Stretch and the Nervous System

By Fernando Pagés Ruiz

Why are you able to gradually bring your head toward your shins once you stop straining? According to science—and many ancient yogis—what was limiting your flexibility most wasn't your body, it was your mind—or, at least, your nervous system.

The Stretch Reflex

According to physiologists who view the nervous system as the major obstacle to increased flexibility, the key to overcoming one's limitations lies in another built-in feature of our neurology: the stretch reflex. Scientists who study flexibility think that the small, progressive steps that allow us to go a little deeper during the course of one session—and that dramatically 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 also 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.

Proprioceptive Neuromuscular Facilitation...What?

Among the recent developments in Western flexibility training are neurological techniques that retrain the stretch reflex, promoting quick, dramatic gains in flexibility. One of these techniques is called—take a deep breath—proprioceptive neuromuscular facilitation. (Fortunately, it's usually just called PNF).

To apply PNF principles to Paschimottanasana, try this: While bending forward, just short of your maximum stretch, engage your hamstrings in an isometric contraction—as if you were trying to draw your heels down through the floor—lasting approximately five to 10 seconds. 

Then release this action, and see if you can move a little deeper into the forward bend.

The PNF method manipulates the stretch reflex by having you contract a muscle while it's at near-maximum length. When you engage your hamstrings, you actually ease the pressure on your muscle spindles, and they send signals that it's safe for the muscle to release further. In a seeming paradox, contracting the muscle actually allows it to lengthen. If you engage and then release your muscle fibers in this way, you will probably discover more comfort in a stretch that was near your maximum just seconds before. Now you're ready to open a little more, taking advantage of a momentary lull in neural activity, deepening the stretch. Your nervous system adjusts, affording you greater range of motion.

"PNF is as close as we've come to scientific stretching," physical therapist Michael Leslie says. Leslie uses combinations of modified PNF techniques to help members of the San Francisco Ballet improve their flexibility. "In my experience it can take weeks of static training to achieve the gains possible in one session of PNF," Leslie says.

As of yet, yoga has not focused systematically on PNF-type techniques. But vinyasa practices that emphasize careful sequencing of asanas and/or repetition of asanas—moving in and out of the same posture several times—tend to promote neurological conditioning.

Gray Kraftsow, founder of the American Viniyoga Institute and one of the most highly respected teachers in the Viniyoga lineage of T.K.V. Desikachar, likens Viniyoga to PNF. "Alternating between contracting and stretching is what changes the muscle," Kraftsow says. "Muscles relax and stretch further after contracting."

Prana & Flexibility

Kraftsow also emphasizes the importance of the breath in any kind of neurological work, pointing out that breathing is a link between our consciousness and our autonomic nervous system. "It's this quality of breathing," Kraftsow says, "that qualifies it as a primary tool in any science of self development."

Pranayama, or breath control, is the fourth limb in a yogi's path toward samadhi. One of the most important yogic practices, it helps the yogi gain control over the movement of prana (life energy) throughout the body. But whether viewed through esoteric yoga physiology or the scientific physiology of the West, the connection between relaxation, stretching, and breathing is well established. Physiologists describe this mechanical and neurological correlation of movement and breath as an instance of synkinesis, the involuntary movement of one part of the body that occurs with the movement of another part.

While you are holding Paschimottanasana, breathing deeply and steadily, you may notice an ebb and flow to your stretching that mirrors the tide of your breath. As you inhale, your muscles tighten slightly, reducing the stretch. As you exhale, slowly and completely, your abdomen moves back toward your spine, the muscles in your lower back seem to grow longer, and you can drop your chest toward your thighs.

It's obvious that exhalation deflates the lungs and lifts your diaphragm into the chest, thereby creating space in the abdominal cavity and making it easier to bend the lumbar spine forward. (Inhalation does the opposite, filling the abdominal cavity like an inflating balloon, making it difficult to fold your spine forward completely.) But you may not realize that exhalation also actually relaxes the muscles of your back and tilts your pelvis forward.

In Paschimottanasana, the musculature of the lower back is in passive tension. According to research cited in Science of Flexibility, every inhalation is accompanied by an active contraction of the lower back—a contraction in direct opposition to the desired forward bend. 

Then exhalation releases the lower back muscles, facilitating the stretch. If you place your palms on your back, just above the hips, and breathe deeply, you can feel the erector spinae on either side of your spinal column engage as you inhale and release as you exhale. If you pay close attention, you'll also notice that each inhalation engages the muscles around the coccyx, at the very tip of your spine, drawing the pelvis back slightly. Each exhalation relaxes these muscles and frees your pelvis, allowing it to rotate around the hip joints.

As your lungs empty and the diaphragm lifts into your chest, your back muscles release and you are able to fold into your ultimate stretch. Once there, you may experience a pleasant, seemingly eternal moment of inner peace, the pacifying of the nervous system traditionally considered one of the benefits of forward bends.

At this point, you may feel especially in touch with the spiritual element of yoga. But Western science also offers a material explanation for this experience. According to Alter's Science of Flexibility, during an exhalation the diaphragm pushes up against the heart, slowing down the heart rate. Blood pressure decreases, as does stress on the rib cage, abdominal walls, and intercostal muscles. Relaxation ensues, and your tolerance to stretching is enhanced—as well as your sense of well-being.

Short Cuts to Flexibility?

But not every moment in yoga is peaceful. At the extreme end of hatha yoga achievement, practitioners can experience breakthroughs that may involve a degree of pain, fear, and risk. (After all, hatha does mean "forceful.") You may have seen the photograph in Light on Yoga of B.K.S. Iyengar poised in Mayurasana (Peacock Pose) on the back of a student in 

Paschimottanasana, forcing her to fold more deeply. Or perhaps you've watched a teacher stand on the thighs of a student in Baddha Konasana (Bound Angle Pose). Such methods might appear dangerous or even cruel to an outsider, but in the hands of an experienced instructor they can be remarkably effective—and they bear a striking resemblance to cutting-edge techniques in Western flexibility training that focus on reconditioning neurological mechanisms.

As I researched this article, a friend told me about a time he accidentally engaged one of these mechanisms and experienced a surprising breakthrough after years of trying to master Hanumanasana (a pose better known in the West as "the splits"). One day, as my friend attempted the posture—left leg forward and right leg back, hands lightly supporting him on the floor—he stretched his legs farther apart than usual, allowing almost the full weight of his torso to rest down through his hips. Suddenly he felt an intense warmth in his pelvic region and a rapid, unexpected release that brought both his sitting bones to the floor. My friend had triggered a physiological reaction rarely encountered while stretching, a neurological "circuit breaker" that opposes and overrides the stretch reflex. While the stretch reflex tenses muscle tissue, this other reflex—technically, it's known as the "inverse myotatic (stretch) reflex"—completely releases muscular tension to protect the tendons.

How does it work? At the ends of every muscle, where fascia and tendons interweave, there are sensory bodies that monitor load. These are the Golgi tendon organs (GTOs). They react when either a muscular contraction or a stretch places too much stress on a tendon.

The huge, state-sponsored sports apparatus of the former Soviet Union developed a neurological flexibility training method based largely on manipulating this GTO reflex. "You already have all the muscle length you need," argues Russian flexibility expert Pavel Tsatsouline, "enough for full splits and most of the difficult asanas. But controlling flexibility requires control of an autonomic function." Tsatsouline makes the point by lifting his leg up on a chair back. "If you can do this," he says, "you've already got enough stretch to do the splits." 

According to Tsatsouline, it's not muscle or connective tissue that's stopping you. "Great flexibility," asserts Tsatsouline, "can be achieved by flicking a few switches in your spinal cord."

But exploiting the GTO mechanism to enhance flexibility entails certain risks, because muscles must be fully extended and under extreme tension to trigger a GTO reflex. Implementing enhanced methods of flexibility training—like the Russian system or advanced yoga techniques—requires an experienced teacher who can make sure your skeleton is correctly aligned and that your body is strong enough to handle the stresses involved. If you don't know what you're doing, it's easy to get hurt.

If used correctly, though, these methods can be extremely effective. Tsatsouline claims he can teach even stiff middle-aged men, with no prior flexibility training, how to do the splits in about six months.

Applied Physiology

By now you may be asking yourself, "What do these Western stretching techniques have to do with yoga?"

On the one hand, of course, stretching is an important component of building the yoga-deha, the yogic body that allows the practitioner to channel ever more prana. That's one reason why the major hatha yoga schools base their practice on the classic asanas, a series of postures that illustrate and encourage the ideal range of human movement.

But any good teacher will also tell you that yoga isn't just about stretching. "Yoga is a discipline that teaches us new ways of experiencing the world," Judith Lasater, Ph.D. and physical therapist, explains, "so that we can give up the attachments to our suffering." According to Lasater, there are only two asanas: conscious or unconscious. In other words, what distinguishes a particular position as an asana is our focus, not simply the outer conformation of the body.

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t's certainly possible to get so caught up in pursuit of physical perfection that we lose sight of the "goal" of asana practice—the state of samadhi. At the same time, though, exploring the limits of the body's flexibility can be a perfect vehicle for developing the one-pointed concentration needed for the "inner limbs" of classical yoga.

And there is certainly nothing inherently contradictory about using the analytical insights of Western science to inform and enhance the empirical insights of millennia of asana practice. In fact, yoga teacher B.K.S. Iyengar, perhaps the most influential figure in the Western assimilation of hatha yoga, has always encouraged scientific inquiry, advocating the application of strict physiological principles to the cultivation of a refined asana practice.

Some yogis are already embracing this synthesis enthusiastically. At the Meridian Stretching Center in Boston, Massachusetts, Bob Cooley is developing and testing a computer program that can diagnose flexibility deficiencies and prescribe asanas. New clients at Cooley's stretching center are asked to assume 16 different yoga postures as Cooley records specific anatomical landmarks on their bodies with a digitizing wand, similar to the ones used in computer-aided drafting. These body-point readings are computed to make comparisons between the client and models of both maximum and average human flexibility. The computer program generates a report that benchmarks and guides the client's progress, spelling out any areas needing improvement and recommending specific asanas.

Cooley uses an amalgamation of what he sees as the best points of Eastern and Western knowledge, combining the classic yoga asana with techniques similar to PNF. (An eclectic experimenter, Cooley incorporates Western psychotherapeutic insights, the Enneagram, and Chinese meridian theory in his approach to yoga.).

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f you're a yoga purist, you may not like the idea of a yoga potpourri that mixes new-fangled scientific insights with time-honed yoga practices. But "new and improved" has always been one of America's national mantras, and blending the best from Eastern experience-based wisdom and Western analytical science may be a principal contribution our country makes to the evolution of yoga.