I can not believe I buy this , it be completely useless for I . I go to the gym regularly , so I know what to expect when they claim it `` dramatically '' increase muscle activity . it do not make my muscle feel `` on fire '' , and I be by no mean a hard core weight lifter . I do follow the video instruction , but it be no better . overall , the product be make of what feel to be a cheap plastic material and the movement of the weight inside be not smooth as you would expect . I try use it a couple time but give up ... it be now sit somewhere in my garage . I should have buy a normal pair of dumbbell .