Intrafusal muscle fiber
File:MuscleSpindle.svg Intrafusal fibers are muscle fibers that comprise the muscle spindle. These fibers are walled off from the rest of the muscle by a collagen sheath. This sheath has a spindle or "fusiform" shape, hence the name "intrafusal." While the intrafusal fibers are wrapped with sensory receptors, their counterpart, extrafusal muscle fibers are the ones responsible for the power-generating component of muscle and are innervated by motor neurons.
It is by the sensory information from these two intrafusal fiber types that one is able to judge the position of their muscle, and the rate at which it is changing.