Performed in a 4-count cadence, this exercise is best known from P90X's Ab Ripper. Start by lying flat with your arms extended above your head. Perform a full sit-up and touch your toes. As you return to the starting position, when your mid-back touches the ground, begin to raise your legs to a 45-degree angle and hold them there. Then, crunch up and touch your toes while keeping your legs in position.