You aren't likely to find a fully built model that is *also* animatable in all possible joints. (You may find a non-animatable version, but it will be unbelievably costly, like hundreds of dollars!) What you can do yourself is focus closely on one or two muscles at a time, with just enough detail to show how this pair of antagonists works. The regular Poser skeleton figure moves nicely, and you can make most muscles from distorted Ball props. Some will need to be shaped in an external program like Amorphium. Just as an example, here's an animation I made quickly from a still image I had put together earlier. It needs improvement, but it illustrates that the task is possible IF you focus on just one joint at a time.

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