Your push-ups hit so much of your upper body — shoulders, triceps, chest, back, and core — that you can't skip it. But concentrate hard on keeping good form. That's going to be arms straight and locked out, hands under shoulders, fingertips pointed forward, and your body in a plank position. Lower yourself down so your chest (not your legs) touches the ground, then shoot yourself back up, locking your elbows at the top. If you're having a tough time, it's fine to drop to your knees. But all the movement from the waist up should remain the same.