Get a Flat Belly in 4 Weeks Diagonal pike legs Lie faceup with legs together and extended straight out. Rotate hips slightly to left so that legs are on a left diagonal. Place left hand lightly on back of head and extend right arm straight out to side, palm facedown (A). Crunch up and lift legs, keeping legs on the diagonal (B). Slowly lower back to floor. Do 30 to 40 reps; repeat on other side. Make sure abs are engaged from the start.