BY FERNANDO PAGÉS RUIZ  |  Originated published here in Yoga Journal

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.

—— Read more here on Yoga Journal.