Yoga and 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)!!!

Why is not recommended to bounce while we work in flexibility:  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.

 

Your Internal Knitting: Muscles, tendons, ligaments and muscle's facsia

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. The three types of connective tissue more important for flexibility are: tendons, ligaments, and muscle fascia.

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.

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."

For that reason is so important to don’t push 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 the posture 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.

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.