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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.
The first of the two main group of stretch receptors wrapping the intrafusal fibers are the Ia fiber, which are the largest and fastest fibers, and they fire when the muscle is stretching. They are characterized by their rapid adaptation, because as soon as the muscle stops changing length, the Ia stop firing and adapt to the new length. Ia fibers essentially supply proprioceptive information about the rate of change of its respective muscle: the derivative of the muscle's length (or position).
The second of the two main groups of stretch receptors are the II fibers, and they are non-adapting, meaning that they keep responding even when the muscle has stopped changing its length. Their firing rate is directly related to the muscle's instantaneous length, or position. This information would indicate the position of one's leg once it has stopped moving.
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