Science of yoga
The science of yoga is the scientific basis of modern yoga as exercise in human sciences such as anatomy, physiology, and psychology. Yoga's effects are to some extent shared with other forms of exercise, though it differs in the amount of stretching involved, and because of its frequent use of long holds and relaxation, in its ability to reduce stress. Yoga is here treated separately from meditation, which has effects of its own, though yoga and meditation are combined in some schools of yoga.
Yoga has been studied scientifically since the 19th century physiology experiments of N. C. Paul. The early 20th century pioneers Yogendra and Kuvalayananda both set up institutes to study yoga systematically.
In the 19th century, N. C. Paul began the study of the physiology of yoga with his 1851 book Treatise on Yoga Philosophy, noting that yoga can raise carbon dioxide levels in the blood (hypercapnia).
Early in the 20th century, two pioneers of yoga as exercise in India, Yogendra and Kuvalayananda, worked to make Haṭha yoga acceptable, seeking scientific evidence for the health benefits of yoga postures (asanas) and yoga breathing (pranayama). In 1918, Yogendra founded The Yoga Institute to carry out research on yoga. Yogendra expressed his intentions in books such as his 1928 Yoga Asanas Simplified and his 1931 Yoga Personal Hygiene. In 1924, Kuvalayananda founded the Kaivalyadhama Health and Yoga Research Center, combining asanas with gymnastics, and like Yogendra seeking a scientific and medical basis for yogic practices.
In 1937, the Yale physiologist Kavoor T. Behanan published his book Yoga: A Scientific Evaluation, reporting that a form of pranayama, Ujjayi ("Victorious breath"), performed at the slow rate of 28 breaths in 22 minutes, could create a deeply relaxed state that he called "an extremely pleasant feeling of quietude", accompanied by a marked slowing of mental performance on tests such as mental sums, recognising colours and solving simple puzzles. The science journalist William Broad notes that this finding contradicted the image of yoga as conferring special powers.
Skeleton and joints
Yoga helps to keep bones and joints in a healthy state. In particular, it helps to maintain bone strength; it also helps to maintain both joint mobility (range of motion), and joint stability. It improves posture, muscular strength, coordination and confidence (reducing anxiety), all of which reduce the risk of injury and bone fracture, and which may therefore be helpful to people with conditions such as osteoporosis. On the other hand, yoga, like any other physical activity, can result in injury; headstand (Sirsasana), shoulder stand (Sarvangasana), and lotus position (Padmasana) are the asanas most often reported as causes of injury. Ann Swanson offers three reasons why yoga may be safer than many sports, namely that it is often slow; it encourages awareness in the moment; and it stresses doing no harm (ahimsa).
Yoga involves both isotonic activity, the shortening of muscles under load, and (unlike many forms of exercise) also a substantial amount of isometric activity, holding still under load, as in any asana which is held for a period. Isometric exercise builds muscle strength.
Breathing and posture affect each other, especially through their effects on the diaphragm. Breathing also affects the autonomic nervous system; quiet breathing slows the heart and reduces blood pressure. Together, these produce a feeling of calmness and relaxation. One way to do this is used in one form of yoga breathing (pranayama); the exhalation is counted to be twice as long as the inhalation, say inhale to a count of 3 and exhale to a count of 6.
Breathing can equally be used to energise the body. The pranayama method of bhastrika (bellows breath) and the satkarma purification of kapalabhati (skull polishing) both energise the body with vigorous abdominal breathing, using the diaphragm to make the abdomen move in and out.
Broad notes the "myth" that yoga, and especially pranayama, increases the supply of oxygen to the body. He writes that instead, fast vigorous breathing as with bhastrika may indeed feel exhilarating, as B. K. S. Iyengar reported, but it lowers the level of carbon dioxide in the blood. This causes blood vessels in the brain to constrict, reducing the brain's uptake of oxygen, resulting in symptoms such as dizziness and fainting. On the other hand, slow pranayama can raise carbon dioxide levels, and increase the uptake of oxygen by the brain.
Yoga can be used as exercise to help maintain physical fitness. A complete yoga session with asanas and pranayama provides 3.3 ± 1.6 METs, on average a moderate workout. Surya Namaskar (the 12-asana Salute to the Sun sequence) ranged from a light 2.9 to a vigorous 7.4 METs;[a] the average for a session of yoga practice without Surya Namaskar was a light 2.9 ± 0.8 METs.[b]
A 2012 survey of yoga in Australia notes that there is "good evidence" that both yoga on its own, and its associated healthy lifestyle—often vegetarian, usually non-smoking, preferring organic food, drinking less or no alcohol—are beneficial for cardiovascular health, but that there was "little apparent uptake of yoga to address [existing] cardiovascular conditions and risk factors". Yoga was cited by respondents as a cause of these lifestyle changes; the survey notes that the relative importance of the various factors had not been assessed.
Yoga sessions often end (and sometimes also begin) with a period of relaxation in corpse pose, Savasana. The activity levels of all the body's muscles, and the motor neurons (nerve cells) that activate them, is reduced as relaxation is practised, except for the diaphragm which is used in breathing; and the breathing rate reduces also.
Systematic reviews have found beneficial effects of yoga on low back pain and to some extent for depression, but despite repeated attempts, little or no evidence for benefit for specific medical conditions. Much of the research on the therapeutic use of yoga, including for depression, has been in the form of preliminary studies or clinical trials of low methodological quality, suffering from small sample sizes, inadequate control and blinding, lack of randomization, and high risk of bias. For example, study of trauma-sensitive yoga has been hampered by weak methodology.
The neurologist and sceptic Steven Novella states that "Yoga .. fits into a more general phenomenon of marketing a specific intervention as if it has specific benefits, when in fact it only has generic benefits." He gives as an example the evidence that yoga helps to relieve low back pain, but that with "a lack of quality studies [by 2013] comparing yoga to other forms of exercise", it is possible that yoga's benefits are just what any form of exercise would provide, generically. Novella points out that yoga also has a spiritual side, so claims made for it can mix science with "a liberal dose of pure pseudoscience and mysticism." He illustrates this by quoting unfounded claims such as that a forward bend squeezes the pancreas and liver, ejecting toxins, and that stretching the lower back is calming because emotional stress accumulates in the lower back muscles. Novella states that "None of those specific claims is based in reality."
- Haskell, curious about the wide range of METs in Surya Namaskar, repeated the study (Mody) which gave the highest value; using "transition jumps, and full pushups", he obtained "agreement" with 6.4 METs.
- Asanas performed individually provide on average 2.2 ± 0.7 METs; pranayama types performed individually provide just 1.3 ± 0.3 METs.
- Cramer, Holger; Lauche, Romy; Haller, Heidemarie; Dobos, Gustav (2013). "A Systematic Review and Meta-analysis of Yoga for Low Back Pain". The Clinical Journal of Pain. 29 (5): 450–460. doi:10.1097/AJP.0b013e31825e1492.
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- DoctorLib2017, p. Chapter 1. Movement: Isotonic and Isometric Activity.
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- DoctorLib2017, p. Chapter 2. Breathing. The Bellows Breath and Kapalabhati.
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- Swanson 2019, p. 27.
- Riley, Kristen E.; Park, Crystal L. (2015). "How does yoga reduce stress? A systematic review of mechanisms of change and guide to future inquiry". Health Psychology Review. 9 (3): 379–396. doi:10.1080/17437199.2014.981778.
- DoctorLib2017, p. Chapter 10. Relaxation and Meditation. The Corpse Posture.
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- Cramer, Holger; Anheyer, Dennis; Lauche, Romy; Dobos, Gustav (2017). "A systematic review of yoga for major depressive disorder". Journal of Affective Disorders. 213: 70–77. doi:10.1016/j.jad.2017.02.006. ISSN 0165-0327.
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- Swanson, Ann (2019). Science of Yoga : understand the anatomy and physiology to perfect your practice. DK Publishing. ISBN 978-1-4654-7935-8.
- Yogendra (1928). Yoga Asanas Simplified. The Yoga Institute.