Doing a lunge on a BOSU works your torso as well as your legs.
To begin, your feet should be shoulder width apart, torso erect with your arms hanging straight at your sides, palms facing in. Place your right foot in front of you on the center of the BOSU, then take a slow, controlled step backward with your left foot.
Lower the hips so that your right thigh is parallel to the floor. At this point, your right knee should be positioned directly over your ankle, and your right foot should be pointing straight ahead. Your left knee should be bent at a 90-degree angle and pointing toward the floor; your left heel should be lifted.
When your left knee is almost touching the floor, pause for one breath, then push up with your left foot and keep your right foot firm on the BOSU. Continue to push through with your left leg until it is high in front of you, bent, while your right leg is straight. Quickly lower your left leg back down to the ground into lunge position. This completes one rep.
Do two sets of 10 reps on each side.