Fast Shredded Abs Race to a six-pack with the springer sit-up.

How to Do It

■ Lie on your back with arms at your sides and legs extended. Explosively sit up as far as you can while driving your left arm forward and your right knee to your chest. Reverse the motion but try not to let your torso or legs rest on the floor for even a moment. Repeat on the other side. The motion looks somewhat like you’re sprinting.

■ The instability created by moving each leg and arm alternately forces the core to work hard to balance you, so while the driving of the knees certainly activates the hip flexors, that won’t take away from the stimulus your abs get. (You’re also doing a full sit-up.) Need more proof? Look at the abs of any champion sprinter. They don’t suck