How Yin Yoga Affects Connective Tissue

The first paper I wrote for the Insight Yoga Institute was an essay on what Yin Yoga is and why anyone would want to practice it. We were supposed to cover the physical, energetic and mindfulness aspects of holding passive postures for several minutes.

I got dinged for suggesting that Yin Yoga stretches connective tissue, and was instructed to use the word stress instead. Of course, “stretching” isn’t the only thing we do in Yin Yoga, but we also have a bit of a semantic problem in English because stress is viewed as a bad thing even if it’s the more accurate thing to say in this case. When we talk about stress in biomechanics, we are talking about a variety of mechanical loads and the term doesn’t have the same negative connotation as it does in the yoga room.

Since then, my antennae are attuned to the language of Yin Yoga, particularly the claims and criticisms, which sometimes have a semantic origin. I’m a naturally skeptical person, and I tend to want someone to prove it before I believe it, no matter what “it” is. It’s not enough for me to know that Yin Yoga targets connective tissue. I want to know the how and the why. I want to get it right. I also want to do no harm.

What I’ve discovered is that there is a lot of misinformation about what Yin Yoga does. Some of the claims are a little exaggerated, which has given Yin Yoga’s critics a lot of ammunition. On the other hand, the critics rely just as much on anecdotes and gut feelings as evidence.

Part of the problem is that there are no studies of Yin Yoga. What we have is a promising amount of research on fascia that we can use to draw some reasonable conclusions about how Yin Yoga might be affecting our tissues. But there is still a lot of uncertainty.

At the end of the day, I think we (both Yin Yoga’s supporters and detractors) want to know if the practice is safe.

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All the connective tissues found in the body, including fascia, tendons, and ligaments, are comprised of cells, fibers (collagen and elastin), and a gel-like fluid called ground substance. The ratio of these elements varies depending on the type of tissue and its location in the body. For instance, your Achilles tendons are thicker and more fibrous than your earlobes. But they are, more or less, the same thing.

The differences in the fiber/fluid make-up of these tissues match their functions. An Achilles tendon is thick and elastic so that it can absorb impact and produce power for running and jumping. But the fascia that wraps around the individual muscles fibers in our thighs is less thick and contains a lot of ground substance to reduce friction.

Researchers have also discovered that the separate connective tissue types respond differently to different loads. What a ligament needs to maintain its optimal function is not the same as a joint capsule. The overall takeaway from the research is that to sustain the robustness of all of our tissues we need to engage in a variety of activities. Healthy loads include the kind of passive, static stretches and compressions of Yin Yoga.

In particular, we know that static stretching stimulates the deep layers of fascia that wrap around the bundles of muscle fibers. Also, the fascia that connects muscles to one another is affected by passive stretches. But, passive loads don’t affect tendons and ligaments. Due to their arrangement, the relaxed muscle fibers absorb most of the tissue lengthening (Schleip, Muller 2012).

But, what do we mean by “stimulates” and “affected by?” What’s really going on here?

The primary cells found in fascia are called fibroblasts, and their main job, among a couple of other things, is to create more fascia (Schleip et al. 2012). The brain doesn’t control the fibroblasts. Instead, fibroblast behavior is determined mainly by the mechanical loads (or lack of loads) placed on them.

In Yin Yoga, we are mainly interested in the effects of compressive and tensile (stretch) loads on our tissues. The sensation you feel in the low back during sphinx or seal pose is a result of compressive forces on the soft tissues and vertebrae. When you fold forward in butterfly pose, you are stretching the back.

The fibroblast cells will adjust the production of collagen, elastin, and ground substance to create an architecture best suited for the demands placed on them (Benjamin et al., 2005). These loads need to be progressive (appropriately increased) and occur over an extended period of time. When it comes to remodeling connective tissue, lengths of time are measured in months and years (Schleip 2012).

Researchers have found that the fibroblasts in tendons and ligaments adapt to compressive forces by producing strong, fibrous collagen that can withstand additional forces (Benjamin et al., 1998). Without specific research, we can reasonably conclude that compressive Yin Yoga poses contribute positively to fascia health.

The effects of tensile loads on connective tissue is less definitive. There just isn’t a lot of research on stretching and what we have has produced mixed results. Multiple variables such as intensity, duration, repetition, etc. make stretching challenging to study. So, we are left with contradictory information on, and no consistent guidelines for, what is beneficial, optimal, or even harmful (Myers 2012). To make it more complicated, we (the royal we) keep changing our minds. One day we aren’t supposed to bounce when stretching, now it’s the next big thing (Schleip et al. 2012).

Can Tissues Lengthen When Stretched?

This seems to be the big point of contention when it comes to Yin Yoga. The short answer seems to be yes, BUT…

The better questions might be: how long does this lengthening last? Can we retain new length permanently? Is there an end point to how much we can stretch before it becomes harmful? Unfortunately, not only are the questions tricky, but we can’t answer them with certainty.

Here are a few things we do know.

Connective tissues do have some plasticity, the ability to undergo change under a load. We know that slow, sustained tensile loads change tissue length more effectively than a quick stretch (Myers 2012). Think of a plastic grocery store bag. If you tug on it quickly, it resists change. But if you pull on it slowly and carefully, it reshapes and permanently deforms. Eventually, it will tear.

The term for our tissues’ ability to stretch is called creep. Creep is measured by the amount of force applied, rate, and length of time. Of interest to Yin yogis: our tissues creep even if you don’t increase the loads (Mitchell 2014).

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Here’s an analogy I like to use: What do you do with a nearly empty bottle of hair conditioner to get the rest of it out? You turn the bottle upside down and wait. What’s happening is that the steady (not increasing) force of gravity will slowly pull the conditioner down to the opening of the bottle. The relationship of time to creep seems to indicate that even mild Yin Yoga poses could lengthen our tissues

I once had a student tell me that she felt like Yin Yoga postures “lasted longer than regular yoga.” But I’ve never heard someone say that the effects last forever, and this certainly hasn’t been my experience.

The plastic bag analogy only goes so far because plastic bags aren’t living tissue. Fascia is also elastic, so when you remove the loads, the tissues gradually recover their original shape as long as they aren’t overstretched and damaged. Some studies have shown that very long holds of around 20 minutes can take hours to recover (Clark 2016). But they do recover eventually.

There is also another factor to consider when we ask ourselves what are we doing in Yin Yoga? Involuntary tissue contraction.

This will surprise very few people, but we humans unconsciously contract our tissues in response to stress (physical, psychological, emotional, etc.). If we are under stress for an extended period of time, we can “forget” how to relax, meaning that our tissues may remain in a constant contractive state even when the stressor has been removed (Myers 2012).

Further, we find that this constant state of contraction leads to a thickening of the fascia (Langevin et al. 2009). The research suggests that many people aren’t feeling an increase in range of motion after stretching, but rather they experience a return to a normal resting state.

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On the cellular level, stretching does produce changes, which over time, can create adaptations and remodeling of the tissues. Just like compression, tensile loads on connective tissues stimulate the fibroblasts to produce more collagen fibers, which increases fascial thickness and strength (Myers 2012). What we don’t know is how long one must hold these loads for meaningful and permanent changes to take hold. Nor do we know how much is too much before we cause damage. Likely this “Goldie Locks” sweet spot is different for everyone.

Other Interesting Benefits of Yin Yoga

Approximately 70% of the volume of fascia is made up of water found in the non-fibrous ground substance. Both stretching and compression play a significant role in tissue hydration. When you load tissue, fluid is squeezed out and into the lymphatic system, including fluids present as a result of swelling (Myers 2012). When the loads are released, and the tissues are left to rest, new fluid is pulled back in like a sponge (Klingler et all 2004). Loading our tissues, especially neglected ones, refreshes them.

We did an informal study of this phenomenon in my Yin Yoga training with Paul Grilley. We measured the height of the same person in the morning, afternoon, and after Yin Yoga practices. Throughout our day, humans lose height as a result of fluid loss in our vertebral discs. When we measured our subject after a Yin Yoga practice, we consistently saw that she regained some of her lost height as her spinal tissues rehydrated.

Well-hydrated tissues have a springy quality that helps them to return to their original shapes after being loaded. Hydrated tissues also respond better and more efficiently to stress. And, well-hydrated tissues can more easily transfer nutrients to the cells and rid the body of toxins (Clark 2012).

Perhaps one of the most intriguing discoveries about fascia is that it is our greatest sensory organ, and it plays a major role in our proprioception (van der Waal 2012) and interoception (Schleip, R., Jager, H. 2012). Movement modalities that emphasize physical awareness and internal listening can help refine the sensory nature of our tissues. Preliminary studies have shown an inverse relationship between pain and proprioception, making this a worthwhile pursuit.

This is particularly exciting for those of us who practice Yin Yoga. When we hold non-neutral shapes for 5+ minutes, we have a lot of opportunity to practice listening to our outer and inner states. Taking our time between poses to notice the before and after effects can also enhance our sensing abilities. Though not impossible, it is much harder to pay attention to how we feel in a more dynamic practice since much of our mental energy is devoted to orchestrating our movements.

Is Yin Yoga Safe?

Critics of Yin Yoga worry that practitioners can overstretch their joints. Because there are no specific studies of Yin Yoga to date and relatively few studies of stretching overall, it’s hard to refute this claim completely. We don’t know what our thresholds are when it comes to stretching.

It is possible that someone could overdo it in a Yin pose. The problem could be that a pose is not appropriate for that person due to her unique anatomy or physiological state. It’s also possible that everything in her movement history up to that moment could make a singular pose the last straw. This could happen in any yoga class as well as a number of other movement modalities. It doesn’t make Yin Yoga inherently more dangerous than other forms of exercise.

It’s also theoretically possible that someone could, over time, practice so much Yin Yoga that he overstretches his tissues to the point of instability. But we should also remember that the loads in Yin Yoga are relatively low, and some might even say that body weight isn’t progressive enough to create lasting change anyway. When you consider that our tissues start to recover to their original starting point as soon as you remove the load, it seems unlikely that a semi-regular Yin Yoga practice would be dangerous. Of course, a lot would depend on what else you’re doing with your body when you’re not practicing Yin Yoga.

In my own purely subjective experience as a woman in her early 40s, I have noticed that I need to target the same area for at least a week before I experience what feels like lasting increases in range of motion. I find this particularly true of side bends. We don’t do a lot of side bends in our daily life, and often the only time I do them is in my yoga practice. If I stop doing them for even just a few days and I return to them later, I find that I lose those flexibility gains.

This is due to a process called contracture, which is a loss of mobility in a joint. Contracture can be a result of an injury or illness, but immobility also causes it. Paul Grilley points out that you rarely, if ever, see very elderly people walking around loose and fancy-free. Instead, as we age the body tends to retract. Our gait becomes shorter because of loss of range of motion, and we shuffle along hunched over.

I observed this in my mother-in-law who, in her early 70s, was a champion rower in the world masters circuit. She also ran regularly and would get down on the floor to crawl around with our then-toddler. A few years ago she suffered a stroke and had to give up all of that. The loss of her mobility and agility was striking. Some of it is due to the effects of the stroke, but a lot of it was the inevitable consequence of immobility.

Our tissues need both compressive and tensile loads to keep us feeling vibrant and youthful.

Creep recovery and contracture make it pretty hard to go overboard with a Yin Yoga practice. I think you would really have to try. This is especially true when you remember that Yin Yoga teachers will remind you to approach the practice with the spirit of being rather than doing. The whole gestalt of Yin Yoga is to do less.

When practiced with this sensibility, I believe that Yin Yoga is about as safe as, if not safer than, any other form of yoga. But I don’t think it is appropriate for everyone. A person’s biology, history, life stage, pathologies, preferences all matter. There is no one size fits all.

Hi! I’m Jennifer O’Sullivan (Sati Yoga). I write about yoga, meditation, stress management, functional anatomy, and bit of this and that about living a healthy life. I teach yoga classes and workshops in the Washington, DC area, and you can find me online at Facebook or www.sati.yoga.
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~~~ Notes ~~~

Benjamin, M., Putz, R., “Molecular parameters indicating adaption to mechanical stress in fibrous connective tissue.” Advances in Anatomy, Embryology and Cell Biology (2005).

Benjamin, M., Ralphs, J.R., “Fibrocartilage in tendons and ligaments — an adaption to compressive load.” Journal of Anatomy (1998).

Clark, B., The Complete Guide to Yin Yoga (2012).

Clark, B. “Creep and Counterposes.” (http://www.yinyoga.com/newsletter34_creep.php#fn12) YinYoga.com (2016)

Klingler, W., Schleip, R., Zorn, A., European Fascia Research Project Report. 5th World Congress Low Back and Pelvic Pain, Melbourne. (2004)

Langevin, M., Bouffard, N., Fox, J., Palmer, B., Wu, J., Latridis, J., Barnes, W., Badger, G., Howe, A., “Fibroblast cytoskeletal remodeling contributes to connective tissue tension.” Journal of Cellular Physiology (2011).

Mitchell, J. “Creep and Recovery.” (http://www.julesmitchell.com/creep-and-recovery/) Jules Mitchell Biomechanics (2014).

Myers, T. Frederick, C., “Stretching and Fascia.” Fascia: The Tensional Network of the Human Body (2012).

Schleip, R. Jager H., Klingler, W. “Fascia is Alive: How cells modulate the tonicity and architecture of fascia cells.” Fascia: The Tensional Network of the Human Body (2012).

Schleip, R., Muller, D.G., “Training principles for fascial connective tissues: Scientific foundation and suggested practical applications.” Journal of Bodywork & Movement Therapies (2012).

Schleip, R. Jager H., “Interoception: A new correlate for intricate connections between fascial receptors, emotion and self recognition.” Fascia: The Tensional Network of the Human Body (2012).

van der Waal, J., “Proprioception.” Fascia: The Tensional Network of the Human Body (2012).