This is a great way to make your pushups harder and get the core and abs working more.
Start in a normal pushup position. Take your right leg underneath your body and lower yourself to ground to do your pushup. As you push yourself back to the top, bring your leg back to the starting position. Repeat on the other side, alternating back and forth. (Remember to keep your leg off the ground once you extend it underneath your body... don't let it rest on the ground!)
