Works: Abs, glutes, quads,calves, hip flexors, balance
Pick up your right leg so it's just a few inches off the ground. Holding a medicine ball, dumb-bell, or other weighted object in both hands (beginners should start without weight and move up in five-pound increments), bend your left knee as low as you can, comfortably. Keep your left knee in line with your left ankle and your spine straight. Stand up, then repeat on right leg. "The weight throws off your natural center of gravity, says Keller, "so it's a more challenging exercise than a simple squat. Do three sets of 10 on each leg.