Lie flat on the floor with your hands behind your ears (a). Raise your legs, head, shoulders, and back off the floor, then use your abs to drive your left elbow toward your right knee, keeping your left leg straight (b). Release, then drive your right elbow toward your left knee, keeping your right leg straight ©. Continue alternating as fast as you can for 20 seconds, then rest for 10. Repeat for four minutes.