How can your body know where an arm is without looking?

An arm can reach for a sleeve without your eyes following every part of the movement. Sensors in muscles, tendons and joints provide the nervous system with information about the body itself. This sense is called proprioception. Vision can help, but it is not the only source of position information.

One sensor is the muscle spindle, a small structure embedded within a skeletal muscle. Its specialized muscle fibers run alongside the ordinary force-producing fibers. This parallel arrangement matters: when the muscle lengthens, the spindle can lengthen with it. The sensor is placed where the change it helps detect actually occurs.

Sensory nerve endings wrap around the spindle's central region. Stretch changes their activity, and the signals carry information about muscle length and how quickly that length is changing toward the spinal cord and brain. The nervous system can also adjust spindle sensitivity, so it is a responsive living sensor rather than a fixed ruler attached to a muscle.

The brain combines many such signals with information from other sensors and its own movement commands. One spindle does not report an entire elbow angle by itself. The invented drawing length of 1.2 simply lengthens the ordinary fibers and their parallel spindle together; it predicts no firing rate. The useful connection is from a local stretch inside a muscle to coordinated movement of a whole arm.