Here are three versions of the plank, a move that works the lower back, chest, and shoulders in addition to the rectus abdominis.
Start in a push-up position, but keep your hands directly under your shoulders instead of outside your chest. Legs are extended straight behind you, with feet together. You are balancing on your palms and the balls of your feet. Hold this position for as long as you can, working your way up to 1 minute. It's a lot harder than it sounds!
x Be sure to focus on keeping your spine neutral (straight).
x Keep your eyes focused on the ground in front of you.
x Don't arch your back. Imagine that you are pulling your belly button up toward the ceiling.
x Until you've mastered them, do these exercises in front of a mirror to check your form.