Millions of people have turned to yoga practices to help them relax. What do we know about what yoga is and whether it really works to decrease stress and anxiety?  In order to answer this question, this paper will look at the nature of stress and its relationship to anxiety, the nature of relaxation, and how yoga postures help us to relax. 

     As early as 1956 Hans Selye popularized the term stress as "the rate of all the wear and tear caused by life" (Rosenzweig, 2005), or the nonspecific response of the body to any demand from a stressor (McCaffrey, 2005).  The term stress is also used to describe the environmental conditions that may result in physical, biochemical, functional or emotional body changes (Benson, 1975).  As such, stress can be understood to be "anything that throws your body out of homeostatic balance" (Sapolsky1994, p.7).  The concept of homeostasis is based on the idea that the body has an ideal state that it tries to maintain with respect to its many physiological processes, including the levels of oxygen, sugar, acidity, temperature, etc. There are many different kinds of stressors, or influences, that might affect the homeostatic balance.  These include acute physical stressors such as injuries, or being out in the cold, or missing a night's sleep, or getting drunk.  There also are chronic physical stressors like chronic illness or malnutrition from poverty.  A third type of stress is psychological or social, like being criticized by your boss at work, or having problems in a marriage relationship, or the death of a loved one.  Finally, a stressor can also be the anticipation of something unpleasant, like having to take a test in school, or it can be the perception that you are facing something that is dangerous, threatening or overwhelming, like seeing a snake that you think is poisonous even if it is not (Bourne, 1995, Sapolsky, 1994).

     The idea that stress can cause distress, or that stress can make you sick, was presented by Hans Selye in the 1930s (Sapolsky, 1994).  However, stress does not inevitably lead to dysfunction, or illness. Stress leads to distress whenever the brain perceives an event or environmental condition to be dangerous or threatening (Khalsa, 2001), or when there is uncertainty about how to gain a positive outcome in response to a situation (Rosenzweig, 2005).  When this happens, a complex physiological response is triggered called the stress response, or the fight or flight mechanism.  This response originated to promote survival by quickly preparing the body for fighting or escaping from a predator. Large muscle groups are prepared for conflict by increasing blood flow to the muscles. Our blood pressure, heart rate, rate of breathing, metabolism and blood sugar levels will increase (Benson, 1975).  Sweat glands are stimulated.  Hyper vigilance is enhanced through the dilation of the pupils of the eyes. In addition, body functions that are not essential for fighting or fleeing are inhibited, including digestion, salivation, growth and reproduction.  Circulation to the skin also decreases.

      The stress response is a function of our autonomic nervous system (as opposed to the somatic nervous system which allows us conscious muscle control). When an event is perceived, it stimulates many areas of the brain.  If a threat is identified, the amygdala conveys the danger message through nerve impulses to the hypothalamus.  The hypothalamus has several pathways through which it takes action.  The quickest responses result from nerve impulses that travel from the brain down the spine and branch out to nearly every organ and blood vessel in the body. The nerve endings release the neurotransmitters adrenaline (in your adrenal glands) and noradrenalin (in all the other parts of the body), also referred to as epinephrine or norepinephrine.  Within seconds these chemical messengers tell the heart, lungs, stomach and other organs in the body how to respond (Sapolsky, 1994). 

     The hypothalamus also masterminds a parallel sympathetic response pathway through the release of CRF (corticotrophin releasing factor) which travels to the pituitary gland. Here the action of the endocrine system begins.  In about fifteen seconds the pituitary releases the hormone ACTH (adrenocorticotropin hormone) into the bloodstream, which travels to the adrenal glands and triggers glucocorticoid release, which is involved in raising the blood sugar level so that energy can be mobilized during stress.  Other hormones are also mobilized in the pituitary, such as endorphins which help to blunt pain perception and allow a type of stress induced analgesia.  Vasopressin (also known as antidiuretic hormone) is also secreted which signals the kidneys to retain water and also acts as a peripheral vasoconstrictor (supporting a rise in blood pressure) (Sapolsky, 1994).  In addition some processes that are not needed in an emergency are suppressed through the action of cortisol, a glucocorticoid which depresses immune function, suppresses appetite and digestion, and inhibits growth.   

STRESS AND ANXIETY

     When the stress response is active we may be experiencing a range of emotional states.  Anxiety may be thought of as a psychological or emotional manifestation of stress (Reilly, 2000).  When anxiety occurs there is usually a sense of apprehension or uneasiness. There may be a worry about a past or future event, or a vague sense of something bad happening, or a generalized fear of losing control.  Physiologically, however, these emotional states show up in the body as variations of the stress response.  The anxious person will report a range of symptoms arising from this stress response, including shortness of breath, heart palpitations, trembling or shaking, sweating, nausea or abdominal distress, numbness, dizziness, feeling of detachment or being out of touch with yourself and fear of dying (Bourne, 1995).  This interrelationship between emotions and physiology exemplifies the growing understanding that mind and body can not be separated.

     Besides being subjectively unpleasant, anxiety can cause many problems.  While it is reasonable to react with some anxiety to life's challenges, anxiety can cause dysfunction if it persists over an extended period of time.  If anxiety is long term, then the stress response physiology which is wonderfully adaptive in a crisis will be active too often, or for an extended period of time.  The wear and tear of this heightened stress response may show up as disease, usually in whatever is the weakest point in your system.  For example, if your weak point is your cardiovascular system, then anxiety, which includes an increase in blood pressure may show up as chronic high blood pressure.  The high cortisol levels that accompany anxiety can lower your resistance to autoimmune diseases. If anxiety weakens the neuroendocrine or neurotransmitter systems of your brain, then you may develop mental illnesses in which anxiety is extremely intense or long lasting such as panic disorder or obsessive compulsive disorder (Bourne, 1995).  (It is important to note here that anxiety caused by stressful events is not the only cause of anxiety disorders.  Other causes include hereditary predispositions to anxiety, traumatic experiences and certain illnesses.) 

RELAXATION

         When a threat or danger is perceived to be over, the input to the sympathetic nervous system decreases and the parasympathetic nervous system becomes more active. These two systems can not be highly operative at the same time (Benson, 1975, Sapolsky, 1994) so relaxation occurs when the parasympathetic nervous system is dominant. Nerve impulses originating from the cortex stimulate the hypothalamus where the parasympathetic response begins. The hypothalamus sends its instructions through nerves to the rest of the body.  The vagus nerve is responsible for most of the parasympathetic innervation of the internal organs (Khalsa, 2002). The vegas nerve sends out a message to slow the heart rate, lower blood pressure, stimulate digestion and normalize blood sugar levels through energy storage. This response was identified by Herbert Benson who popularized it as the relaxation response (Benson, 1975).

      Benson (1975) defines relaxation as "a state of decreased psychophysiological arousal: a calming state.  Coulter (2001) defines relaxation as a multifaceted process involving conscious control of the somatic nervous system and its innervation of skeletal muscle, regulation of the autonomic nervous system and its control of smooth and cardiac muscle, and the reigning in of emotion and mental activity.  Both definitions recognize both the physiological and the mood related aspects of relaxation.  How we experience relaxation is quite subjective, including a range of feelings that might include sleepiness, disengagement, physical relaxation, mental quiet, refreshed, at ease/peace, energized, joyful, thankfulness and love, awe, prayerfulness, timeless and aware (Ghoncheh, 2004).
     There are many different ways to relax.  Health professionals have identified six major approaches: progressive muscle relaxation (PMR), autogenic training, breathing exercises, yoga stretching, imagery and meditation.  It is important to note that the boundaries between the different types of relaxation are not always clear.  Yoga asanas are often accompanied by breathing techniques or imagery, and yoga classes often include meditation. The technique most widely used by therapists is PMR.  This technique involves tensing and then letting go of muscle groups (feet, legs, hands, arms, face, etc.) one at a time, throughout the body. The most popular approach for the general public is yoga stretching (Ghoncheh, 2003).   

     It is often a challenge to make ourselves relax.  If the circuits in the cortex of our brain that are used to will ourselves to relax are not used regularly, they gradually become dysfunctional, and the input from unconscious regions of the brain gets relatively more powerful.  The more we practice relaxation, the more established are the patterns in our brain that can remember the experience, and relaxation becomes more accessible (Coulter, 2001).

MUSCLE TENSION /RELAXATION.

     There are two functional divisions of our nervous system, both of which are involved in regulation of tension and relaxation.  The autonomic nervous system discussed above controls the function of organs, blood vessels and glands.  The somatic nervous system is concerned with voluntary skeletal muscle activity and conscious sensations including touch, vision, pain, pressure and audition.  While these systems are relatively autonomous they also are constantly interacting, exchanging information vital to smooth coordination of all our body functions.  The most important function in the body where these systems collaborate is respiration, the rate and depth of breathing.  This relationship will be explained in further detail in the breathing section of this paper. 

     Skeletal muscle tension, or physical strain, is observed in and reported by many people during stress or anxiety.  It is largely responsible for the feeling of being tense.  This may be observed as clenched fists, clenched jaw, and repetitive voluntary movements.  All these muscular manifestations of stress are observed to decrease when relaxation occurs (Reilly, 2000). The mechanics of muscle function will be discussed below in this paper in the stretching section.

RESEARCH

     Yoga originated thousands of years ago.   Many different styles and techniques of yoga have evolved, making the study of yoga a daunting task. However, since yoga became more popular in the west in the 1970s, many research studies have attempted to identify the effects of yoga and the neurophysiologic processes through which it works.  In order to study the effects of yoga on the level of anxiety or relaxation that a person is experiencing, researchers measure changes in any of the many physiological processes that are involved in the stress response. 

     One study in Thailand looked at the effects of yoga on hypertension.  Yoga pranayama (breath control) and asanas were taught in an 8 week program.  The experimental group (as compared to a control group) showed a significant decrease in scores on a stress assessment questionnaire, as well as showing a decrease in blood pressure and heart rate (McCaffrey, 2005).
      Another study, done by P. K. Vedanthan, M.D. of the Northern Colorado Allergy and Asthma Clinic taught yoga (including asanas, pranayama and meditation) for 45 minutes three times per week to a group of asthma sufferers. After four months they reported significantly more relaxation and a more positive attitude, and tended to use their inhalers less, than a control group (Black, 20001).

     In another study, conducted by Thomas Jefferson Medical College, a yoga class was given to healthy new yogis every day for seven days.  The researchers measured the subjects' cortisol levels before and after the yoga class and discovered a significant decrease after the class (Knittel, 2003).

     Brown and Gerbarg studied Sudarshan Kriya Yoga (SKY) breathing and its effect on anxiety.  The SKY program involves a series of asanas that use highly regulated forms of voluntarily controlled breathing or pranayama. One component of their research studied ujjayi breathing which involves slow breathing, laryngeal contracture, inspiration against airway resistance, prolonged expiration against airway resistance and breath holding. The slow breathing is done in three different asana positions, with hands held at different levels - waist, chest, upper back- to maximize awareness of each lobe of the lungs. One of several indicators that Brown and Gerbarg studied was heart rate.  Our heart rate is higher when we inhale than when we exhale, a phenomenon referred to as respiratory sinus arrhythmia (RSA).  It has been observed that the difference between the heart rates of the two phases (RSA variability) is greater under parasympathetic dominance than under sympathetic dominance. In fact, RSA variability is low in individuals with anxiety and panic disorders.  During the ujjayi breathing exercises there was an increase in RSA variability, indicating an increase in vagal tone (activity of the vegas nerve reflecting parasympathetic activity).  During the ujjayi subjects reported feeling calm but alert and attentive (Brown, 2005). 

HOW DO YOGA ASANAS HELP US RELAX?

     All approaches to relaxation appear to involve physical stillness and some kind of mental control (Watts, 2000).  Herbert Benson suggests that in order to evoke the relaxation response there must be a passive attitude and a mental device such as a sound, a word or a phrase, repeated silently or aloud, or fixed gazing at an object (Benson, 1975).  With PMR the focus is tensing and letting go of a series of voluntary skeletal muscle groups.  Yoga asanas, or physical positions, focus on creating and maintaining a stretch (Ghoncheh, 2003).  Yoga appears to be one of the more accessible of the relaxation techniques because of its somatic focus (Watts, 2000). Which technique is preferable may vary from one person to the next; however John Kabat-Zinn has suggested that patients suffering from anxiety and depression appear to prefer the body-focused approach of yoga (Kabat-Zinn 1992). Important elements of yoga asanas that will be presented in this paper include posture, breathing, and stretching/exercise.

POSTURE

     The focus of yoga is a physical posture.  Sensations are held as constant as possible by maintaining a posture, or else the sensation of the posture is stabilized by repetition.  Stability is important because our attention is most easily drawn to changes in our environment, and when changes occur our brains are processing many different elements of information related to the changes.  Stability in our peripheral sensations allows our central cognitive processes to move more independently, in a way that is different from normal, and seems to allow a heightened degree of vividness on whatever is our focus (Watts, 2000).  With yoga asanas the focus is posture, muscle stretching, and breathing.  As well, the emphasis in yoga is usually not on a struggle to force control in a particular position, but instead the intent is to be relaxed and subjectively effortless.  This type of control, willful but at the same time free of struggle, seems to allow a sense of harmony and integration within the mind (Watts, 2000). 

     Another aspect of posture is that the actual physical position that the body is in will have an influence on our physiology.  Internal organs may be squeezed or expanded by the body position.  B.K.S. Iyengar states that when organs are compressed the blood or lymphatic fluids are squeezed out, and when the pressure is released the area is soaked with fresh oxygenated blood or fresh fluid.  For example, forward bends will reduce pressure on the adrenals while backbends will compress them.  It appears that keeping the internal organs of the body refreshed in this way will optimize their functioning, supporting a sense of well being (Sparrowe, 2001).  Stated another way, P.R. Sarkar explains that all asanas have either a pressurizing or depressurizing effect on the glands (Sarkar, 1994), and regular practice of asanas allows the body to stay healthy and hardy, and many diseases are cured (Sarkar, 1992).

     Posture also has an effect on breathing.  A backward bend will expand the abdomen and lung cavity, allowing greater intake of oxygen, while a forward bend will do the opposite.  This is further discussed in the breathing section of this paper.

 YOGIC BREATHING

     During yoga asanas there is typically an instruction regarding breathing, or pranayama.  Many different breathing techniques are taught with the many types of yoga that exist today.   Some are calming and some are energizing.  The most basic modification that facilitates relaxation involves shifting our automatic breathing, which is often shallow, to abdominal or diaphragmatic breathing.  In abdominal breathing we make a conscious effort to inhale deeply enough that the diaphragm contracts and the abdomen rises, allowing more space for air to enter the lungs.  The inhalation and exhalation are both slowed and take about the same amount of time.  How does this help us relax?
      Most of the time our breathing is unconscious.  Respiratory control centers in the brain stem, (the pons and the medulla) regulate our breathing through input to the nerves which control the skeletal muscle activities of respiration.  This pathway is part of the somatic nervous system.  If the level of tension in our body changes, our breathing patterns change through automatic responses under the control of the autonomic nervous system in coordination with the somatic nervous system(Coulter, 2001). When a person is stressed the sympathetic nervous system is dominant and breathing becomes shallow and rapid.  When relaxed, our body shifts to parasympathetic dominance and breathing is slow and deep.

     When we are in a stressed breathing pattern, our conscious minds are able to take notice and we have the ability to decide to slow our breathing down (Hendricks, 1995). This is possible because the muscles involved in inhalation and exhalation (the respiratory diaphragm, the abdominal muscles and the intercostal muscles) are under both voluntary and involuntary control.  If we make a decision  to voluntarily slow down the action of these muscles through deep, slow and controlled breathing we can promote a shift in the balance between the sympathetic and parasympathetic nervous systems, increasing the action of the parasympathetic system, producing a sense of calm and relaxation (Coulter, 2001, Brown, 2005, Khalsa, 2002).

     An important aspect of the shift to parasympathetic dominance through controlled breathing is related to the basic function of breathing, which is to oxygenate our blood.  When we breathe shallowly we are mostly using our intercostal muscles.  When we employ deep breathing, we emphasize the muscles of the abdomen and the respiratory diaphragm and the oxygen flows into a greater area of our lungs.  In addition, the rate of blood flow in the bottom of our lungs is significantly greater than the rate at the top of our lungs.  When we breathe deeply our blood is getting more deeply oxygenated so our lungs don't have to work as hard, and our breath rate will slow down.  Heart rate can also slow down because the heart does not have to pump as much blood to get a full supply of oxygen to the body (Hendricks, 1995).  Here we see the place where voluntary control of breathing, a function of the somatic nervous system, has a direct affect on our heart rate and breath rate, functions of the autonomic nervous system.  The information about blood oxygenation and heart and respiration rate is picked up and relayed through the autonomic nervous system, and coordinated through a complex series of feedback mechanisms in the somatic and autonomic nervous systems.    

    Another aspect of breathing while doing an asana is the kumbhak, or breath holding between the inhalation and exhalation phases of pranayama.  In one study heart rate increased during two varieties of kumbhak that had no end-inspiratory and short end-expiratory breath holding.  In contrast, heart rate did not increase with two varieties of kumbhak that did have end-expiratory and end-inspiratory breath holding.  The lower heart rate is an indicator of a relatively higher parasympathetic activity, implying greater relaxation as a result of the associated breath pattern.  The variation of heart rate seen here is a powerful reminder of the complexity and subtlety underlying the performance of a single yoga asana (Brown, 2005). 

STRETCHING and EXERCISE

     When a muscle contracts, tension is generated, and the resulting action is that it may shorten or lengthen, depending on the work it is trying to do (Appleton, 1994).  Relaxation is the reversal of the contraction (Thibodeau, 1987).  When a muscle is stretched an interesting phenomena occurs.  As the stretching muscle lengthens, a nerve impulse sends the information about the change in muscle length to the spine and triggers a stretch reflex, causing a contraction of the stretched muscle.  This stretch reflex helps to maintain muscle tone and protect the body from injury.  The more sudden the change in muscle length, the stronger the stretch reflex will be.  In yoga we hold the stretch for a prolonged period of time.  The benefit of this is that the muscle habituates, or becomes accustomed to the new length, and then reduces the signaling which is triggering the contraction of the stretch reflex.  Over time the stretch receptors in your muscles will tolerate more tension and allow a greater lengthening of the muscles being stretched.  In addition, when muscle tension reaches a certain threshold, another reaction called the lengthening reaction occurs.  This inhibits the muscles from contracting and causes the muscles to relax (Appleton, 1994).  Holding a yoga asana for a period of time also allows the lengthening reaction to occur, and the stretched muscle relaxes.  As mentioned earlier, when our muscles are free of tension, we associate this body state with relaxation.

     Some asanas, especially forward folds are especially relaxing.  Other asanas, like back bends or a sun salutation sequence, require our muscles to do a fair amount of work, and can be quite energizing, increasing our heart rate and respiration.  How can this be relaxing?  Exercise is well known to help relieve stress and anxiety (Bourne, 1995).  Stress often causes us to hold tension in our muscles, so that they are in a state of mild contraction, and the muscles lose their normal resting muscle tone.  The work involved in physical activity allows muscles to release stored energy and to return to their normal resting tone, reducing muscular tension (Randolfi, 2005).  This promotes a feeling of relaxation after exercise. In addition, remember that an anxious person may have an ongoing, low level activation of the stress response. When we exercise we simulate the fighting or running that the stress response has prepared us for.  Heart rate, respiration rate and blood circulation increase.  This helps to remove (metabolize) excess adrenaline and other hormones or neurotransmitters are helping to maintain the anxious state.  This allows the body to return to homeostasis faster and reduces the impact of the stress (Bourne, 1995, Randolfi, 2005).  The balance of the stimulating and the relaxing aspects of yoga asanas produces a calm but alert state and contributes to the "feel good" sensation that we have after yoga practice (Weintraub, 2004).

PUTTING IT ALL TOGETHER

    Breathing, stretching, posture and relaxation are intimately connected. One place to see this is in forward folds, such as pascimottanasana or yoga mudra.  When we do a forward fold we exhale.  The lungs deflate and the diaphragm lifts into the chest creating more space in the abdominal cavity.  This makes it easier to do a forward bend.  In addition, exhalation also relaxes the muscles of your back and tilts your pelvis forward.  When we inhale, there is an active contraction of the lower back.  This works in opposition to a forward bend.  So if we are sitting in an asana that involves a forward bend, each time we exhale our back muscles release, and slowly we are able to fold further forward.  As we hold the pose the stretch reflex also relaxes, permitting the muscles to stretch further.  And yet another process is also cooperating with this effort.  When we exhale, the heart rate slows.  Blood pressure then decreases, as does stress on the rib cage, abdominal walls and intercostal muscles.  The nervous system is pacified, the muscles have released their tension and we may experience a pleasant sense of deep relaxation or well being (Ruiz, 2000).

     Shavasana or corpse pose also brings together several elements of relaxation.  In shavasana, the yoga student is supine (lying on their back), breathing deeply and remaining as motionless as possible.  Abdominal breathing in the corpse pose is the most relaxing of any breathing in any posture. When we are lying down, and the chest and spine are fixed, the abdominal wall yields most easily to the action of the diaphragm, and the breath becomes very delicate (Coulter, 2001).  As a result, respiration produces little movement in the rest of the body, minimizing muscle tension.  In addition, shavasana is done at the end of a course of yoga postures when we are already in a pleasant mental and physical state, making it easier for relaxation to occur (Coulter, 2001).  For all these reasons shavasana is an ideal position for relaxation.  

     Yoga asanas provide an easily accessible form of relaxation which is effective and enjoyable.  However, yoga can be much more than that. The word yoga means union, referring to the union of mind, body and spirit. When accompanied by a spiritual ideation yoga can pull your mind inward toward higher states of consciousness. Subtle energies that we do not yet understand may be involved in the way that yoga affects us.  P. R. Sarkar suggests that hatha yoga is a form of yoga that strives for perfection of the practice of yoga, where the goal is a complete suspension of the mind and its psychic propensities (an ultimate relaxation, perhaps?).  However, he says that if yoga is treated as a means of realizing Parma Pursusa (God), then this is the real yoga . (Sarkar, 1979).

                                                  Selected References

Appleton, B. (1994). Stretching and Flexibility.  http://www.cmcrossroads.com/bradapp/docs/rec/stretching.

Benson, H., 1975. The Relaxation Response. New York: Avon Books.

Black, K. (winter 2000-2001). Yoga Under the Microscope.  Yoga Journal.com,   http://www.yogajournal.com/health/114_1.cfm.

Bourne, E.J. (1995). The Anxiety & Phobia Workbook. Oakland, Ca., USA: New Harbinger Publications, Inc.

Brown, R. & Gerbarg, P. (2005).  Sudarshan Kriya Yogic Breathing in the Treatment of Stress, Anxiety, and Depression: Part I - Neurophysiologic Model.  The Journal of Alternative and Complementary Medicine, Vol. 11(1), 189-201.

Coulter, H. David. (2001). Anatomy of Hatha Yoga. Body and Breath: Honesdale, Pa., USA

Ghoncheh, S. & Smith, J. (2004). Progressive Muscle Relaxation, Yoga Stretching, and ABC Relaxation Theory.  Journal of Clinical Psychology, Vol.60(1), 131-136.

Hendricks, G., 1995. Conscious Breathing. New York: Bantam.

Kabat-Zinn, J. Massion, A.O., Kristeller, J., Petersen, L.G., Fletcher, K.E., Pbert, Khalsa, D. Singh & Stauth, C., 2002, Meditation as Medicine.  New York: Fireside.

Knittel, L. (2003).  Beginner's Bliss.  Yoga Journal.com, http://www.yogajournal.com/health/1064_1.cfm.

McCaffrey, R., Ruknui, P., Hatthakit, U., & Kasetsomboon, P. (2005).  The Effects of Yoga on Hypertensive Persons in Thailand.  Holistic Nursing Practice, July/August 2005.

McCall, T. (2004).  Can you prove that yoga works? Yoga Journal.com, http://www.yogajournal.com/newtoyoga/1266_1.cfm.

Ransolfi, E. (2005). Exercise as a Stess Mangement Modality. Optimal Health Concepts.com.

Reilly, C. (2000). Relaxation: A Concept Analysis, Graduate Research in Nursing 2,1, www.graduateresearch.com/reilly.htm.

Rosenzweig, M., 2005. Biological Psychology, Fourth Edition. Sunderland, Ma.: Sinauer Associates.

Ruiz, F. (2000) What Science Can Teach us about Flexibility. Yoga Journal.com, http://www.yogajournal.com/practice/209_1.cfm.

Sapolsky, R., 1994. Why Zebras Don't Get Ulcers. New York: W.H. Freeman and Company.

Sarkar, P.R., 1979 Ananda Purnima, printed in  Subhasita Samgraha part 12

Sarkar, P.R., 1992. Ananda Marga Caryacarya part III. Calcutta: Ananda printers.

Sarkar, P.R.,1994. Yoga Psychology. Calcutta: Ananda Marga Publications.

http://www.yogajournal.com/health/535_1.cfm

Sparrowe, L. (2001). Good to the Bone.  Yoga Journal.com

Thibedeau, G. (1987). Anatomy and Physiology. St. Lewis/Toronto/Santa Clara: Times Mirror/Mosby College Publishing.

Watts, F. (2000). Psychological research questions about yoga. Mental Health, Religion and Culture, Vol. 3, Number 1.

Weintraub, A. (2004). Yoga for Depression. New York: Broadway Books.