How Muscles Respond
The skeletal muscle uses two types of receptors to relay information to the brain. There are muscle spindles, which are located in between the muscle fibers themselves and Golgi tendon organs (GTO's). They are located where the tendon meets the muscle fibers. The muscle spindles detect change in muscle length and rate or speed of length change, and GTO's detect muscular tension or force.
The Stretch Reflex
The stretch reflex is the body's natural defense mechanism against injury due to over stretching. When a muscle is stretched too far too fast the stretch reflex is triggered: the muscle being stretched contracts. This occurs when a muscle is stretched such that the primary sensory fibers of the muscle spindle, located in the muscle belly, respond to both the velocity and the degree of stretch, and send this information to the spinal cord. Likewise, secondary sensory fibers detect and send information about the degree of stretch (but not the velocity) to the central nervous system. The conveyance of this information to the motor nerves activates the extrafusal fibers of the muscle, causing them to contract, thereby reducing or even stopping the stretch. The knee jerk test is good example, when the patellar tendon (below kneecap) is lightly tapped the quadriceps contracts.
Skeletal muscle often operates in pairs, when one muscle group contracts (agonist); the opposing muscle group (antagonist) relaxes, allowing movement. For example, when you flex your quadriceps, the hamstrings must relax to allow the leg to extend. We can use this phenomenon to induce relaxation in the muscle group, which is being stretched.
Inverse Myotatic Reflex
The inverse myotatic reflex is yet another natural defense mechanism against injury, designed to prevent tendons and muscles from tearing away from their attachments. In this reflex when a tendon is stretched beyond a certain threshold the GTO's, fearing injury, fire and cause the muscle to relax. This mechanism allows the muscle to be stretched further than with static stretching alone. Proprioceptive Neuromuscular Facilitation (PNF) stretching, with the use of a partner, can take advantage of this reflex to increase gains in flexibility.