By Bernie Clark, March 20, 2020
This simple concern takes the form
There is no scientific evidence that Yin Yoga works.
We could refute this claim with the simple observation that the absence of proof is not proof of absence. Just because there is no scientific evidence that something does not work does not mean that it doesn’t work. It simply means it has not been proven yet. There could be many reasons for this lack of proof, including the simple fact that the topic has not yet been studied scientifically. However, we do not need to use this argument, because Yin Yoga has been studied and found effective. It does work!
Investigations into the science behind how Yin Yoga works is addressed in a Forum section at www.YinYoga.com called The Science Behind Yin Yoga and in an article called A Scientific Basis for Yin Yoga. This article explains that there are four levels of evidence that can be offered for any scientific assertion or theory:
1) Testimony (anecdote)
2) Argument (hypothesis)
3) Correlation
4) Experimentation
These are listed in order of increasing validity and acceptability: the anecdote is the weakest form of proof, while an experiment is the strongest. While weak, an anecdote is still evidence, and if your personal experience is that Yin Yoga works for you, makes you healthier or cured a specific ailment, then what more proof do you really need? Your personal, anecdotal experience should be considered strong proof that, for you at least, Yin Yoga is efficacious.
The strongest form of evidence is a double blind, controlled experiment: this is the “gold standard” in the medical industry. And there have been a couple of such experiments done with Yin Yoga (see the YOMI studies below). In addition, there are many other studies that, while not specifically investigating Yin Yoga, do investigate the effects of long-held, static stresses. Through these findings, hypotheses can be constructed that show that if the interventions in these experiments worked to improve the health and well-being of the subjects, then applying similar interventions in a Yin Yoga practice should have similar benefits.
Following this line of reasoning, the rest of this section lists studies that investigated either Yin Yoga directly or long-held static stretching and found positive results. Links are provided for the interested reader to look deeper into the research. The point should become clear: Yin Yoga does have significant scientific evidence to support its claim that it is beneficial for most practitioners.
The YOMI studies
A Swedish group looked at the relationship between Yin Yoga, mindfulness and stress and worry. They called their program YOMI.
The YOMI program is a psychoeducational training and physical practice-based program that bridges knowledge from evidence-based psychotherapy with the practice of mindfulness and yin yoga.[1]
The YOMI studies have shown directly that Yin Yoga can trigger the parasympathetic nervous system and provides many health benefits.
Their 2017 study found
[Yin] yoga stimulates the parasympathetic nervous system, which slows down breathing, heart rate and relaxes the body…[2]
Their 2018 study[3] found that a 5-week Yin Yoga-based intervention appeared to reduce both the physiological and psychological risk factors known to be associated with non-communicable diseases. The study suggests that incorporating Yin Yoga could be an easy and low-cost method of limiting the negative health effects associated with high stress.
For more details on the YOMI findings and to read the lead author (Frida Hylander) directly, visit the YOMI website.
Acute effects of Yin Yoga and aerobic exercise on anxiety
Another study, done in 2019, looked at the effects of Yin Yoga on anxiety and came to the conclusion:
…there was a significant reduction found in state anxiety and trait anxiety after the yin yoga, with no changes in trait mindfulness. The results indicate that yin yoga has an anxiolytic effect, although the uneven number of participants made comparison difficult. Yin yoga should be considered as a potential treatment option for anxiety.[4]
Historical precedence
The 19th century German anatomist, Julius Wolff, discovered that bone subject to constant or repetitive loading will become stronger, and thus more able to resist the deformation caused by the load. This observation became known as Wolff’s Law. The inverse is also true: an unloaded bone will become weaker. The mechanism by which the bone grows stronger is called mechanotransduction, which is a process through which forces or other mechanical signals are converted to biochemical signals. Simply put, bones need stress to be healthy and Yin Yoga can provide one form of the necessary stress.
Similarly, there is a law governing soft tissue development named after the early 20th century America orthopedic surgeon, Henry Davis. This is known as Davis’s Law and states,
Ligaments, or any soft tissue, when put under even a moderate degree of tension, if that tension is unremitting, will elongate by the addition of new material; on the contrary, when ligaments, or rather soft tissues, remain uninterruptedly in a loose or lax state, they will gradually shorten, as the effete material is removed, until they come to maintain the same relation to the bony structures with which they are united that they did before their shortening. Nature never wastes her time and material in maintaining a muscle or ligament at its original length when the distance between their points of origin and insertion is for any considerable time, without interruption, shortened.[5]
In more plain language, stresses are needed to maintain and regain optimal health and strength of both bony and soft tissues. One way to apply this stress is via long-held, static stresses such as those employed in Yin Yoga.
While all this has been known for over a century, there is more recent evidence supporting these findings, as shown in the next few sections.
Studies on static progress stretches
Static progressive stretch (SPS) is one of a group of therapeutic interventions used to help patients regain lost range of motion while decreasing pain, stiffness and swelling. It is used with patients who have undergone surgery or trauma that required immobilization of a joint. With these devices the joint is taken to its tolerable limit of movement (end range) for a period of time. Once the sensation ebbs, the patient then is free to increase the stress, taking the joint to a new end range, and again hold there. Through this progressive approach, at each setting the joint is subjected to a constant level of movement, and as the joint is held at this position, the stress level within the tissues lessens. Thus we have the name static progressive stretch. This is very similar what we do in the Yin Yoga practice. (More information on the similarities between Yin Yoga and SPS can be found in the article Static Progressive Stretch and Yin Yoga.)
The following are studies that have shown the effectiveness of SPS:
- R. Costa, M.J. McElroy, A. Johnson et al., “Use of a static progressive stretch orthosis to treat post-traumatic ankle stiffness,”BMC Research Notes 5 (2012): 348.
- M. Bonutti, G.A. Marulanda, M.S. McGrath et al., “Static progressive stretch improves range of motion in arthrofibrosis following total knee arthroplasty,” Knee Surgery, Sports Traumatology, Arthroscopy 18.2 (2010): 194–199.
- D. Ulrich, P.M. Bonutti, T.M. Seyler et al., “Restoring range of motion via stress relaxation and static progressive stretch in posttraumatic elbow contractures,” Journal of Shoulder and Elbow Surgery 19.2 (2010): 196–201.
- Ibrahim, A. Johnson, R. Pivec et al., “Treatment of adhesive capsulitis of the shoulder with a static progressive stretch device: a prospective, randomized study,” Journal of Long-Term Effects of Medical Implants 22.4 (2012): 281–291.
Splinting can also create long-held, static stresses and help to avoid contracture
A 1987 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 researcher 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.[6]
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.[7]
In this case, Yin Yoga-like stresses proved more efficacious for joints suffering contracture than yang forms of stresses. Another way to state the findings is,
Time is more important than intensity!
Studies on acupressure and long-held stresses
Teams lead by Helen Langevin, director of the National Center for Complementary and Integrative Health and past professor at Brigham and Women’s Hospital – Harvard Medical School, have shown how acupressure and acupuncture work their magic. These studies employ long-held, static stresses of 10 minutes once or twice a day over a period of a few weeks in mice. The results of these practices are fascinating.
1) Sarah M. Corey, Margaret A. Vizzard, Nicole A. Bouffard, Gary J. Badger and Helene M. Langevin, “Stretching of the Back Improves Gait, Mechanical Sensitivity and Connective Tissue Inflammation in a Rodent Model,” PLoS One 7.1 (2012): e29831.
This study used mechanical stretches lasting 10 minutes each, twice a day for 12 days. The tissue stretch mitigated inflammation-induced changes leading to restored stride length and intrastep distance, decreased mechanical sensitivity of the back and reduced macrophage expression in the nonspecialized connective tissues of the low back.
2) L. Berrueta, J. Bergholz, D. Munoz, I. Muskaj, G. J. Badger, A. Shukla, H. J. Kim, J. J. Zhao and H. M. Langevin, “Stretching Reduces Tumor Growth in a Mouse Breast Cancer Model,” Scientific Reports 8 (May 18, 2018).
In this study, mice were injected with breast cancer tumors and divided into a control group and a stretch group. The stretch group was subjected to one full body stretch, held for 10 minutes, once a day for 4 weeks. After the intervention, the stretch group’s tumors were 52% smaller than those in the control group.
3) Ying Xiong, Lisbeth Berrueta, Katia Urso, Sara Olenich, Igla Muskaj, Gary J. Badger, Antonios Aliprantis, Robert Lafyatis and Helene M. Langevin, “Stretching Reduces Skin Thickness and Improves Subcutaneous Tissue Mobility in a Murine Model of Systemic Sclerosis,” Frontiers in Immunology 8 (February 16, 2017).
This study investigated whether stretching would affect scleroderma, which is an autoimmune disorder that can alter skin, blood vessels, muscles and internal organs, making them weaker and thicker. Holding a full body stretch for 10 minutes, once per day, over 4 weeks improved skin thickness and mobility.
4) Highly recommended reading is an article Langevin wrote in May 2013 called The Science of Stretch, published in The Scientist. She discusses her research into long-held (up to 30 minutes) stresses on connective tissues and how it can cause a remodeling of the tissue and the architecture of the fibroblasts. She also discusses the mechanism by which acupuncture or acupressure reduces pain.
Studies of myofascial release
A 2015 study was done to answer the question of how much stretch and for how long is optimal to reduce pain, increase range of motion and improve physiologic function of joints. Myofascial release (MFR) is a physical therapy using long-held, static stretches. It was employed to stress joints and was shown to reduce inflammation and improve healing response to injury.
The researchers found that a strain (elongation) of 3% held for 5 minutes is ideal. Stresses above that level, which created a strain of 9% or 12% (that is a lot!), can cause 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 1 to 5 minutes. This, in our Yin Yoga context, means less effort and longer holds are better. The researchers concluded, “Longer duration of MFR resulted in rapid decreases in wound size.”[8]
Jules Mitchell, a yoga teacher and researcher, commented on this study: “Fibroblasts are highly responsive to differences in strain magnitude and duration. It appears that low magnitude and long duration is most effective for tissue repair.”[9] Once again we find that time is more important than intensity. That is Yin Yoga!
Fascial fitness training can employ long-held, static stresses
Robert Schleip is a director of the Fascia Research Group, Division of Neurophysiology at the University of Ulm in Germany and author of the book Fascial Fitness.[10] He studies how fascia works and how to make it work better. Fascial training may use small, elastic bounces while our muscles are in a stiffened position as well as slow, relaxed, static stretching of the kind we do in a Yin Yoga practice.
In Schleip’s book he reports that fascial exercises ideally include both tensile stresses, which can cause a stretch, and compressive stresses, which squeeze our tissues. Compression and shearing stresses, which move two areas of the body in the opposite directions, stimulate metabolism and the supply of fluid in the tissues. Tissues are first dehydrated: water that contains inflammation-causing free radicals is squeezed out. Then, after the stresses have ended, the tissues slowly rebound, rehydrating with clearer water. Particularly effective in dehydrating and rehydrating tissues is the application of slow, persistent, yet gentle pressure, the kinds of stresses found in several forms of massage and in Yin Yoga. These forms of fascial exercise stimulate the release of anti-inflammatory messengers, which help to reduce friction and stuckness between fascial layers.
Schleip’s book is based on many researchers’ findings as well as his own studies. He and colleagues found that when mice lumbodorsal fascia were isometrically stretched for 15 minutes followed by a 30 minute rest, an increase in stiffness was observed. Subject to similar loading procedures, tissues showed decreases in fluid content immediately post-stretch and increases during rest phases. When allowed sufficient resting time, a super-compensation phenomenon was observed, characterized by matrix hydration higher than initial levels and increases in tissue stiffness. In other words, an alteration of the water content was observed.[11]
The switching of the state of water, from a gel-like state (think of Jell-O) to a solution state (liquid) and back again is a necessary component of healthy living. Yin Yoga is one way to help trigger these transitions.
Return to Topics
[1] Frida Hylander et. al., “Yin yoga and mindfulness: a five week randomized controlled study evaluating the effects of the YOMI program on stress and worry,” Anxiety, Stress & Coping 30.4 (2017): 365–378.
[2] Ibid.
[3] See Daiva Daukantaitė et al., “Five-week yin yoga-based interventions decreased plasma adrenomedullin and increased psychological health in stressed adults: A randomized controlled trial,” PLoS One (July 18, 2018).
[4] Denis Winroth et. al., “Acute Effects of Yin Yoga and Aerobic Exercise on Anxiety,” Alternative & Integrative Medicine 82.2: 278.
[5] From John Nutt, Diseases and Deformities of the Foot (E.B. Treat and Company, 1913).
[6] George R Hepburn, “Contracture and Stiff Joint Management with Dynasplint,” Journal of Orthopaedic & Sports Physical Therapy 8.10 (1987): 498–504.
[7] Ibid.
[8] Thanh V. Cao et al., “Duration and Magnitude of Myofascial Release in 3-Dimensional Bioengineered Tendons: Effects on Wound Healing,” The Journal of the American Osteopathic Association 115.2 (2015): 72–82.
[9] See Jules Mitchell, Yoga Biomechanics: Stretching Redefined (Handspring Publishing, 2018), 131.
[10] See Robert Schleip, Fascial Fitness (Lotus Publishing, 2017).
[11] See R. Schleip, L. Duerselen, A. Vleeming et al. “Strain hardening of fascia: static stretching of dense fibrous connective tissues can induce a temporary stiffness increase accompanied by enhanced matrix hydration,” Journal of Bodywork and Movement Therapies 16(2012): 94–100.