If you're interested in working your butt a little more this week, this exercise will do it. Like traditional lunges, this move engages the glutes, quads and hamstrings. Unlike traditional lunges, however, this move not only challenges your balance, it engages the upper body, making this a whole body exercise.
I have to warn you, this is a tough exercise and you'll wobble all over the place the first time around. I recommend standing next to a rail or wall to hold onto as you practice this move. Try to keep your knee in line with the toe as you lunge and engage the abs to protect your back.
Do it right: Place the left foot/shin on the ball and hold a light weight in your right hand. Bend the right knee into a lunge as you roll the ball out with your left leg until it's straight and the front knee is bent to about 90 degrees (knee behind the toe). At the same time, reach the right hand out, keeping the abs engaged. Squeeze the right leg to roll the ball back to starting position. Repeat for 10-16 reps and switch sides.
For more, check out this Lower Body Opposing Muscle Groups Workout.