"I know that I feel great after doing my Yin Yoga practice, but I wonder if Yin Yoga really works. Is there any scientific evidence you can provide that proves Yin Yoga is good for me?"
This question, and others just like it, is often asked in emails I receive from students and teachers. Another similar question goes something like this: "I heard from another yoga teacher that Yin Yoga is dangerous and I shouldn't be stressing my joints or compressing my lower back. Do you have any scientific evidence that Yin Yoga is healthy that I can take back to my teacher?"
The biopsychosocial model of health
When surrounded by doubt, it is logical to seek some assurance that you are on the right track, but to fully answer these concerns we have to step back and look at a bigger question: Why do you do yoga? If your intention is to gain or maintain optimal health, we then have to ask, "What is health?" A new model of medicine is becoming commonly used called the biopsychosocial model. This model recognizes that health is not simply having a physically sound body. Indeed, the word "health" comes from an old English word that means "whole," and optimal health is wholeness of mind, body, and soul within your physical and social environment. All of these are factors to consider when we look at how our yoga practice helps us gain and maintain health.
We could spend some time discussing the psychological benefits of Yin Yoga, but this is not the kind of proof the students and teachers asking the above questions are seeking. They are seeking proof of a biological benefit for Yin Yoga. However, the psychological benefits are very important and worth addressing at least superficially before delving deeper into the biological basis for Yin Yoga's efficacy. Remember, while you are marinating in your Yin Yoga postures, you are provided a wonderful opportunity to deepen your meditation or mindfulness practice. Many studies since the 1970's have shown that a regular meditation or mindfulness practice reduces stress, stress-related illness, reduces blood pressure, heart disease, reduces depression, anxiety, the problems of diabetes, turns off the flight-or-fight response, improves wound healing, interpersonal relationships, coping skills, increases cognitive abilities, thickens the prefrontal cortex of the brain, ... and on and on. While the studies documenting these benefits have been conducted on meditation, it is not a great leap to conclude that the time spent doing Yin Yoga will tap into the same well of healing and wholeness that meditation provides.
Unfortunately, when students ask, "Yes, but - does Yin Yoga really work?" they are not asking about the mental or emotional benefits, despite they fact that these are real and are valuable. They want proof of the physical benefits.
The levels of scientific evidence
There are four levels of evidence that can be offered for any scientific assertion or theory:
This bears repeating: if your experience is that Yin Yoga works for you, why would you doubt your own experience? It is interesting to understand why it works for you, but don't deny the evidence in front of your own eyes simply because someone else is not convinced. Students who are insecure may devalue their own life experience in the face of an authoritative figure (like a senior yoga teacher or doctor who expresses strong convictions), but your own experience is the ultimate teacher. If something works for you, it is probably good; and if something doesn't work for you, no matter how prestigious the teacher or doctor suggesting it, drop it.
Science begins with the facts; data are observations, empirically derived and undeniable. From the facts must flow the theories, not the other way around. This is not to imply that looking for a deeper understanding as to why the facts are they way they are pointless --- far from it! Understanding the "Why does this work?" can help to improve the process and obtain even greater benefits. The point is to ensure that we do not let hypothesis and experimentation on others diminish the reality that you experience for yourself.
With this understanding, our question becomes, "Why does Yin Yoga work for me?" From a physiological perspective there are several possible avenues we can explore: effects in the connective tissues, effects within the muscles, effects within the nervous system, the immune system, the endocrine system and even within the epigenetic system. To fully appreciate everything Yin Yoga does for us in all these areas would take a book; so let's restrict our investigation to the connective tissue. This reduces our question to a more specific "Why does Yin Yoga make my connective tissues healthier?"
"Why does Yin Yoga make my connective tissues healthier?
"Unfortunately, I am aware of no studies or experiments evaluating Yin Yoga specifically. There are many studies looking at Yoga or stretching, but all of these, from what I have seen, employ relatively short stress times, on the order of 20 seconds or so. Some of these studies cast yoga in a negative light and conclude that yoga before an athletic event is counterproductive. For example, one study found that stretching before the main sporting activity reduced performance, and thus one should not stretch as part of the warm up. From this, the conclusion arose in the minds of some readers that yoga will not increase athletic performance. However, this is not a legitimate conclusion given the nature of the study: the study did not look at the long term effects on health and performance of a regular yoga practice, it only looked at the effects of short term stretching immediately before an athletic event. Most yoga teachers would probably agree that doing a yoga practice right before a sporting event is not ideal. For example, it would not be a good idea to do a Yin Yoga practice just before playing a game of hockey. Reflexes will be considerably slower. ("Oh look ... there ... is ... the ... puck ...")
In any case, all of these studies on yoga or stretching employed short hold times. Yin Yoga regularly employs much longer stress times, ranging from 1 to 10 minutes or more, but usually the postures are held from 3 ~ 5 minutes. Since we can not point to any Yin Yoga specific studies, we have to extrapolate from studies that look at the effects of longer held stresses on the connective tissues. Fortunately, there are a few studies available.
Yin Yoga-like studies
One technique used in Yin Yoga is the application of a distraction force. Distraction (often called traction) is the application of a long-held stress that tends to pull bones apart. One study found that distraction stimulates both the growth of bones and their associated ligaments. When we do a forward fold, like we do in Caterpillar or Butterfly Pose, we apply a distraction stress along the spine. This study suggests that this stress can help strengthen the bones and ligaments along the spine. However, again, we can only make this a conjecture, using the second level of scientific evidence, because the study did not look specifically at Yin Yoga.
Going in the other direction, if a joint is overly contracted and range of motion is limited, the treatment is not surprising for any student of yoga: mobilization. A study of contracture repair contrasted short, intense stresses like we find in an active yoga practices with long held, mild stresses like we find in a Yin Yoga practice. The researchers concluded, "The longest period of low force stretch produces the greatest amount of permanent elongation, with the least amount of trauma and structural weakening of the connective tissues." The shorter, more intense stresses (more yang-like) were observed to have resulted in "a higher proportion of elastic response, less remodeling, and greater trauma and weakening of the tissue."? In this case, Yin Yoga-like stresses proved more efficacious for joints suffering contracture than yang forms of stresses.
Another study utilized stress in exactly the same way that we do in Yin Yoga, in fact this particular study looked at the effects of a 5 minute stretch of the whole body, done twice a day, and found, "Mechanical input in the form of static tissue stretch has been shown ... to have anti-inflammatory and anti-fibrotic effects." In other words, holding a whole body stretch for 5 minutes helped improve the body's immune response and reduced fibrotic restrictions to movement. Unfortunately, while the posture used in the study was definitely Yin Yoga-like, the student was a rodent, and we must extrapolate the benefits of rat yoga to humans cautiously.
In the world of physiotherapy, long held stress results in myofascial release (MFR) of tension, which has been shown to improve healing response. Studies have shown that the lower the amount of stress and the longer its duration, the better the wound healing response. The specific myofascial release technique employs slowly applying an external mechanical load that overcomes the fascia or tendon's intrinsic tension to lengthen the collagen fibers. Myofascial release is designed to stretch and elongate the fascia and underlying soft tissue to release areas of decreased fascial motion. This is very similar to what we do in our Yin Yoga practice: we come to an edge, not the deepest possible position, but a place where we feel some challenge, and we soften and hold for time. One particular study, done in vitro (which means outside the body, in a petri dish) found several very technical, but important results of MFR, including improved physiological function, pain reduction and increased range 
One particular posture used in Yin Yoga is the Sphinx Pose: lying on your belly, resting on your elbows, which raises your torso up into a mild backbend. This position is also used in a form of physiotherapy called McKenzie Therapy, and can assist people who suffer from a bulging disc. Spine biomechanic and university professor Stuart McGill notes, "we now have proof that the extended postures [of the spine] can drive the nucleus material that is in the delaminated pockets of the posterior nucleus back towards the central part of the disc." In other words, the Yin Yoga pose called Sphinx can help some people with their bulging discs (but not everyone!)
The benefits of yoga in general
Because there are no studies looking specifically at Yin Yoga, we have to extrapolate from experiments using Yin Yoga-like techniques. This is the second form of evidence; it is not ideal but it does offer some proof that Yin Yoga does work. There are studies of the yang practice of yoga, however. A meta-study (which is a study of studies) done in 2011 summarized several significant benefits from a regular yoga practice. These included
1) There are many resources an interested reader can review for further details, such as Herbert Benson's book, The Relaxation Revolution; the studies by Jon Kabat-Zinn, or an interview with researcher Sara Lazar in the Washington Post.
2) These levels were developed by Dr. Roy Walford and are described in his book 120 Year Diet, pages 22 - 25.
3) From The Biomechanics of Stretching by Duane Knudson in the Journal of Exercise Science & Physiotherapy, Vol. 2: 3-12, 2006: "It is clear that from the standpoint of maximizing muscular performance, stretching creates an acute decrease in performance, therefore stretching should not normally be recommended prior to exercise with apparently healthy individuals, but be programmed during the cool-down after exercise training."
4) See The biology of distraction osteogenesis for correction of mandibular and craniomaxillofacial defects: A review by Subodh Shankar Natu et al in Dental Research Journal 2014 Jan-Feb; 11(1): 1626.
5) Contracture and Stiff Joint Management with Dynasplint by George R Hepburn.
6) The elastic response occurs when the tissues return to the original lengths.
7) See Stretching of the Back Improves Gait, Mechanical Sensitivity and Connective Tissue Inflammation in a Rodent Model by Sarah M. Corey et al in PLoS ONE | www.plosone.org January 2012 | Volume 7 | Issue 1 | e29831
8) The above study looked at in vivo effects in rats of static stresses, and was very yin-like: the stretches were held for 5 ~ 10 minute, twice a day, each day for 12 days in a row. The stretch was a whole-body stretch. The results showed that in healthy rats, this exercise improved mechanical sensitivity to low back pain. Rats with simulated inflammation showed more robust benefits from the exercises in sensitivity to pain, gait and superficial fascia thickness (the rats doing the exercises had thinner fascia than the control group, presumably due to less inflammation.) This study seems to imply that long held, static stresses may help with non-discogenic low back pain, but we cannot infer from this one study whether the cause of the findings are due to localized effects in the fascia or systemic effects from the whole body stress.
9) From Duration and Magnitude of Myofascial Release in 3-Dimensional Bioengineered Tendons: Effects on Wound Healing by Thanh V. Cao et al in J Am Osteopath Assoc. 2015;115(2):72-82: "Documented clinical outcomes associated with MFR include improved physiologic function, reduction of pain, and increased range of motion in the affected joint. Our previous findings revealed that modeled MFR applied in vitro inhibits the cytotoxic effects of fibroblasts, alters fibroblast actin architecture, and induces the expression of various anti-inflammatory and growth factors. We have also shown that MFR downregulates inhibitory factors on protein kinase C and phosphoinositide 3-kinase to sensitize fibroblasts to nitric oxide and improve wound healing. Recent data from our laboratory have also shown that in a 3-dimensional environment, fibroblast cytokine secretion is dependent on MFR strain duration and magnitude, suggesting a correlation between physiologic response and dosed MFR."
In this study the researchers found that a strain of 3% is ideal. Stresses above that level, which creates a strain of 9% or 12% (which is a lot!), can cause worse problems, but a stress resulting in only a 3% strain can accelerate healing. The longer the stress is applied the better. The lengths of time varied from 0, 1, 2, 3, 4, or 5 minutes, and was then slowly released back to baseline. This, in our Yin Yoga context, means less effort and longer holds is better. They concluded, "Longer duration of MFR resulted in rapid decreases in wound size."
10) See Low Back Disorders by Stuart McGill, pages 43 and 47.
11) But McGill also warns, "I am cautious about having people routinely engage in these postures following recovery from the acute episode-the spine may pay a price ... Some individuals have "kissing spines" in which adjacent posterior spines collide in full extension at one level. The involved level is usually due to a simple case of anatomic variation. These may become more frequent in people with disc height loss where the posterior spine becomes more approximated." Professor McGill is echoing the words of wisdom of Paul Grilley: everybody is different and for some people Sphinx pose may be excellent, but for others it is not a great idea.
12) From Exploring the therapeutic effects of yoga and its ability to increase quality of life by Catherine Woodyard in Int J Yoga. 2011 Jul?Dec; 4(2): 49-54
ARTICLE SOURCE: https://www.yinyoga.com/newsletter29_science.php