Another way you can add variety to your push-ups, is simply by changing the speed or the tempo of the movement. So you could take your push-up really slowly, and work a little bit more on stability and control. So if you are coming back into that push-up. Again whether you choose to do a knee push-up or full push-up, choose whatever push-up that you can do maintaining the best ranges of motion and the best core postural positioning to your spine. So if you are doing a slow push-up; you are gonna inhale as you lower your body down, and then exhale slowly pushing up. There's basically two major parts of a muscle contraction, the concentric phase which is where you are coming up and the eccentric phase also known as the negative phase where you are lowering down. What I'd like to do is focus on both phases. So for example I could do slow on the way down and then a big powerful push to come up. Inhale lower slow; exhale, powerful push to come up. You could also do the reverse. Where you are going, fast down and then slow pushing your body back up. Fast down and then slow pushing your body back up. Basically just adding variation. Because as your body gets used to one way of moving whether it's in direction or speed, it's gonna adapt to it. And what keepd your muscles always surprised and therefore keeps your body always changing is literally changing their neuromuscular pathways. So thats a great way to do that.