For most major actions in the body, the brain decides what movement must be taken, the nerve impulses are transmitted out of the brain, down the spinal cord and out to the intended receiver. Then when the action is carried out the impulse return back via the reverse pathway to tell the brain it was completed. This is the path for any brain-controlled, conscious action in the body.
However, there are many processes in the body that do not require this dialogue. The heart functions, breathing, metabolic processes, disease fighting and many other autonomic processes happen automatically in the body. The body uses signals to increase, decrease, or maintain many of these actions. If the carbon dioxide levels in the body begin to rise, for example, the autonomic nervous system calls for an increase in respiratory rate.
Another automatic response by the nervous system is the reflex. The body reacts in a predetermined way based on specific stimulus. This may be a practiced response or a pre-programmed one. The stretch reflex (or myotatic reflex) is one of those responses.
Through constant consideration in our yoga practice we can bring about changes in the nervous system to overcome many obstacles to increased flexibility however in the longer term, the key to overcoming one’s limitations lies in another built-in feature of our neurology: the stretch reflex. The small, progressive steps that allow us to go a little deeper into postures and improve our flexibility over a life of yoga practice are in large part the result of retraining this reflex.
To get an understanding of the stretch reflex, picture yourself walking in a winter landscape. Suddenly you step on a patch of ice, and your feet start to splay apart. Immediately your muscles fire into action, tensing to draw your legs back together and regain control. What just happened in your nerves and muscles?
Every muscle fiber has a network of sensors called muscle spindles. They run perpendicular to the muscle fibers, sensing how far and fast the fibers are elongating. As muscle fibers extend, stress on these muscle spindles increases.
When this stress comes too fast, or goes too far, muscle spindles fire an urgent neurological “SOS,” activating a reflex loop that triggers an immediate, protective contraction.
That’s what happens when the doctor thumps with a small rubber mallet on the tendon just below your kneecap, stretching your quadriceps abruptly. This rapid stretch stimulates the muscle spindles in your quadriceps, signaling the spinal cord. An instant later the neurological loop ends with a brief contraction of your quadriceps, producing the well known “knee jerk reaction.”
That’s how the stretch reflex protects your muscles. And that’s why most experts caution against bouncing while stretching. Bouncing in and out of a stretch causes the rapid stimulation of muscle spindles that triggers reflexive tightening, and can increase your chances of injury.
Slow, static stretching triggers the stretch reflex, but not as abruptly. When you fold forward into Paschimottanasana, the muscle spindles in your hamstrings begin to call for resistance, producing tension in the very muscles you’re trying to extend. That’s why improving flexibility through static stretching takes a long time. The improvement comes through slow conditioning of your muscle spindles, training them to tolerate more tension before applying the neuro-brakes.
While yoga isn’t actually about becoming more flexible, having a good understanding of what flexibility is, and why it’s important, can help you take your yoga practice to the next level.