The stretch reflex is an automatic reflex that all skeletal muscles use to protect themselves from injury. This reflex causes muscle tissue to contract in response to increased lengthening or tension on that muscle. The faster and stronger a muscle length or tension changes, the stronger the reaction by the stretch reflex.
The stretching or tensing of a muscle stimulates sensory receptors called muscle spindles, which are located throughout the belly of the muscle. Muscle spindles monitor and detect change in muscle length and tension. When the muscle stretches or becomes more tense, the muscle spindle signals the brain and spinal cord to tell the stretching muscle to contract, essentially stopping the muscle from lengthening. This is the body’s way of protecting itself from allowing the muscle to over-stretch or tear. After all, in the past if our body became injured from a muscle tear, we would have run the risk of being eaten by a bigger, stronger, faster, hungrier creature due to our own immobility. Its simple reflexes like these that emphasize how amazingly smart our bodies are all on their own.
The key to relaxing, or reassuring, the muscle spindle and stopping it from triggering a strong stretch reflex is to be gentle and slow when stretching your body. If you move too quickly into a stretch or try and force a muscle to lengthen, the muscle spindle will cause a stronger and stronger muscle contraction. Its important to remind yourself that the body does this automatically the same way your heart beats on its own and your blood vessels dilate or constrict on their own. You cannot force or will it to stop by stretching more aggressively. You have to coax the muscle to lengthen with time and gentleness.
Other muscle reflexes like reciprocal inhibition or the golgi tendon organ work in tandem with the stretch reflex. More on those later