Add a dynamic component to your hamstring workout by swapping the leg curl machine for a fitness ball, says Irv Rubenstein, Ph.D., exercise physiologist and founder of S.T.E.P.S., a fitness facility in Nashville, Tenn. Lie on your back with legs extended, lower leg on an exercise ball. Extend arms out to sides. Raise your hips up off the ground by pressing down on the ball with your lower legs and heels until your body forms a straight line: shoulders, hips and ankles should line up. Roll the ball towards you by bending your knees; pull your heels toward your butt. Allow your feet to roll up on to ball. Slowly lower to original position by straightening knees; repeat 12 to 15 times.