I know, five-minute abs sounds like the latest infomercial promise, but if you put in that much time per day, I guarantee you'll get a tight, toned tummy fast. Especially if you spend it doing bicycle crunches. If you're going to do only one ab move for the rest of your life, make it this one.

The bicycle is the most efficient ab sculptor because you're continually moving, so muscles never rest, explains Peter Francis, Ph.D., a professor emeritus of exercise science at San Diego State University who compared 13 common ab moves. Plus, it works both abs and obliques simultaneously.

Try it: Lie faceup, hands behind you head, elbows out. Lift shoulder blades and left leg about 6 inches off the floor as you bring right knee toward chest and twist torso toward right knee so left elbow barely touches it. Switch sides, lowering right leg to 6 inches off the floor and bringing left knee in while twisting torso left for one rep.

Make it harder: Hold a 3- to 6-pound medicine ball at chest. Continue alternating in a pedaling motion. Try two sets of 12 reps. See a video of the move here.