Killer Core

Everybody wants a strong core. You'll never get bored as you work your way toward strong foundation
Here are three versions of the plank, a move that works the lower back, chest, and shoulders in addition to the rectus abdominis.
Basic Plank (beginner) 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.
Plank Twists (intermediate) Start in plank position. Maintaining this position, exhale and rotate your torso by bringing your right knee in toward your left armpit. Return to the starting position, then repeat, bringing the left knee toward the right armpit. Plank twists work not only the rectus abdominis but also the internal obliques.
Extended Plank (advanced)Start in a basic plank position, but instead of placing your hands under your shoulders, bring your hands together and place them on the floor in front of you (about 3 inches in front of your head). This variation places much greater emphasis and intensity on the rectus abdominis because you're not able to use your arms as pillars to support your body weight.