We tend to think of muscles as the individual, isolated structures that we see in our anatomy books - the biceps, the hamstrings, the psoas (or to be super geeky, *psoai* if we're talking about 2 of them), etc. But did you know that this is NOT how your brain understands muscles?
Your brain doesn't actually know what a "psoas" or a "biceps" is. These are arbitrary names that we give to our parts so that we can learn and communicate about them (which is a good thing!) But the brain doesn't think in terms of individual muscles - instead, it perceives and directs *motor units*, which are tiny subsets of individual muscles.
In this sense, we could really think about each muscle as actually being made up of thousands of smaller muscles, which are where movement truly happens in the body.