Yin Yoga is a slow-paced style of yoga with postures or asanas that are held for comparatively long periods of time--five minutes or longer per pose is typical. It was founded and first taught in the United States in the late 1970s by martial arts expert and Taoist yoga teacher Paulie Zink. Yin-style yoga is now being taught across North America and in Europe, due in large part to the widespread teaching activities of Yin Yoga teachers and developers Paul Grilley and Sarah Powers.
Yin Yoga poses apply moderate stress to the connective tissues—the tendons, fascia, and ligaments—with the aim of increasing circulation in the joints and improving flexibility. Yin Yoga poses are also designed to improve the flow of qi, the subtle energy said in Chinese medicine to run through the meridian pathways of the body. Improved flow of qi is hypothesized to improve organ health, immunity, and emotional well-being. Yin Yoga as taught by Paul Grilley and Sarah Powers is not intended as a complete practice in itself, but rather as a complementary practice to balance the effects of more active forms of yoga and exercise. Paulie Zink's approach includes the full range of Daoist yoga and is intended to be a complete practice in itself.:21 Sarah Powers has developed a system called Insight Yoga which features both Yin poses and more active Yang poses.
The practice of holding yoga poses or asanas for long periods of time has always been a significant part of traditional yoga practice, both in the Hatha yoga tradition of India and in the Taoist yoga tradition of the greater China area. Some regard Yin yoga is the oldest form of Hatha yoga, since it is an effective method of physical conditioning for prolonged meditation, which was the principal concern of ancient yogic practitioners.:13 Contemporary schools of hatha yoga have also advocated holding some poses for relatively long periods of time. For example, BKS Iyengar recommends holding the Supta Virasana asana for 10–15 minutes.:12 Taoist yoga practices from China also included yin-style poses in the Taoist system of “Internal Alchemy”—practices for the purpose of improving health and longevity.:15 For that matter, long-held stretches have been and are commonly recommended in other physical disciplines, such as gymnastics and dance, to increase flexibility. It is traditional for ballerinas, for example, to open their hips by approximating the splits position for long periods of time.:28
Techniques for stretching of this type have been practiced for centuries in China and Taiwan as part of Daoist Yoga, which was sometimes known as Dao Yin. Taoist priests taught this knowledge, along with breathing techniques, to Kung Fu practitioners beginning 2000 years ago, to help them fully develop their martial arts skills. What later came to be known as Yin Yoga, in which a series of long-held poses are performed one after the other, was introduced in North America in the late 1970’s by Paulie Zink, a martial arts champion and Taoist yoga teacher.
Zink began studying Kung Fu as a teenager and after five years of training was chosen as a student by Cho Chat Ling, a Kung-Fu and Qigong master from Hong Kong. Cho Chat Ling was a master of a style known as Tai shing pek kwar, which combines three different disciplines of Kung Fu. (In Kung Fu, the movements of animals are closely studied and emulated, inspiring many of the movements taught in Kung Fu, and also giving rise to five different schools within the discipline.) In time, the master Cho decided to train Zink as his protégé and successor. For ten years, in daily classes lasting six to eight hours, he instructed Zink in Kung-Fu and Taoist yoga, in roughly equal proportions.:20 At the end of the decade, Cho Chat Ling formally recognized Zink as a master in his own right, and asked him to take part in martial arts competitions. Zink entered the Long Beach International Karate Championships in 1981, 1982 and 1983 and won Grand Champion in the weapons forms category in all three years, and was also Grand Champion in the “empty hands” category in two of those years. Black Belt magazine named him Kung Fu artist of the year in 1989. Noted in the Kung Fu community for his achievement of exceptional personal flexibility, Zink also emphasized flexibility training in his martial arts classes as a method to develop agility, power and endurance.
In the late 70’s, Zink began, separate from his martial arts classes, to teach a synthesis of Hatha Yoga with a full range of disciplines from Daoist Yoga, as well as postures, movements and insights that he had developed himself. This later became known as Yin Yoga.:19 Paul Grilley, a yoga teacher then living in California, who later became a major proponent of Yin Yoga, first saw Paulie Zink on a public access television talk show in 1988, performing a demonstration of Taoist yoga practices that he used in teaching martial arts. Impressed by both his skillfulness in yoga and his gentle, restrained manner, devoid of the arrogance he had observed in other martial artists, he sought Zink out in 1988 and studied with him for about one year. Paulie Zink, even more than his master, was convinced of the effectiveness of holding yoga poses for long periods of time in order to improve flexibility.:20 The poses Zink taught were sometimes similar, but often different from the poses known in Indian hatha yoga. Zink also included “yang”-style yoga in his classes—poses involving more movement and strength.:20
In 1989, Grilley met Hiroshi Motoyama, a Japanese scholar and yoga adept, who was researching the physiology of Traditional Chinese Medicine. Motoyama was especially interested in physiology of the meridians, or subtle pathways and vessels, and the qi or subtle energy hypothesized to flow through or get stored in them. These are both fundamental concepts in Chinese medicine and acupuncture. He related these to the parallel concepts of the nadi pathways and chakras of Indian yoga, and the prana said to be carried within them. He theorized that the connective tissues—the tendons, ligaments and fascia—are especially rich in these meridian pathways.:xiv
Grilley, who had previously been teaching yoga classes for seven years, now began to teach the long held poses he had learned from Zink, informed by his earlier study of anatomy with Garry Parker and the theories of Motoyama. He taught—as had Paulie Zink earlier—that the long-held Yin Yoga poses, which stress the connective tissues, would in addition to improving flexibility, gently stimulate the flow of qi in the meridian pathways, which would in turn nourish the organs and the whole body. Grilley at first called it Taoist yoga, in deference to the term Paulie Zink had used. Then one of Grilley’s students, Sarah Powers, a nationally-known US yoga teacher, also began teaching yoga in Grilley’s style, and decided to call it Yin Yoga in order to differentiate it from the various styles of Taoist yoga available. Grilley and Zink also soon adopted this name.
Sarah Powers traveled widely teaching Yin Yoga, and when her students asked for more information, she would refer them to Paul Grilley, who began receiving requests to travel and offer seminars. Paulie Zink continued teaching Yin Yoga in his own style, combined with other elements of Taoist yoga. Sarah Powers incorporated Buddhist psychology and meditation in her teaching of Yin Yoga.:24 Grilley and Powers began offering Yin Yoga teacher training courses, as did Paulie Zink. Over the next 10 years, Yin Yoga became available all over North America and in Europe, though yoga classes, and via DVD’s and books. In 2002, Grilley published the book, “Yin Yoga: A Quiet Practice” (and in 2012 a revised edition titled, Yin Yoga Principles and Practice). In 2008, Sarah Powers published the book, “Insight Yoga,” which features Yin Yoga along with principles of Chinese medicine and more active (or yang) yoga sequences that she says are complementary to Yin Yoga poses. Both Grilley and Powers have also released instructional DVD's on Yin Yoga, as has Paulie Zink. Grilley and others credit Sarah Powers with the widespread practice of Yin Yoga today.:xiii
Yin Yoga is based on the Taoist concept of yin and yang, opposite and complementary principles in nature. Yin could be described as stable, immobile, feminine, passive, cold, and downward moving. Yang is understood to be changing, mobile, masculine, active, hot, and upward moving. The sun is considered Yang, the moon Yin. In the body, the relatively stiff connective tissues (tendons, ligaments, fascia) are yin, while the more mobile and pliable muscles and blood are yang. More passive asanas in yoga are considered yin, whereas the more active, dynamic asanas are yang, because they stimulate the muscles and generate heat.
Sarah Powers and her colleagues say that Yin Yoga is most effective when more active forms of yoga or exercise are also practiced, either immediately before or after a Yin session, or separately.
Yin Yoga sessions as taught by Paul Grilley and Sarah Powers consist of a series of long-held, passive floor poses that mainly work the lower part of the body—the hips, pelvis, inner thighs, lower spine. These areas are especially rich in connective tissues, the systematic loading of which (yin yoga teachers avoid the word “stretching”) is a main focus in this style of yoga. However, Paulie Zink's approach to Yin offers a wider range of postures, including ones that work the upper body.:20
In a Yin Yoga class, the poses are held for up to five minutes and possibly longer. This is done, said Paulie Zink, to gently stress the connective tissue, which tend to get stiff and immobile with age. Zink originally learned to stretch for long periods as part of his martial arts training. In time, this practice is said to make the joints more elastic. Paul Grilley compares Yin Yoga practice to “joint rehabilitation,” and says that working the joints in this way acts as a preventive health technique, that it makes the joints and their core components stronger and more durable, parallel to the way in which aerobic exercise strengthens the cardiovascular system.
The second reason for loading the connective tissues is to stimulate the flow of qi or “life force” in much the same way as would occur in an acupuncture session. Yin Yoga employs specific sequences of poses geared toward stimulating particular energy channels (known as meridians in Chinese medicine, or nadis in Indian yoga), which are said to revitalize particular organ systems, for example, the kidneys, or the liver. This, in turn, is said to strengthen the immune system and enhance emotional well-being.
Although Yin Yoga poses or asanas closely resemble many of the asanas of Indian hatha yoga, they are performed differently, and have different names, in part to alert those who are familiar with similar poses in hatha yoga not to perform them the same way. For example, in the Cobra or Bhujangasana of hatha yoga, the practitioner lies prone and lifts the chest, curving the spine in an arc, reaching the legs back strongly. However in Yin Yoga, in the similar Seal pose, the legs are relaxed, and the upward movement of the trunk is entirely supported by the arms.
Paulie Zink's classes are quite different from the Yin Yoga taught by Grilley and Powers, in that he offers the full range of Taoist yoga practices, including poses and movements related to the five elements (see Facets of Yin Yoga below). :21
There are four main tenets in practicing Yin Yoga: 1) “Find an appropriate edge”: The practitioner moves slowly and gently into the pose, and looks for an appropriate amount of intensity, never stretching so far as to cause pain; 2) Stillness: the practitioner should consciously try to release into the pose, and to remain still, without shifting position; 3) Hold the position: Sarah Powers recommends that beginning practitioners hold for 1 to 3 minutes, and advanced practitioners five minutes or more. (It is advisable to use a timer so one can relax, unconcerned about the time); 4) Release with care.
In keeping with its roots in Taoist yoga, Paulie Zink says that Yin has a deeper level: to “open the heart and “invoke the primal self.”  Sarah Powers also emphasizes the subtler aspects of the practice, stating that it is effective in cultivating inner stillness, which she says nourishes creativity. She also says that because Yin poses by nature are often moderately uncomfortable, their practice cultivates the ability to distance oneself from the ups and downs of experience, and so builds inner strength.
Facets of Yin Yoga 
The complete art of Yin yoga as taught by Paulie Zink encompasses the following components:
Yin yoga: Mostly sitting or lying postures for promoting growth, clearing energetic blockages, and enhancing circulation.
Yang Yoga: More strenuous postures for developing core strength and muscle tone, balance, and stamina.
Taoist Flow yoga: This includes both Yin and Yang yoga postures practiced in continuous, smooth and circular motions. The technique of transition from pose to pose is integral to the practice.
Chi Kung: These exercises involve simple and gentle movement and breathing techniques.
Taoist alchemy: The principles of Taoist Alchemy are based upon the Taoist theory of the five elements that is used in Chinese medicine. As it is applied to Yin yoga, Taoist Alchemy is a method of embodying the energetic attributes of various animals and enlivening the five alchemical elements believed to be contained in the body’s energetic field. The five transforming energies of Earth, Metal, Water, Wood, and Fire animate distinct qualities in the body such as calm, strength, fluidity, springiness and lightness, respectively.:19
The Physiology of Yin Yoga 
Yin yoga targets connective tissue, specifically ligaments and tendons in the joints and spine. Over time, practice of Yin yoga can lengthen these tissues, increasing range of motion. To give an idea of the role that connective tissue plays in determining range of motion: muscles account for about forty percent of the resistance against the body's flexibility, while connective tissue accounts for about fifty percent. The intensity and physical benefits of the practice depend on two variables: duration of the asana, and the temperature of the muscle.
Duration of Asana 
In order to lengthen the connective tissue, the practitioner holds an asana, engaging in static stretching. This applies stress, in the form of tension, to the muscle and connective tissue in the targeted region. The muscle, more elastic than the connective tissue, responds immediately, lengthening to its limit. When the muscle is fully stretched, the stress reaches the connective tissue, which is not elastic and does not immediately lengthen. In order to affect the connective tissue, stress must be applied for several minutes at a time. In Yin yoga, asanas are usually held for three to five minutes, but can be held for as long as twenty minutes. Because of the long duration of asanas, patience is another of the key values cultivated in the practice of Yin yoga.
Temperature of Muscle 
If the muscles are cold, they will be less elastic, and more stress will be transferred to the connective tissue. For this reason, it is recommended that Yin yoga be practiced when the muscles are not yet warmed up. However, this is a general rule and for some people, it is better to stay a bit warm while practicing. Because this style of yoga does not generate bodily heat, it is good to keep the temperature of the room a little higher than usual.:33
Yin Asanas 
Yin yoga is almost entirely passive, although some Yin asanas contain some Yang elements. During the asanas, muscles are relaxed to avoid tetany, or muscle spasm, which could result from engaging muscles for long periods. Yin yoga as taught by Paulie Zink includes more than a hundred asanas; however, Paul Grilley, Sarah Powers and teachers taught by them put much less emphasis on variety, teaching in the range of 18 - 24 postures.:63
Influence on biochemistry and the meridiens 
According to the research of Hiroshi Motoyama and James Oschman, loading the connective tissue during yin or yang yoga stimulates fibroblasts to produce more hyaluronic acid (HA). Motoyama and Oschman say that HA may be the key to understanding the acupuncture meridian system. They explain that HA is a primary component of synovial fluid, and has the property of strongly attracting water, a good electrical conductor. If the theory is correct, it would explain why, by stimulating the production of HA, Yin Yoga strengthens both the body’s meridian system and the joints.
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