proprioception

Proprioception is your body's quiet sense of itself — knowing where your arms, legs, and other parts are and how they are moving, even with your eyes shut. Close your eyes and touch your nose, or walk up stairs in the dark without looking at your feet: you can do it because your body is constantly reporting the angle of each joint and the stretch of each muscle. It is the unseen sense that lets you type without watching your fingers and keeps you steady when you stand still.

This sense comes from tiny receptors buried inside your muscles, tendons, and joints. Special structures called muscle spindles sit among the muscle fibers and detect how much a muscle is stretched and how fast; other sensors in the tendons measure how hard the muscle is pulling. These detectors send a steady stream of signals up your nerves to the spinal cord and brain, which blend them into a moment-by-moment map of your body's shape and motion. Because this happens automatically, you rarely notice proprioception — until it fails, after which simple acts like walking or reaching can suddenly require careful, deliberate effort.