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

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.

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