Intrafusal muscle fibers are skeletal muscle fibers that comprise the muscle spindle and are innervated by gamma motor neurons. These fibers are a proprioceptor that detect the amount and rate of change of length in a muscle. 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."
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.