Oh wow, it seems I am behind on the old nuclear weapons front, thanks for pointing it out 
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Whot? I am perfectly able to clench my...I tried but it didn't seem to work...
In my experience it is not possible for a man to clench his butt cheeks together to hold in a fart, let alone tense everything up all at once. 
If you mean contract all your voluntary (skeletal) muscles voluntarily, the answer is no. Even an anatomist, that could identify every skeletal muscle in the body, wouldn't be able to contract them all, even one at a time.Thanks SteveFish  great answer
Further, during normal muscle contraction all of the muscle fibers in a single muscle do not contract all at once (NOTE- skeletal muscle cells are often called fibers because of their unusual appearance). When you make a controlled movement, such as pick up a delicate and light weight object, you don't crush it and fling it up because only a few muscle fibers in the appropriate muscles were contracted. When you hold a heavy weight up for an extended period, a larger portion of muscle fibers are used, but never all because the fibers tire quickly and they have to, sort of, take turns.
Ah, interesting, thanks for that Bill.D.Katt. As a firm believer in empirical study, I'll just go outside into the rain and...
I heard that if you are tasered, all your voluntary muscles clench and unclench. I don't think you would want to try this though.
Someone threw a snowball at me the other day. It was that rotten little heifer across the street.
Similar thing happened to me a heifer dozen times!