Add Yahoo as a preferred source to see more of our stories on Google. Your favorite ab-building machine at the gym may be causing you more harm than good. That's right—we're talking about the torso ...

According to experts, it takes just four core workouts to absolutely rip your top half to shreds, so you can get stronger.

What it's supposed to do: Train the quadriceps. What it actually does: It strengthens a motion your legs aren't actually designed to do, and can put undue strain on the ligaments and tendons ...