YO, WHERE' MY ARMS AT?!?

Proprioception is "the process by which the body can vary muscle contraction in immediate response to incoming information regarding external forces," by utilizing stretch receptors in the muscles to keep track of the joint position in the body "
Some kid from Bryn Mawr wrote a really interesting paper on this. it also explains why a legless (as in actually legless, not drunk) Ronald Reagan says "Where's the rest of me?" in King's Row.