Well, they work completely different parts of the body. You could work your way up with pushups and supplement the rest with situps or squats or something similar. If you can't do a regular push up, do however many girl pushups you can do, and if you dont make it to 20 do the rest in sit ups or squats. Then just keep trying to do more and more each time until you can end up doing all of whatever you want. I have been switching between 10 pushups and 20 crunches for each post. Yay, pushups this time.