand up. Two reps, stand up. Three reps, stand up... and so on up to 10 reps. Then, repeat 9 reps, stand up. 8 reps, stand up... back down to 1 rep.
Note that your "rest" period is the time it takes to stand up and get back down into the pushup position. The other rest position, is holding the plank at the top off the pushup. Yeah...
The Challenge:
Perform the full ladder, up to 10 reps, in under 5 minutes.
Good. Luck. And enjoy the pump!
If you can't perform it in under 5 minutes, well, keep practicing. If you can't finish the ladder, go as far as you can with good form. My advice to complete the challenge? Get stronger.
Below is a video of me doing the challenge, if you want to watch me struggle, feel free to watch the whole thing. I start to sag a bit towards the end and I was hurtin'. I should note that, in my defense, that I naturally have a more pronounced kyphosis (upper back rounding) and my shirt only accents it so it looks like I'm reaching my neck forward. Trust me, that's as far back as my neck will go, it's a perpetual struggle.