be very careful . I have work out for 25 year . I have never have a injury. , , until use the shake weight . I get it as a impulse because it look different and I be curious . I like it at first . the same day that I have use it for the first time , I feel neck twinge . now I have severe neck pain that have go on for 4 day . I cant turn my head . I cant drive or sleep at night . maybe my fault , as I do the workout and do not keep my foot flat on the ground in a stable way . I walk around the room while do it . bad idea . it create a whiplash effect .