Works: Abs, glutes, hamstrings
With the back of your head, arms, and shoulders on thefloor, put the soles of both feet on a Swiss ball. Contract your glutes to raise your pelvis off the floor. From this position, take your left leg, knee slightly bent, off the ball.Pause, return your foot to the ball, and switch to the rightleg. Do three sets of 20 on each leg.Extra Credit: With one leg on the ball, roll the ball awayfrom your glutes and back again. Switch to the opposite leg."It's like you're doing a one-legged hamstring curl, lifting allof your body weight, says Keller.