What causes these natural body positions (see details)?

Start by opening your hand all the way. Now slowly close it, stopping briefly at natural stopping points. I count three such positions. For the last one, the hand is almost but not quite closed. I find a similar situation for any arm or leg movement and also for opening and closing my mouth.

What causes the muscles to be able to relax in these positions? I would guess that we move to these positions when we want to hold a pose, for example a batter in baseball waiting for the next pitch.