Dumbbell side bends work the muscles of the trunk, particularly the deep spine muscles, explains Len Kravitz, PhD, Program Coordinator of Exercise Science and researcher at the University of New Mexico. Although they can help contribute to a stronger core, reduce the risk of back pain and improve functional movements such as a serving a tennis ball or carrying a heavy object on one side of the body, dumbbell side bends won't create a slim and toned midsection -- only diet and cardiovascular exercise can help you do that. If you do include dumbbell side bends in your routine, do them at least two times per week on nonconsecutive days.
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Why Not Everyday?
The main muscle activated during dumbbell side bends is the quadratus lumborum, which is located in the lower back on either side of the spine. This muscle, just like any muscle, needs rest between workouts to recover and grow stronger. When you add dumbbells to the side bend, you overload the muscle more and break down the fibers. Strength and function improve when these fibers are repaired by the body. The muscles need 48 to 72 hours to do this. So, at most, you can work them every other day.