It’s a weight that you shake and even though it seems a daft idea, the Shake Weight is actually starting to prove quite popular. It looks like a single dumbbell and the idea is you stand and hold it in certain positions and shake it as hard as you can. These little vibrating moves tone the muscles in your arms, chest and shoulders (depending on which position you hold it in).
Laugh at it or not, it actually does work; instructors have known for a long time that pulsing your muscles quickly produces muscle fatigue and therefore the muscle strength increases to cope with the work. But be aware the first time you do a workout, your muscles will complain the day after — at least that’s what Ellen Degeneres found. She started out by laughing at the Shake Weight, shook it around a bit and the next day complained of aching muscles, would you believe? So it will work if you use it.
Contrary to the belief of some people, it does not shake on its own! I think certain people are mixing it up with the vibro machines that are advertised that do the vibrating for you; but you have to vibrate the Shake Weight all by yourself to get any action 
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The Shake Weight only weighs 2.5 Lb so can easily be stored and carried, and the marketers reckon that shaking it around for six minutes a day will produce measurable results. Well it might, but you really have to do other exercise as well — I’m sure you know that!
It will give you some toning in chest, arms and shoulders. Hold it in front of you in both hands and shake to target your chest muscles and biceps; hold it above your head to target shoulders and take your arm behind you, bend your elbow and shake it to target your triceps. I don’t think it is any way as good as a press up for toning those areas, but using the Shake Weight at least means you don’t have to get down on the floor!
I like it; it’s not expensive, it’s easy to use, you’re unlikely to injure yourself and is better than not exercising at all.
However for less than $40 you might consider taking a look at a product that will not only tone your under arms but your whole body as well it’s called The Truth About Abs and will provide you with all you need to ignite your fat burning and weight loss goals.