What is the stretch reflex?
As a muscle is stretched, so is the muscle spindle (which runs parallel to muscle fibers). The muscle spindle records the change in length (and speed of length change) and transmits this signal to the spinal cord. This triggers the stretch reflex (also called the myotatic stretch reflex), which initially attempts to oppose the change in muscle length by causing the stretched muscle to contract. The more sudden the change in muscle length, the stronger the contraction is. Thus the muscle spindle attempts to protect the muscle from injury. One of the reasons for holding a stretch for a sustained period of time (15–30 seconds) is that the muscle spindle gradually becomes accustomed to the new length and reduces its opposing signaling, thus allowing for greater muscle lengthening.
