We don't do very much thinking about it because we have an automatic feedback system, an afferent nerve chain going back to the brain. So we have a message going out to the arm muscles, and a message coming back, a feedback system, We have a two way communication. There are two different sets of nerves to do these functions, the efferent and afferent. What you have in the muscles to generate the feedback signals are proprioceptors. The ones we primarily use in the muscles are called spindle cells.
Muscle fibers tend to be long spaghetti like masses that contract.
Spindle cells are also like long thin cells that are mixed in with the muscle cells. When the muscle gets shorter, the spindle cells shorten up and send a message back to the brain saying that the muscle is getting shorter. Then again there is a communication going on here. When we have muscle aches and pains, limited range of motion is because this communication is not happening properly. This is another level of communication .