You need your erector spinae to be strong, latissimus dorsae, and qaudraus lumborums for front and back, a good work out for back muscles (and this is hard to do without the right leg gripping support, but it's the opposite of a cruch, it's where you lay on an edge of a cushioned massage table or other soft flat surface, and brace your lower body, hang off the edge til your head, shoulder, abs, and gripping the hips to the edge, face down bend over the edge going towards the floor, and come up. Repeating this until you feel the burn. Hard to explain, but can you imagine it? And for cruches, laying on floor, cross one leg over the other, and bring your legs up, have your arms flat bracing the floor by your sides and do this both ways with legs swapping to the other. When you do a simple crunch, sitting up kind, you're aiming for the upper abs, the lower abs get work with the leg lifts.