I get this from a groupon deal . at first I do some research online and like many other , I think it look ... well ... dirty . but I get it anyways . the first time use it , it as my only workout of the day . get a little burn here and there , not too tiring . but a couple of day later , I use it after a 2 mile run , I could really feel the difference . I really feel the burn in my arm and shoulder . however , it be only a good tool . if you want to look buff like the guy in the infomercial from use shake weight , unlikely . to gain muscle mass , you need wide range of body movement with heaier weight . Shake weight be good for burn off the energy that be leave in you arm . but if use with weight training , cardio , and good diet , you will feel the difference .