The Physiology of Stretching: tricks to lengthen muscles during yoga

Readers - stand up! Get out of your chair. Join us for a simple stretch. Interlace your fingers and press your palms into the sky, feeling the shoulders and torso lengthening as you press into your heels. Ahhhhhhhhh..... Feels good, right? Stretching releases endorphins, breaks up fascial adhesions, resets resting muscle length and with time, can correct out of whack body alignment. But did you know that stretching is a complex physiological process that involves not just muscles and tendons, but also the nervous system? Here we'll walk you through a few different automatic nerve reflexes that cascade through the body and spinal cord when you stretch. And we'll show you some tricks for safely deepening your stretches and improving your yoga practice and posture.

Our body is peppered with nerve receptors - they are located within our joints, tendons, and muscles. These nerves connect our muscles to the spinal cord. The receptors detect movement and changes in muscle tension and length, and signal the central nervous system, which responds by regulating the contractile state of the muscles - either by contracting or relaxing them. Muscles shorten or lengthen, which affects the range of motion of a given joint. This regulation happens automatically, in response to our bodily movements, without our awareness - during yoga practice and all day long.

Below are the three major reflexes that govern the contractile state of muscles:

1. the Muscle spindle stretch receptor: located in the belly of skeletal muscle. When a muscle is stretched, it sends a signal to the spinal cord to contract and resist the stretch. This is the body's way of protecting the muscle from overstretching or tearing.
   
   What does this mean for yoga practitioners and teachers? Go slow. Breathe into it. Back off a little. Then take another gentle run at the stretch. Never try to manhandle your muscles - or your students - into a stretch. Aggression is counterproductive, as it intensifies the firing of the muscle spindle, causing the muscle to contract. This mechanism can block deepening of a stretch. Instead, dissolve tightness slowly by coaxing the nerve reflex into decreasing contraction, and then go deeper into pose. Holding a stretch for 30 - 60 seconds causes the muscle spindle to decrease firing, and the muscle then begins to relax. Backing part way out of stretch also decreases firing, allowing a deeper stretch.
2. Reciprocal inhibition: a kind of biomechanical yin/yang. Reciprocal inhibition is the interplay of agonists (the muscle responsible for a given movement) and antagonists (the "opposite" muscle or muscle group that resists the movement). Simply put, when the agonist contracts, its antagonist relaxes. Muscles on one side of a joint relax to accommodate contraction on the other side of that joint.
   
   You can easily envision how this principle applies to yoga. For example, when you contract the quadriceps in Paschimottanasana (forward fold), your nervous system signals the hamstrings to relax to allow this extension of the knee joint. You can test this reaction out with any agonist/antagonist grouping: biceps/triceps. Abdominals/spine extensors. Hip flexors / glutes.
3. Golgi tendon organ: This sensory receptor is located where the muscle and tendon are joined. It detects changes in tension, and when tension increases, it signals the muscle to relax. Essentially, this receptor is a circuit breaker, preventing injury to the tendon when the tension caused by contracting a muscle becomes too high. This contrasts with muscle spindle receptor (described in 1 above), which detects change in length and tension in the body of the muscle and signals the muscle to contract.
   
   How can you use this mechanism to your advantage during yoga? When you temporarily contract a muscle you're trying to stretch, you are stimulating the golgi tendon organ. The nervous system tells the muscle to relax. This creates slack in the muscle that we can take up by going deeper. This mechanism is also known as the "relaxation response" or proprioceptive neuromuscular facilitation (PNF). It can be mindfully used to dissolve blocks and deepen yoga poses.

   

   Blue: contracting. Red: stretching

   
   

   
   
   A note of caution: don't overdo it with this technique, as you can damage tendons if too aggressive. Use no more than 20% of your maximum force to contract the target muscle, and hold for a maximum of 10 seconds. Then relax the muscle for one breath. Then move back deeper into your pose. Be very mindful of proper joint alignment, and back off if you feel any joint pain. Use sparingly: only one pose per session, for 2 - 3 cycles, and rest for 48 hours between PNF sessions.

Now we'll look at how to applying all three techniques to achieve a deeper Janu Sirsanana (knee to head pose), which requires lengthening the hamstrings.

Janu Sirsasana - left leg extended

1: Take the general form of Janu Sirsasana. The muscle spindle fires, resulting in reflex contraction of hamstrings.

2. Bend the extended knee to lighten the stretch, and hold this relaxed position for 2-3 breaths as the muscle spindle winds down.

3. Contract the quads to straighten knee and lengthen hamstrings. This signals hamstrings to relax via reciprocal inhibition.

4. Press the forward heel into the ground to contract the hamstrings. This increases tension at muscle-tendon junction and stimulates the golgi tendon organ. The nervous system then signals the hamstrings to relax.

5. Contract the quads once again to straighten the knee, taking up the slack created by hamstring relaxation. Contracting the quads creates reciprocal inhibition, further relaxing the hamstrings.

Try out these subtle techniques in your own yoga practice to mindfully address tightness, asymmetries and limitations. 

For an excellent scientific discourse of the muscles used in yoga, check out Ray Long's website at www.bandhayoga.com 

by Katherine Girling Zuna Yoga offers yoga teacher trainings in Bali, Gili Meno and Cambodia