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.