ONE-LEG HAMSTRING CURLS ON BODY BALL
Lie with your legs outstretched and your feet and calves on a Swiss ball. Raise your left leg straight into the air so it's perpendicular to the floor. Press your right heel into the ball, bend your knee, and roll the ball toward your butt until your right foot is flat on the ball. Pause and return to the start. Don't allow your butt to rest on the floor between repetitions. Switch sides and repeat.