How Does Stretching Work, and How Can We Go Deeper?

A reflex is a response to a stimulus that doesn’t require the involvement of consciousness, and consists of two parts.

Afferent – brings information about a stimulus into the CNS. Detected, brought in by a neuron.

Efferent – carries information away from CNS to cause a response somewhere in the periphery, carried again by a neuron.

Generally, reflexes occur in the spine or the brain stem. Higher parts of the brain are not required, thus the definition.

Muscle Stretch Reflex (MSR)

If a skeletal muscle is rapidly stretched the MSR will cause it to contract very quickly afterwards, presumably as a protective response to prevent injury. The receptors that detect the rapid stretch are called muscle spindles (inside the muscle belly in-between muscle fibers [individual muscle cells]), which contain specialized fibers that stretch with the muscles. These spindles are covered in neuron axons that can detect the stretch and send the information back through the CNS (afferent, somatosensory neurons). This creates an excitatory synapse inside the CNS with another neuron that will send an axon back out to the rapidly stretched muscle and excite muscle cells inside the muscle belly causing a response, a contraction (efferent, lower motor neurons).

In addition, the automatic contraction of one muscle causes the complete relaxation of the antagonist muscle. When the efferent synapse (contraction) occurs it also sends inhibitory neurons to the antagonist so it’s not fighting against the contraction. This is reciprocal inhibition. We can also cause this ourselves – if we want to stretch the hamstrings, actively engage the quadriceps for 5-10 seconds then release, and work deeper into the hamstring stretch.

There is another part to the MSR. The Golgi Tendon Organ (GTO) lives in the tendon rather than the muscle. The tendon doesn’t get shorter or longer, so the GTO doesn’t measure a change in length, rather it senses tension in the tendon. It has one protective response, autogenic inhibition. The GTO senses so much tension there is concern for injury and eventually inhibits the muscle-tendon complex it is in. After six seconds it will inhibit the muscle, as in it will weaken it’s contraction to decrease the tension. 

Autogenic inhibition and the muscle stretch response can happen at the same time. After about six seconds the GTO overrides the muscle spindles. 

What does this mean for stretching? If we are stretching muscle spindles are going to fire, and if it’s a deep stretch the GTO will also fire. To cause change (think length) in the complex (muscle-tendon) it is important to hold a stretch for longer than six seconds, or how long it would take the GTO to take over and cause a release of tension. 

It is critical to keep in mind that we are not going to grit our teeth and bear through it. It must be done thoughtfully and intelligently, with attention directed towards our physical sensations. Discomfort is fine, it is typically a requirement for growth, but pain is a message from our body to our brain that damage is being done. This causes a stress response. If this is how we practice, or how our students practice if we are teachers, every time we/they stretch, we/they are creating a chronic stress response and could eventually develop hyporeflexia, a condition in which our muscle stretch response is dulled. This condition would take work with a PT to redevelop.

We are never trying to override our body. We want to work with these reflexes and responses to deepen the work. Our body is not a problem to be solved, it is an incredible gift, a tool full of innate wisdom that we must honor if we want to progress into our full, truly limitless potential.

Proprioceptive Neuromuscular Facilitation (PNF)

Putting it all together… we have PNF.

The exercise: A 30 second passive stretch of a muscle followed by a 10 second isometric contraction (muscle doesn’t shorten or lengthen when engaged), followed once more by a 30 second passive stretch. This creates an increase in the range of movement of the muscle between the two passive stretches.

The science: during the first passive stretch muscle spindles are activated and an impulse is sent to the cerebellum to create a concentric contraction. So we let the muscle contract isometrically inside of the stretch (AFTER 30 seconds) for 10 seconds, which activates the Golgi Tendon Organ because of the tension built. Here the brain is going to get slightly confused because we are asking it to contract and release simultaneously. In effect, it ignores both reflexes. This is the cause of autogenic inhibition, or a reduction in the excitability of the muscle. The result is that during the second round of a 30 second passive stretch there is no muscle spindle warning against too much stretch. That is why there is a significant stretch increase. 

This can be detrimental if not done with care. That is why it must be done intelligently to slowly increase ROM without permanently numbing the reflex. This kind of stretching is NOT meant to be done as a warm-up. The muscles should already be warmed up, full of blood - this is good to do towards the end of an asana practice, or after a workout. One time per muscle is enough.

Done more easily with a partner, otherwise props can be incredibly useful and provide the passivity for us. Think of using a strap for hamstring stretches. 

If you are still confused, there are countless resources available online. Take the time to ensure the resource is reliable, valid, and then have at it. Please, do not use these techniques if you are confused.