An example of stretch reflex triggered by passive muscle movement...

A stretch reflex is the contraction of a muscle in response to a passive stretching of a muscle spindle within a muscle. This reflex enables the body to correct any unintended changes in body posture in a timely manner without waiting for input from the cerebral cortex as well as keeps appropriate tone in muscles. A well known example of stretch reflex is patellar reflex or knee jerk reflex, i.e., sudden kicking movement of the lower leg in response to a sharp tap on patellar tendon.