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What Science Can Teach Us About Flexibility

In recent years, biomedical research has begun to investigate and appreciate what yogis have known for centuries: Stretching keeps us limber, youthful, and healthy.

By Fernando Pagés Ruiz

pigeon pose

If you're already practicing yoga, you don't need exercise scientists and physiologists to convince you of the benefits of stretching. Instead, you'd probably like them to tell you if there's anything in their flexibility research that can help you go deeper in your asanas. For example, when you fold into a forward bend and are brought up short by the tightness in the back of your legs, can science tell you what's going on? And can that knowledge help you go deeper?

The answer to both questions is "Yes." A knowledge of physiology can help you visualize the inner workings of your body and focus on the specific mechanisms that help you stretch. You can optimize your efforts if you know whether the tightness in your legs is due to poor skeletal alignment, stiff connective tissues, or nerve reflexes designed to keep you from hurting yourself. And if you know whether any uncomfortable sensations you feel are warnings that you're about to do damage, or whether they're just notices that you're entering exciting new territory, you can make an intelligent choice between pushing on or backing off—and avoid injuries.

In addition, new scientific research may even have the potential to extend the wisdom of yoga. If we understand more clearly the complex physiology involved in yogic practices, we may be able refine our techniques for opening our bodies.

Why Stretch?

Of course, yoga does far more than keep us limber. It releases tensions from our bodies and minds, allowing us to drop more deeply into meditation. In yoga, "flexibility" is an attitude that invests and transforms the mind as well as the body.

But in Western, physiological terms, "flexibility" is just the ability to move muscles and joints through their complete range. It's an ability we're born with, but that most of us lose. "Our lives are restricted and sedentary," explains Dr. Thomas Green, a chiropractor in Lincoln, Nebraska, "so our bodies get lazy, muscles atrophy, and our joints settle into a limited range."

Back when we were hunter-gatherers, we got the daily exercise we needed to keep our bodies flexible and healthy. But modern, sedentary life is not the only culprit that constricts muscles and joints. Even if you're active, your body will dehydrate and stiffen with age. By the time you become an adult, your tissues have lost about 15 percent of their moisture content, becoming less supple and more prone to injury. Your muscle fibers have begun to adhere to each other, developing cellular cross-links that prevent parallel fibers from moving independently. Slowly our elastic fibers get bound up with collagenous connective tissue and become more and more unyielding. This normal aging of tissues is distressingly similar to the process that turns animal hides into leather. Unless we stretch, we dry up and tan! Stretching slows this process of dehydration by stimulating the production of tissue lubricants. It pulls the interwoven cellular cross-links apart and helps muscles rebuild with healthy parallel cellular structure.

Remember the cheesy '70s sci-fi flick in which Raquel Welch and her miniaturized submarine crew get injected into someone's bloodstream? To really grasp how Western physiology can benefit asana practice, we need to go on our own internal odyssey, diving deep into the body to examine how muscles work.

Muscles are organs—biological units built from various specialized tissues that are integrated to perform a single function. (Physiologists divide muscles into three types: the smooth muscles of the viscera; the specialized cardiac muscles of the heart; and the striated muscles of the skeleton—but in this article we'll focus only on skeletal muscles, those familiar pulleys that move the bony levers of our bodies.)

The specific function of muscles, of course, is movement which is produced by muscle fibers, bundles of specialized cells that change shape by contracting or relaxing. Muscle groups operate in concert, alternately contracting and stretching in precise, coordinated sequences to produce the wide range of movements of which our bodies are capable.

In skeletal movements, the working muscles—the ones that contract to move your bones—are called the "agonists." The opposing groups of muscles—the ones that must release and elongate to allow movement—are called the "antagonists." Almost every movement of the skeleton involves the coordinated action of agonist and antagonist muscle groups: They're the yang and yin of our movement anatomy.

But although stretching—the lengthening of antagonist muscles—is half the equation in skeletal movement, most exercise physiologists believe that increasing the elasticity of healthy muscle fiber is not an important factor in improving flexibility. According to Michael Alter, author of Science of Flexibility (Human Kinetics, 1998), current research demonstrates that individual muscle fibers can be stretched to approximately 150 percent of their resting length before tearing. This extendibility enables muscles to move through a wide range of motion, sufficient for most stretches—even the most difficult asanas.

If your muscle fibers don't limit your ability to stretch, what does? There are two major schools of scientific thought on what actually most limits flexibility and what should be done to improve it. The first school focuses not on stretching muscle fiber itself but on increasing the elasticity of connective tissues, the cells that bind muscle fibers together, encapsulate them, and network them with other organs; the second addresses the "stretch reflex" and other functions of the autonomic (involuntary) nervous system. Yoga works on both. That's why it's such an effective method for increasing flexibility.

Your Internal Knitting

Connective tissues include a variety of cell groups that specialize in binding our anatomy into a cohesive whole. It is the most abundant tissue in the body, forming an intricate mesh that connects all our body parts and compartmentalizes them into discrete bundles of anatomical structure—bones, muscles, organs, etc. Almost every yoga asana exercises and improves the cellular quality of this varied and vital tissue, which transmits movement and provides our muscles with lubricants and healing agents. But in the study of flexibility we are concerned with only three types of connective tissue: tendons, ligaments, and muscle fascia. Let's explore each of them briefly.

Tendons transmit force by connecting bones to muscle. They are relatively stiff. If they weren't, fine motor coordination like playing piano or performing eye surgery would be impossible. While tendons have enormous tensile strength, they have very little tolerance to stretching. Beyond a 4 percent stretch, tendons can tear or lengthen beyond their ability to recoil, leaving us with lax and less responsive muscle-to-bone connections.

Ligaments can safely stretch a bit more than tendons—but not much. Ligaments bind bone to bone inside joint capsules. They play a useful role in limiting flexibility, and it is generally recommended that you avoid stretching them. Stretching ligaments can destabilize joints, compromising their efficiency and increasing your likelihood of injury. That's why you should flex your knees slightly—rather than hyperextending them—in Paschimottanasana (Seated Forward Bend), releasing tension on posterior knee ligaments (and also on the ligaments of the lower spine).

Muscle fascia is the third connective tissue that affects flexibility, and by far the most important. Fascia makes up as much as 30 percent of a muscle's total mass, and, according to studies cited in Science of Flexibility, it accounts for approximately 41 percent of a muscle's total resistance to movement. Fascia is the stuff that separates individual muscle fibers and bundles them into working units, providing structure and transmitting force.

Many of the benefits derived from stretching—joint lubrication, improved healing, better circulation, and enhanced mobility—are related to the healthy stimulation of fascia. Of all the structural components of your body which limit your flexibility, it is the only one that you can stretch safely. Anatomist David Coulter, author of Anatomy of Hatha Yoga, reflects this in his description of the asanas as "a careful tending to your internal knitting."

Now let's apply this physiology lesson to a basic but very powerful posture: Paschimottanasana. We'll begin with the anatomy of the asana.

The name of this pose combines three words: "Paschima," the Sanskrit word for "west"; "uttana," which means "intense stretch"; and "asana," or "posture." Since yogis traditionally practiced facing east toward the sun, "west" refers to the entire back of the human body.

This seated forward bend stretches a muscle chain that begins at the Achilles tendon, extends up the back of the legs and pelvis, then continues up along the spine to end at the base of your head. According to yoga lore, this asana rejuvenates the vertebral column and tones the internal organs, massaging the heart, kidneys, and abdomen.

Imagine you're lying on your back in yoga class, getting ready to fold up and over into Paschimottanasana. Your arms are relatively relaxed, palms on your thighs. Your head is resting comfortably on the floor; your cervical spine is soft, but awake. The instructor asks you to lift your trunk slowly, reaching out through your tailbone and up through the crown of your head, being careful not to overarch and strain your lower back as you move up and forward. She suggests that you picture an imaginary string attached to your chest, gently pulling you out and up—opening anahata chakra, the heart center—as you rotate through the hips into a seated position.

The image your teacher is using is not just poetic, it's also anatomically accurate. The primary muscles at work during this first phase of a forward bend are the rectus abdominis that run along the front of your trunk. Attached to your ribs just below your heart and anchored to your pubic bone, these muscles are the anatomical string that literally pulls you forward from the heart chakra.

The secondary muscles working to pull your torso up run through your pelvis and along the front of your legs: the psoas, linking torso and legs, the quadriceps on the front of your thighs, and the muscles adjacent to your shin bones.

In Paschimottanasana, the muscles running from heart to toe along the front of your body are the agonists. They're the muscles that contract to pull you forward. Along the back of your torso and legs are the opposing, or complementary, groups of muscles, which must elongate and release before you can move forward.

By now, you've stretched forward and settled into the pose completely, backing off slightly from your maximum stretch and breathing deeply and steadily. Your mind focuses on the subtle (or perhaps not so subtle) messages from your body. You feel a pleasant pull along the full length of your hamstrings. Your pelvis is tilted forward, your spinal column is lengthening, and you perceive a gentle increase in the spaces between each of your vertebrae.

Your instructor is quiet now, not pushing you to stretch further but allowing you to go deeper into the posture at your own pace. You're getting to know the posture and getting comfortable with it. Perhaps you even feel like a timelessly serene statue as you hold Paschimottanasana for several minutes.

In this kind of practice, you're maintaining the posture long enough to affect the plastic quality of your connective tissues. Prolonged stretches like this can produce healthful, permanent changes in the quality of the fascia that binds your muscles. Julie Gudmestad, a physical therapist and certified Iyengar instructor, uses prolonged asanas with patients at her clinic in Portland, Oregon. "If they hold the poses for shorter periods, people get a nice sense of release," Gudmestad explains, "but they aren't necessarily going to get the structural changes that add up to a permanent increase in flexibility."

According to Gudmestad, stretches should be held 90 to 120 seconds to change the "ground substance" of connective tissue. Ground substance is the nonfibrous, gel-like binding agent in which fibrous connective tissues like collagen and elastin are embedded. Ground substance stabilizes and lubricates connective tissue. And it is commonly believed that restrictions in this substance can limit flexibility, especially as we age.

By combining precise postural alignment with the use of props, Gudmestad positions her patients to relax into asanas so they can remain long enough to make lasting change. "We make sure people aren't in pain," Gudmestad says, "so they can breathe and hold a stretch longer."

Reciprocal Inhibition

Along with stretching connective tissue, much of the work we do in yoga aims to enlist the neurological mechanisms that allow our muscles to release and extend. One such mechanism is "reciprocal inhibition." Whenever one set of muscles (the agonists) contracts, this built-in feature of the autonomic nervous system causes the opposing muscles (the antagonists) to release. Yogis have been using this mechanism for millennia to facilitate stretching.

To experience reciprocal inhibition firsthand, sit down in front of a table and gently press the edge of your hand, karate-chop style, onto the tabletop. If you touch the back of your upper arm—your triceps muscle—you'll notice that it's firmly engaged. If you touch the opposing muscles, the biceps (the big muscles on the front of your upper arm), they should feel relaxed.

In Paschimottanasana the same mechanism is at play. Your hamstrings are released when you engage their opposing muscle group, the quadriceps.

David Sheer, an orthopedic manual therapist in Nashville, Tennessee, uses the principle of reciprocal inhibition to help patients safely improve their range of motion. If you went to Sheer to improve your hamstring flexibility, he would work the quadriceps, developing strength in the front thigh to help relax the hamstrings. Then, when the hamstrings have achieved their maximum range for the day, Sheer would strengthen them with weight-bearing, isometric, or isotonic exercises.

At the Yoga Room of Nashville, Betty Larson, a certified Iyengar instructor, uses the principles of reciprocal inhibition to help yoga students release their hamstrings in Paschimottanasana.

"I remind my students to contract their quads," says Larson, "lifting up the entire length of the front of the leg, so the back of the leg is loosened." Larson also includes backbends in her classes to strengthen her students' hamstrings and backs. She feels it's extremely important to develop strength in the muscles you are stretching. Like many teachers, Larson is using ancient yogic techniques that apply physiological principles only recently understood by modern science.

According to Sheer, she's doing the right thing. He claims the best type of flexibility combines improved range of motion with improved strength. "It's useful flexibility," says Sheer. "If you only increase your passive flexibility without developing the strength to control it, you make yourself more vulnerable to a serious joint injury."

Let's return to your Paschimottanasana. Imagine that this time, as you pivot from your pelvis and reach your trunk forward, your hamstrings are unusually tight. You can't seem to move as deeply into the pose as you would like, and the harder you try, the tighter your hamstrings feel. Then your instructor reminds you to continue breathing and relax every muscle that's not actively engaged in sustaining the pose.

You give up trying to match your personal best. You relax into the posture, without judgment, and slowly your hamstrings begin to release.

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Reader Comments

Natalie, RYT

I find the facts are outdated in this article. New science shows us that static stretching is not beneficial. Unless you are a dancer or Circus Soleil performer you do not need to sit in a split. Furthermore the static stretching reduces the strength of the muscle.So, strength training after the static stretch sounds like oxymoron. Let us think what is really beneficial for our students. Is it the ego boosting pretzel or the longevity and the vitality of the body?

A Yoga Teacher

Wonderful article, which I would share with my students if it weren't for the massive number of distracting advertisements all around it, obscuring the small black text of the article.

A young dancer

Being a dancer I find this information to be very useful and will use this information for better things

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