Sit-ups shouldn't be your go-to ab exercise — and not just because they're boring. If you're only doing sit-ups, you're only training one part of your abs, your rectus abdominis or "the six-pack muscles" that run from the bottom of your chest to your pubic bone. That leaves the transverse abdominis, the deep layer of muscle that wraps around your torso, and the obliques, muscles that run down your sides, untouched. Neglecting these muscles leads to dysfunction and even injury, not to mention that you'll never look quite as good as you could with your shirt off.
But the worst part of sit-ups isn't what they're missing, it's what they're acting on — namely, the spine. Imagine that you're standing up, and throwing your torso forward to touch your toes, over and over. It's easy to see the havoc that wreaks on the lower back, and it's similar to what's happening with most sit-ups. While performing a slow and controlled sit-up can work fine to build one section of your abs, there are far superior alternatives that can target the entire core, while strengthening other parts of your body and even burning fat.