How do we know how to move our muscles

While not claiming to be an expert, I would say we learn. A newborn doesn't know how to make muscles do what he wants, but as he flails about, his brain gets constant feedback. "When I do this, that happens". I remember watching my infant firstborn. We had him on a blanket with one of those activity centers over him. As he moved about, his hand hit one of the toys hanging down on the side. He froze and stared at it. Then he started moving just that hand while staring at the toy. Eventually he hit it again. I could see the connections being made in real time.