Unbalanced muscle training not only occurs in the body of fitness novice, but even experienced drivers cannot escape this problem. The muscles of the left and right arms or legs are unbalanced. When training on one side, it is necessary to strengthen the weak link, and dumbbells are sufficient for fitness practitioners. It can not only make up for the exercise, but also maintain the balance of the muscles on both sides of the thigh. http://www.luchfitness.com/free-weight/free-weight-accessories/