Yoga Physiology

Yoga physiology, anatomy and movement science

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News about the blog

My apologies as I’ve been lax about keeping up the blog. Following Hurricane Sandy, which gave me more time to write, I’m re-committing to blogging. I plan on writing posts more regularly, but I’ve decided to switch the blog over to my own domain: Just the name is different. The subject matter will be the same. I’ll still be writing about yoga anatomy and physiology and practice, movement science and health.

Unfortunately, if you’re a subscriber, I haven’t figured out to re-subscribe you to the new blog. I’m sorry about that, but if you want to continue to follow the blog, just go to the new URL ( and re-subscribe.

Thank you for reading for what I’ve written so far, and I hope you’ll follow me to the new site.


Research review: The effects of bhastrika in the elderly, part 3

If you haven’t read the previous posts about this study, you should read them first.

In my last post about this study (1), I described how four months of pranayama practice affected respiratory function in elderly yoga practitioners. Today, we take a look at how the practice of bhastrika impacted heart rate variability and the functioning of the autonomic nervous system.

Before delving into heart rate variability, a review of the autonomic nervous system is in order. The autonomic nervous system regulates numerous systems of the body, controlling circulatory, respiratory, digestive, endocrine, immune and metabolic processes. In general, it manages internal functions that occur without conscious control. That’s where the name comes from: early researchers envisioned it as acting autonomously, or independently, of conscious volition. In fact, however, all parts of the nervous system are integrated and function cohesively. It’s true that most people can’t voluntarily alter their heartbeat as you might vary the rhythm of a tapping finger. Nevertheless, input from the conscious part of your brain as well as from emotional centers affects the autonomic nervous system, and vice versa. Think of how your heart beats as you wait to go on stage to make a major presentation, and how you could choose to slow it down by taking several long, conscious breaths. That indirect linkage between volitional action and autonomic function explains how practices such as yoga can influence internal physiology.

The autonomic nervous system

The autonomic nervous system is generally divided into two major divisions, the parasympathetic and the sympathetic. You’ll often hear the parasympathetic division referred to as the “rest and digest” system, and the sympathetic division as the “fight or flight” system. As the catch phrase suggests, the sympathetic division prepares your body for action, priming you to react to dangerous or stressful conditions. Your liver releases glucose (blood sugar), your breathing speeds up, air passages in your lungs widen, your heart pounds, and systolic blood pressure rises. Those responses are vital for survival when you’re faced with an immediate threat. Because exercise is also a stressor, albeit a controlled one, the same mechanisms kick in when you’re moving through a series of vigorous sun salutations or working out in the weight room.

Problems start when stress becomes overwhelming or overly prolonged. Stressors like car alarms, stock market crashes and tax deadlines aren’t physically threatening, yet the sympathetic nervous system reacts as if they were. And although short-lived spikes in glucose, breathing rate or blood pressure are healthy and necessary when dealing with a real threat, if they become chronic they cause serious health problems.

The parasympathetic nervous system brings us back to normal when danger has passed. While the sympathetic nervous system responds to external events, marshaling resources to deal with threats, the parasympathetic system maintains the body’s normal internal environment, what French physiologist Claude Bernard called the “milieu interieur.” Many aspects of that environment must be maintained within narrow limits for us to function and stay healthy, a process termed homeostasis (2). For example, the parasympathetic system stimulates digestion, keeps heart rate and blood pressure within normal levels, and promotes healthy immune function

How does heart rate variability fit into this?

The primary pacemaker of the heart is the sino-atrial node, a region of specialized cardiac muscle cells in the right atrium of the heart. The sino-atrial node rhythmically fires at a rate of about 60 to 100 beats per minutes, creating electrical signals that propagate throughout the heart, stimulating contraction.

Inputs from the autonomic nervous system modify that basic sinus rhythm, particularly inputs from the vagus nerve, the major nerve of the parasympathetic system. When vagal (parasympathetic) tone increases, heart rate slows down. As that input is withdrawn, the sino-atrial node returns to its baseline firing rate. When the sympathetic system kicks in, heart rate can speed up above 100 beats per minute.

Left to its own devices, the sino-atrial node would beat out a regular rhythm like a metronome. But, largely because of vagal stimulation, a healthy person’s heart rate actually varies considerably under normal circumstances. It isn’t regular, and shouldn’t be (3). In fact, too regular a heartbeat is a sign of an unhealthy autonomic nervous system and a risk factor for death from heart disease (4).

You may be able to feel the normal variation in your heartbeat that accompanies breathing—termed respiratory sinus arrhythmia—when your heart is beating strongly after a few rounds of vigorous sun salutations. Pause in tadasana and place your hand over your heart (under the left nipple is the best place to find the heart beat). Take a few long, slow breaths and notice how your heart rate accelerates as you inhale and decreases as you exhale.

Slow breathing increases respiratory sinus arrhythmia, making it easier to sense, but even when you’re sitting quietly and breathing normally, your heart rate varies with the breathing cycle. As you exhale, parasympathetic tone increases and your heart rate slows. When you inhale, there is a decrease in vagal input, and your heart rate speeds up. It’s not clear why that is, but one theory suggests that soaking the lungs with extra blood during an inhalation uses the heart’s output more efficiently.

Whatever the reason, the fact that heart rate variability is tied to autonomic tone makes it a useful marker for measuring the balance between the sympathetic and parasympathetic divisions. The Rev. Stephen Hales first described heart rate variability in 1733, but it was only with the development of modern digital processors that it became a useful measure. Today researchers use complex mathematical models to identify several frequency components of heart rate variability (5). The low-frequency range is generally a marker of sympathetic tone while the high-frequency band is linked to parasympathetic activation. The ratio between the two describes the relative balance between the two autonomic divisions (3).

In this study, Santaella and his colleagues found that the group who practiced pranayama experienced a reduction in the low-frequency band as well as in the low-to-high-frequency ratio, suggesting a shift from a sympathetic to a more parasympathetic state. In other words, the bhastrika practitioners were less stressed than their counterparts in the control group.

So what can we take from these results?

One result that I found curious was that only the pranayama group showed improvements, since both groups participated in regular yoga classes. The authors don’t say much about those classes other than that they included asanas and stretching exercises. However, other studies suggest that asana practice can also improve cardiovascular function and induce more parasympathetic dominance. So why the control group didn’t show some significant improvements isn’t clear.

Like just about every scientific research study you’ll ever read, this one ends with the comment that further research is necessary. That’s not just because scientists are a bunch of killjoys. In fact, the conclusions we can draw from any individual scientific study are pretty narrow. We can conclude that this particular practice (rounds of kapalabhati alternating with inhalations through the right nostril followed by breath retentions) for this particular amount of time (four months) has these particular effects (improved respiratory pressures and heart rate variability) in this particular population (healthy senior citizens). Would other pranayama practices in other populations have the same effects? We don’t really know from this study. Scientific knowledge grows as the result of many, many studies, some of which confirm and some of which contradict each other.

On the other hand, we don’t need certain knowledge to make educated guesses for ourselves. The results from this study agree with those from other studies suggesting that pranayama improves respiratory function and autonomic balance. You may not have scientific certainty that it will have the same effects for you, but it seems like a pretty good bet.


  1. Santaella DF, Devesa CRS, Rojo MR, et al. Yoga respiratory training improves respiratory function and cardiac sympathovagal balance in elderly subjects:
a randomised controlled trial. BMJ Open 2011;1:e000085. doi:10.1136/ bmjopen-2011-000085
  2. Porges SW. Cardiac Vagal Tone:
A Physiological Index of Stress. Neurosci Biobehav Rev. 1995 Summer;19(2):225-33 
  3. Parati G, et al. Point: cardiovascular variability is/is not an index of autonomic control of circulation. J Appl Physiol. 2006 Aug;101(2):676-8
  4. Tsuji H, et al. Reduced heart rate variability and mortality risk in an elderly cohort. The Framingham Heart Study. Circulation. 1994 Aug;90(2):878-83
  5. Billman GE. Heart rate variability – a historical perspective. Front Physio. 2:86. doi: 10.3389/fphys.2011.00086

Self magazine interview

I was just interviewed by Self magazine about why and how to modify your yoga practice. The interview is posted on their blog here.

What does stretching do for you?

Is there any reason to stretch? And is there such a thing as too much stretching? I’ve been thinking about these questions lately, after starting to lift weights again following a hiatus of 20-some years.

My motivation when I began weight lifting in my twenties was basically cosmetic; I wanted to get bigger, which I failed at doing completely. These days I’m more concerned with my health (and with having fun).

I saw how quickly my already thin father wasted away with illness before he died last year, and I was determined to have more reserves available when I reached his age. People lose muscle as they grow older, a condition called sarcopenia, and, unfortunately, muscle mass is a predictor of longevity. We don’t store protein as we store glycogen and fat; protein gets put to use. Thus, the only significant source that the protein-hungry immune system can call upon to resupply amino acids during severe illness is working muscle. Fortunately, sarcopenia can be slowed with activity, particularly with resistance training.

Also, as thin as I am, I’m more at risk of osteoporosis than other males. I’ve been practicing yoga for long enough that it no longer presents much of a physical challenge for me, or for my bones. There is a minimal essential stress necessary to stimulate bone growth. Of course, there are asanas that remain difficult for me, but overall I doubt that my yoga practice still stresses my bones as it did when I began. Heavy deadlifts certainly do.

And an unexpected benefit of weight lifting is that it’s turned out to be fun. It’s a challenge, especially Olympic lifting, which I’ve been learning lately. I get to feel like a beginner–clumsy, weak and uncoordinated–which I don’t get in my yoga practice, as much as I may try to approach it with a beginner’s mind. So when I actually manage to hoist a heavy barbell up, it’s a thrill.

However, the primary reason I started lifting was that I felt my body had become unbalanced from asana practice. I had become hypermobile, overflexible, at the expense of strength. This was an interesting shift for me. One of the reasons I started practicing yoga seriously in the first place was to restore flexibility that I had lost with weight training back in my twenties. The situation was now reversed. I needed to tighten up to return to balance.

And now, in another interesting shift, as my muscles feel tighter, stretching feels good again. I took a yoga class the other day focusing on deep, slow stretches for the gluteal muscles and hamstrings. Mine felt like they sorely (and I mean sorely) needed the attention after a session of heavy deadlifts the day before. I had forgotten that feeling of tight hamstrings. It felt good to stretch them out, and by the end of class uttanasana wasn’t quite so much the exercise in humility it had been at the beginning of class. The stretching felt like a necessary corrective, rather than over-pulling already loose muscles.

Which brings me to the question I began with. As good as it may feel, is it in fact beneficial to stretch? Is there evidence that it actually does anything for you?

Perhaps surprisingly, in recent years exercise physiologists studying this question have largely come to the conclusion that there isn’t much evidence that it does.

It’s long been assumed that stretching could help prevent injuries among athletes. While that might be true for some specific individuals with some specific injuries, there’s little evidence that it prevents injuries overall. Athletes who don’t stretch are no more (or less) likely to be injured than athletes who do. And, although the evidence is also equivocal, some studies suggest that being hypermobile can increase your risk of injury.

Likewise, in general, stretching doesn’t seem to enhance athletic performance. In fact, stretching immediately before an athletic event probably actually decreases performance.

However, a recent study in the Journal of Strength and Conditioning Research suggests another reason stretching may be beneficial: it may help you relax.

The researchers gave 10 young men with low flexibility a short routine of three stretches, held for 30 seconds and repeated three times. The subjects’ heart rate variability was measured for 30 minutes before stretching, while stretching, and for 30 minutes afterwards.

In simple terms, heart rate variability is an indicator of the state of an individual’s autonomic nervous system. The results of the study indicated that in the period following the stretching routine the subjects had enhanced parasympathetic tone. In other words, they were less stressed and more relaxed than when the test began.

There are reasons to take these results with a grain of salt, or at least to be cautious about how generally applicable they are. For one thing, there was no control group, so we don’t know whether, if the subjects had just sat and relaxed for the same period of time, they would have seen the same results.

In addition, the men were selected for low flexibility, so we don’t know if the results would have been similar for those with more flexibility. (In fact, it’s not even clear that they were generally inflexible. They were tested with the sit-and-reach test, which is considered a standard measure of flexibility—basically, they sat in paschimottanasana, and researchers measured how far they could reach with their hands. Does this test indicate overall flexibility? It might for some individuals, but it only measures hip and spinal flexion and says nothing about other joints or other ranges of motion. However, since the stretching routine the subjects practiced was focused on the hamstrings and trunk, it was probably a valid measure for this study).

Despite the unanswered questions, this study is still intriguing. Perhaps most researchers studying stretching have been barking up the wrong tree. Maybe its benefits have less to do with effects on specific muscles, and more to do with the nervous system as a whole through inducing the relaxation response—which probably comes as no surprise to many yogis.

Upcoming workshop: Detox the Holidays

I will be teaching a workshop entitled “Detox the Holidays” this Saturday, January 8, from 9:30 am to 12 noon, at OM yoga Center in New York City (826 Broadway at 12th Street, 6th floor). Join me for  a vigorous asana and pranayama practice to help ease the transition into winter.

Hyperventilation: Why breathing more isn’t necessarily better (Part 1)

This is part one of a series of posts I plan to write on the physiology of hyperventilation.

Recently, when I was in California I spent an evening practicing holotropic breathwork. I didn’t know much about this beforehand, and you might not either, so I’ll just set the scene. There were about 20 or so of us in a circle, along with a facilitator (whose instructions generally only increased my mystification). After a rambling introduction, he switched on some music, and we divided into pairs. One member of each pair was to be a sitter, whose only job was to observe. The others—the breathers—lay supine on a bed of cushions and blankets. That was me.

Our job as breathers was to breathe as rapidly and deeply as possible, preferably making lots of noise. As a physiology geek, this was an interesting experiment to me, so I was diligent in trying to stay with it. It was much harder than it might sound, though, particularly since we had to keep it up for well over an hour. The longer it went on the more spaced out I became, and the more difficult it became to remember what I was supposed to be doing. Eventually I started to experience muscle spasms; my arms and legs began to jerk about uncontrollably.

In the end, I couldn’t sustain it. Despite the urging of the facilitator, who periodically came around to rally me with loud whooshing breathing sounds, I found myself falling into longer and longer periods of almost involuntary apnea (cessation of breathing). My friend who was observing told me afterward that my reactions were mild compared to others’. “It was like an exorcism,” she said: thrashing limbs, clenching fists, loud crying and sobbing.

My purpose isn’t really to write about holotropic breathwork, which, as I said, I don’t know much about anyway. My point here is that the effects I felt—mental fogginess, involuntary muscle contractions, long spells of apnea—are typical physiological responses to hyperventilation (defined as breathing in excess of physiological needs).

So what does this have to do with yoga? I’ll get to that, but for now I’ll just note that many yoga practitioners and teachers—naively, I think—believe that when it comes to breathing, more is better. As you’ll see, however, that’s not necessarily the case; in fact, over breathing can have very deleterious effects.

You might think the problem is too much oxygen. But in healthy people arterial blood leaving the lungs is already nearly fully saturated with oxygen, even during quiet breathing. In other words, you can’t take in too much oxygen.

Rather, the problem with hyperventilation comes from blowing off too much carbon dioxide. In normal breathing, small amounts of incoming air are mixed with a much larger volume of air remaining within the alveoli (the air sacs in the lungs where gas exchange with the blood takes place). This has the effect of maintaining an internal atmosphere within the lungs that contains a much higher percentage of CO2 than does the air outside (carbon dioxide makes up only a very small percentage of atmospheric air). CO2 levels in the bloodstream closely reflect the composition of this internal atmosphere, so that during hyperventilation, as more carbon dioxide is expelled from the alveoli, CO2 levels in the blood also fall.

We tend to see CO2 as a waste product, something to be disposed of. But in fact we need to keep  CO2 in the blood within a certain range, because it plays an important role in maintaining blood pH. As CO2 levels drop during hyperventilation, the result is a higher, or more alkaline, pH.

Because of this rise in blood pH, hyperventilation has an effect that might at first seem paradoxical—by over breathing, we actually reduce the amount of oxygen getting to the brain, a situation termed cerebral hypoxia.

This became an issue during World War II. Hyperventilating military pilots tended to become confused and disoriented. Researchers studying conscientious objectors in the laboratory were able to confirm that hyperventilation led to a reduction in blood flow to the brain, which had long been suspected. Their supposition was this was due to constriction of cerebral blood vessels. This has subsequently been shown to be the case, and although the exact mechanism is still debated, it’s clear that a rise in blood pH  triggers arterial constriction.

What’s worse is that when blood becomes more alkaline, hemoglobin (the molecule that transports oxygen in the red blood cells) tends to hold on more tightly to oxygen. This makes sense in the context of allocating oxygen to the tissues that need it most: metabolically active tissues that have consumed a lot of oxygen and need to replenish it will have also produced a lot of CO2. This creates a more acidic local environment, which in turn causes hemoglobin to release more oxygen. The reverse is true when conditions are more alkaline; tissues don’t get as much oxygen, because hemoglobin hangs on to it.

Thus, the brain gets hit with a double whammy during hyperventilation—less blood flow, plus less oxygen being released from the blood. No wonder those World War II pilots were so disoriented, or that my brain was so foggy that evening in California.

In the next installment of this series, I’ll discuss more physiological effects of hyperventilation.


Kety SS & Schmidt CF. The effects of active and passive hyperventilation on cerebral blood flow, cerebral oxygen consumption, cardiac output, and blood pressure of normal young men. J Clin Invest. 25:107-19, 1946

Raichle ME & Plum F. Hyperventilation and Cerebral Blood Flow. Stroke. 3:566-575, 1972

Tomashefski JF, et al. Carbon dioxide and acid-base transients during hyperventilation. J Appl Physiol. 17(2): 228-232, 1962