Vectors. Consider every muscle strand to be a vector. It is very much similar to this, after all. Each muscle has a direction and a weight.
For my purposes, the "weight" of each vector is it's resting force towards movement.
Imagine a person holding your arms at your side as you try to lift them. You are let go after 30 seconds and told to relax your arms, but they continue to rise without your input. [It's almost like an after image of the force you applied earlier.] Your resting force towards movement would be what your muscles want to do just from your normal actions in a day.
So, now consider each of these muscle strands as a quantified vector. When you are at rest, the sum of all these vectors would be zero for a body in balance.
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Dammit, I had my next thought planned, and in the time it took to put up a page break, it was gone.
My train of thought has derailed.
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It does get more complicated, and gravity (more specifically, how your muscles deal with it) is most definitely a huge factor.
But I got the paradigm out there, before it was lost to the next bombardment of thoughts, a win in itself.