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Knee-In Crunch with Medicine Ball and RotationMain Muscle: Abdominals Mechanics Type: Compound Lie flat down on the floor with the feet flat and knees bent. Position a medicine ball (or weight) above the head at eye level. Keep your arms straight and over the head. With the feet about 8-12 inches apart, pull in the knees until the knees are directly over the hips. Your lower legs should be horizontal to the floor and the knees should bend at a 90-degree angle. Slowly rotate your knees to one side while trying to keep both hips on the floor. After rotating to both the right and left sides, without pausing in the middle, lower to starting position until your feet almost touch the floor. Repeat the same exercise, with rotation. Do not rest the feet until you have finished an entire set. Focus on proper breathing and do not hold your breath.