What is the stretch reflex (myotatic reflex)?
From a young age, we’re taught that stretching is a necessary part of any workout routine. If we’re involved in sports or physical training, we stretch to warm up, cool down, and during breaks to help us stay loose. Unfortunately, stretching usually doesn’t accomplish much, mainly due to the myotatic reflex, more commonly referred to as the stretch reflex.
The stretch reflex is a function of the gamma loop, a feedback loop in our nervous system that regulates the level of tension in our muscles. If you’re not familiar with the gamma loop, you can read What is the Gamma Loop?
Muscle spindles play an important role in our stretch reflex. Muscle spindles are sensory receptors located within our skeletal muscles (the type of muscles that move our skeleton around, in contrast to the smooth muscle of our internal organs). Muscle spindles detect changes in the length of our muscles, and for this reason they’re also referred to as stretch receptors.
When one of your skeletal muscles is stretched—either by you pulling on it, someone else pulling on it, or by someone giving you a deep massage—the muscle spindles within that muscle are stretched too.
As you can see in the diagram below, the sensory axons wrapped around the muscle spindle sense the increase in length and send that information to the alpha motor neuron in the spine. The alpha motor neuron then tells the skeletal muscle fibers to contract. Why? To keep us standing upright and to prevent our muscles from being torn.