Final answer:
Muscle spindles contract at the same rate as the muscle through the stretch reflex, a reflex that activates muscle spindle receptors and causes direct muscle contraction.
Step-by-step explanation:
Muscle spindles are stretch receptors that detect the amount of stretch, or lengthening of muscles. They allow a muscle spindle to contract at the same rate as the muscle where it resides through the stretch reflex. When a skeletal muscle is stretched, a muscle spindle receptor is activated, and the axon from this receptor causes direct contraction of the muscle. This reflex helps to maintain muscles at a constant length.