I have to agree with Jedi, TL. A tip I picked up was to use action lines.
I did a quickie with them in aqua to help you out. Basically they dictate a predetermined shape for the body, and the more curvaceous, the more action you will get out of your pose and the less 'stiffness'. You can still make the picture look like exertion is happening with the lines you use over top to show taughtness, but stiffness is a bad thing in fighting and general life, unless of course you're getting some nookie.
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this is a proper tutorial to help as well.