This abdominal exercise will engage those hard-to-target lower abdominals, and burn away that last bit of belly fat that’s keeping you from your six pack. On the function side, you’ll work the five muscles that make up the hip flexor: the psoas major, iliacus, rectus femoris, pectineus, and sartorius. Don’t forget to flex your abs and keep those legs straight throughout. Lay on your back, feet six inches off the ground. Tuck your hands under your butt. Keeping your legs stiff as a board, begin moving your feet up and down without touching the ground.