Who know how to implement the 2D bone animation showed in the game?

I wonder how do they implement the bone animation in the flash game http://www.foddy.net/athletics.swf Do you know any study materials which I can start to learn 2D bone system from? I just implemented a avatar system by compose multiple bitmaps in each frame(similar with maple 开发者_JS百科story), but some guys tell me that, a bone system can save more art resources, so I want to learn some thing about that.

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Try Box2D. It's a 2D physics engine that does what you want.

Here's a link: http://www.box2d.org/

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Have a look at at Dragonbones to do bone animations with your images. It's easy, FREE and can export to multiple file formats. You import your images into a library, create an armiture of bones and then pose the armiture in one or more animations.