Day no. 22 – Stability Ball Leg Curls. Lie on your back with your heels on top of a stability ball. Press your heels into the ball and lift your body off the floor to form a straight bridge with your body. Powerfully contract your butt and hamstrings, and curl the ball inwards until your knees form a right angle. Hold the contraction for a few seconds, roll the ball back out to the full bridge, and repeat. Complete 2 sets of 30 reps.