Physiological Effects of Yogic Practices

Since yoga aims at perfection of the body and mind, it is natural to ask whether the progress towards perfection is reflected in objective reproducible changes in physiological variables. Besides this simple approach, there are several other aspects of yoga which have fascinated, and even provoked, scientists. The amazing claims made by some yogis, such as their ability to stay underground for long periods, or to stop the heart beat voluntarily, have posed a challenge to scientists to either confirm or refute these claims conclusively. Further, the claims regarding wide-ranging therapeutic benefits of yoga have prompted scientists to examine whether there is a physiological basis for such claims.

Difficulties And Limitations
Performing scientific studies on yoga is not easy. First, sincere and sustained pursuit of yoga is not very common. Secondly, sincere and serious yogis shun the publicity that accompanies scientific investigation because, being an ego-boosting exercise, it interferes with their sadhana. Finally, some of the most significant changes induced by yoga are either attitudinal and hence cannot be examined objectively, or are subjective and hence can only be experienced by the yogi. Experiences are difficult even to describe in words, leave alone to examine in terms of physiological or biochemical variables. In spite of all these limitation, several studies have been conducted on the physiological effects of yogic practices. But because of these limitations, the studies conducted have been far from ideal. The studies generally fall in two categories, both with pitfalls of their own. One category of studies have been conducted on accomplished yogis, but the number of subjects is too small for statistical analysis. The second category of studies, far more numerous than the first, has been conducted on large numbers of students, soldiers or patients. In these studies, yoga has generally meant a few weeks of asanas or pranayama or meditation or various combinations of these. Obviously, these studies demonstrate only the effect of isolated, half-hearted practice of a few yogic techniques. One exception probably has been the large number of studies conducted on the effects of 'transcendental meditation' (TM) by Robert Keith Wallace and his colleagues. Similar studies have recently begun on a breathing technique, sudarshan kriya (SK). In these studies the practice of TM or SK is serious and sustained, but the limitation is that the effects are those of only one yogic practice. Studies initiated about ten years ago at Swami Vivekananda Yoga Research
Foundation, Bangalore, have tried to overcome these limitations by studying the effects of an integrated package of yoga on large numbers of well-motivated subjects. However, much of the existing literature is still handicapped by the limitations mentioned above. Hence the effects described below should be interpreted keeping these limitations in mind. It may be presumed, however, that if these limitations did not exist, the physiological effects would be more marked, not less.

Physiological Effects
Cardiovascular Effects
In general, yogic practices reduce the resting heart rate and blood pressure. Further following these practices, a given level of exercise is associated with a smaller rise in the heart rate and blood pressure. Regular practice of a combination of asanas for 3 weeks was found by Dr. W. Selvamurthy and his colleagues at Defense institute of Physiology and Allied Sciences to restore baroreflex sensitivity towards normal in patients having essential hypertension.

Respiratory Effects
Yogic asanas and pranayama have been shown to reduce the resting respiratory rate. Further, they increase the vital capacity, timed vital capacity, maximum voluntary ventilation, breath holding time, and maximal inspiratory and expiratory pressures. TM has been shown to reduce the resting respiratory rate, tidal volume and minute volume. In a study involving comparison of routine National Defence Academy (NDA) training, athletics and yogic exercises, Commander H.S. Nayar and his colleagues found that the yogic exercise group showed maximum improvement in respiratory function.

Effect on Metabolic Rate
Prof. B.K. Anand and his colleagues at All India Institute of Medical Sciences published 1n 1961, a classical study on a yogi who could spend long periods of time underground. The study was performed by keeping him in an airtight box while his oxygen consumption and carbon dioxide production were measured. It was found that during his stay in the box, the yogi could lower his metabolic rate to nearly half his 'basal' metabolic rate.
A reduction in oxygen consumption has also been demonstrated during TM.
A recent study done at Swami Vivekananda Kendra has shown that contrary to expectations, the reduction in oxygen consumption following 'cyclic meditation' (yoga postures interspersed with shavasana) is greater than that following shavasana alone.

Effects on the Nervous System
Prof. B.K. Anand and his colleagues observed a preponderance of alpha waves in the EEG of yogis, indicating a more relaxed state of mind. However, it might be argued that people with a higher alpha activity are attracted to yogic practices. Hence the observation may not reflect an 'effect' of yoga. Further, it was observed that sensory stimuli, such as a loud bang or an ice cold/ hot object, which normally block the alpha rhythm, chould not do so in yogis during meditation. This indicates that the yogis do not get easily distracted by sensory stimuli while they are meditating. Power spectral analysis of 24-hour EEG in those who have been practicing TM has shown an increase in the alpha/ delta power and a decrease in the beta/ alpha power. The increase in alpha/ delta power indicates reduction in time spent sleeping. The decrease in beta/ alpha power indicates a more relaxed mind during the awake period. Further, it was found that there was a better balance and synchrony in the EEG recorded from the left and right side, and from the frontal and occipital leads. This has been interpreted to indicate enhanced creativity, but that is open to question.
Comparable results were obtained in a recent study by Dr. Shirley Telles and her colleagues at Swami Vivekananda Yoga Research Foundation in which they found that during meditation there was significant reduction in heart rate but an increase in cutaneous peripheral vascular resistance, indicating a physiologically relaxed state but increased mental alertness.
Some recent studies have shown that unilateral forced nostril breathing (UFNB) affects cerebral hemispherical dominance. Left-sided UFNB leads to right-hemisphere dominance and improves spatial skills. On the other hand, right-sided UFNB induces left-hemisphere dominance and increases verbal skills. These studies have made use of newer techniques of measuring cerebral blood flow or neuronal activity. Such techniques have also shown the pattern of cerebral blood flow to be different during meditation as compared to the resting phase of the normal awake state.
Studies on autonomic functions indicate that yogic practices, in general, bring about a tilt towards parasympathetic dominance. While this is the overall effect of long-term practice of a combination of asanas, pranayama and meditation, individual practices may act differently. For example kapalabhati (rapid breathing) reduces vagal efferent activity to the heart. On the subject of voluntary control of autonomic functions, Dr. B.K. Anand and his colleagues conducted studies on a few yogis who thought they could stop the heart at will. Only one of them could voluntarily slow down the heart rate and even achieve a disappearance of P-wave in the EKG for as long as 16 cycles. The others performed Valsalva's maneuver, as a result of which the pulse became impalpable but the heart continued to beat.
In a recent study by Herbert Benson's group on Chinese Taichi and Kundalini yoga meditation it was observed that the amplitude of heart rate oscillations was greater during meditation than before meditation as well as in non-meditational control groups. This finding challenges the notion of meditation being only an autonomically quiescent state.
Galvanic skin response (GSR), also called electro dermal response (EDR) has frequently been used in studies on autonomic function. Sympathetic activity following an alerting stimulus, e.g. a sudden loud sound, leads to palmar sweating. Sweat, being a good conductor, leads to a fall in the electrical resistance of the skin. The response is seen within seconds of the stimulus and is over within half minute. The magnitude of the response is considered an indicator of sympathetic reactivity. It has been shown that those who practice TM have fewer spontaneous GSRs than non-meditating controls, indicating lower reactivity to stressful stimuli. This confirmed when the subjects were exposed to a series of auditory stresses (loud sounds). As compared to non-meditators, the meditators got habituated to the stimuli faster, and their GSRs were smoother. The results indicate lower reactivity and greater stability of the sympathetic nervous system in meditators. In a recent study don at Swami Vivekananda Kendra, mental repetition of 'Om', a meaningful symbol led to reduction in skin resistance whereas there was no change during repetition of a neutral syllable or during non-targeted thinking.
Some recent studies by Dr. Shirley Telles and her colleagues are also worth mentioning. Practising asanas/ pranayama, meditation and tratakas, and attending devotional sessions for 10 days led to a significant improvement in fine coordinated movements. Similar practices for a month led to a reduction in the degree of optical illusion created by Muller-Lyer lines. The same group of workers also found a combination of yogasanas to raise the critical fusion frequency. Apart from better neural performance, higher CFF indicates reduced fatigue and stress level.

Endocrine and Metabolic Effects
Yogic practices have been shown to reduce baseline and average glucocorticoid levels. But the glucocorticoid response to an acute challenge is enhanced. These findings indicate a lower level of stress and an enhanced capacity to cope up with a challenge. Certain asanas and also TM have been shown to reduce the fasting blood glucose and serum cholesterol levels. These are favorable biochemical indicators suggesting reduced risk of diabetes and atherosclerosis. A recent study has shown night plasma melatonin levels to be higher immediately following meditation as compared with those at the same time on a control night. Higher melatonin levels might be one mechanism through which the claimed health-promoting effects of meditation occur.

Mechanisms Underlying the Effects
The mechanisms underlying many of the physiological effects are not well understood. Sometimes, it is difficult even to determine what can be termed a mechanism. For example, the higher alpha index in the EEC may be considered the 'mechanism' underlying the calm disposition of yogis. But it may be argued quite correctly that higher alpha index is also an effect, with the only difference that it is an effect observed only in the laboratory whereas calm disposition is an effect which may be observed anywhere because it needs no equipment. On the other hand, there are 'mechanisms' in search of an effect. For example, it has been shown by position emission tomography (PET) that yoga nidra (yogic 'sleep') a guided meditative technique-increases cerebral blood flow in the sensory and association cortices in the posterior parts of the brain. In contrast, in the normal resting state, there is high cerebral blood flow in the frontal lobe, basal ganglia, thalamus and cerebellum. What effect the difference in regional blood flow explains, and why, is difficult to elaborate upon on the basis of known physiology.
The effects of asanas to increase the strength and efficiency of muscles and to improve the flexibility of the body is similar to that of other physical exercises and needs no special explanation. However, the improvement in cardio respiratory function may seem out of proportion to the intensity of the yogic exercises. This may be because in yogasanas the regulation of breathing may lead to great changes in thoracic or abdominal pressure, although the intensity of exercise in terms of energy expenditure is not very high. Hence the degree of stimulation of cardio respiratory reflexes may be comparable to that during intense exercise. It is a well-known dictum that frequent use of a physiological process strengthens it.  This dictum may apply also to these reflexes. That may be the mechanism by which regular practice of a few selected asanas restores the normal sensitivity of baroreflexes in hypertensive patients. A shift in autonomic balance towards parasympathetic dominance may explain the reduction in heart rate, fall in blood pressure and improvement in gastrointestinal function. The increased glucocorticoid secretion in response to an acute challenge may explain the improved ability to cope up with stress. The physical exercise, dietary changes, and stress reduction associated with yogic practices may explain the fall in fasting plasma glucose and serum cholesterol levels.

Conclusion
It would be surprising if a discipline involving the body as well as mind, and aiming at a total transformation of both, did not affect various physiological variables. Further research is likely to reveal more and more effects, and hopefully, also their mechanisms. However, the effects on the mind are likely to remain enigmatic due to limitations of the tools available to science. For example, these tools are unsuitable for demonstrating a change in the level of consciousness. But it is being increasingly accepted that human consciousness can be expanded, depended and elevated. And, it is also now considered possible that a change in the level of consciousness may affect cellular function at molecular level.

Questions and Problems
Q. 1 Does yoga induce permanent changes in the brain?
Q. 2 What are the possibilities and limitations of research on consciousness?

Answers and Solutions
The classical image evoked by the brain is that of a robust structure made up of a set of neurons of which neither the number nor the connections or neurotransmitters can be altered, except in disease. In view of this image, several studies on the relationship between yoga and the brain have concentrated on investigating the regions or circuits which show a change of activity during various yogic practices. But this image is no longer valid. Recent studies indicate that while the broad outline of neural connections is genetically determined, finer details are mutable, and are influenced by various factors. Further, besides connections, the efficacy of synaptic transmission and neurotransmitter profile are also subject to change. This flexibility in neural structure and function is called neural plasticity. Besides developmental and regeneration-related plasticity, there is also experience-related plasticity. Yogic practices involve unique sensory, motor and learning experiences. Therefore it is reasonable to assume that, as in other types of sensory-motor experiences and learning, the changes associated with yoga would also involve synaptic plasticity as well as altered connectivity of neurons.
Consciousness, in physiology, means awareness of a sensory stimulus. The ascending sensory pathways as well as the representation of various parts of the body in the somatosensory cortex had been worked out by classical neuroanatomical and neurophysiological techniques quite some time back. The neuronal organization associated with perception of different sensory modalities and sub-models is also -known in considerable detail now. The neuronal networks responsible for cognition have recently been unraveled also in the normal living human brain using techniques such as positron emission tomography (PET) and functional magnetic resonance imaging (FMRI). Functional MRI systems currently in use give a spatial resolution of about 2 mm. Further improvements are imminent, and a resolution of 0.5 mm seems to be within reach. But what all these techniques tell us is more and more accurately the regions of the brain which show a change in activity when some cognitive event takes place. But we are still far from understanding how exactly feeble electrical impulses and regulated release of neurotransmitters and neuromodulators are transformed, into subjective experiences, like a beautiful red rose, or sweet soothing music. Whether we ever will understand it is a matter of debate. There are scientists, such as Fralcis Crick, and philosophers such as Daniel Dennett, who feel all mental events are the results of neuronal activity. It is only a matter of time, they believe, that a day will come when we will know enough about neuronal function to understand how physicochemical events can generate subjective feelings. But there are others, particularly philosophers, such as Colin McGinn, who believe that consciousness is not amenable to empirical study. The basic problem, as pointed out by Searle, is that we do not have even a theoretical model of how objective phenomena, such as electrical activity of neurons, can give rise to a subjective experience. Subjectivity is irreducible whereas science, as it is currently practiced, has a reductionist approach. However, most neurobiologists are prepared to wait for the larger picture to emerge, and are at present content to know more and more about the specific neuronal events associated with well-defined mental events. Neurochemistry has added another hopeful dimension to this approach by showing that specific neurotransmitters, such as dopamine or serotonin, are associated with predictable alterations in mood or states of alertness.
A source of confusion in discussions on consciousness is that biological and spiritual literature use the same word for entirely different concepts. In biology, consciousness approximates awareness. In spirituality, Consciousness (spelt with capital C) is used as an equivalent of the Absolute. In that sense, the entire universe is a manifestation of the supreme Consciousness. In matter the Consciousness, is dormant in animals, a bit of the Consciousness is expressed; and in man, a little of it more is expressed. Yoga is an effort at elevating human consciousness towards Consciousness. The journey of yoga, for which a whole lifetime is usually grossly inadequate, and the mighty evolutionary urge, are both merely about converting the small c into the capital C.

Courtesy from - Understanding Medical Physiology (IIIrd Edition)