Depending on how quickly you decide to do this exercise, it can be a great cardio exercise. It's a great way to target your obliques.
- Stand with feet slightly more than shoulder-width apart, hands behind head, elbows out to sides.
- Lift left knee to waist level or higher in front of you, as you rotate torso to left, touching right elbow to knee; return to start position.
- Immediately switch sides and repeat.
- Continue alternating from side to side, touching opposite knee to elbow.