Yoga Therapy : An evidence-based Medicine
Although yoga is historically a spiritual discipline, it has also been used clinically as a therapeutic intervention. A bibliometric analysis on the biomedical journal literature involving research on the clinical application of yoga has revealed an increase in publication frequency over the past three decades with a substantial and growing use of randomized controlled trials. The scientific research literature contains over 300 scientific articles on Yoga covering its effects on a wide range of health conditions. Research began as early as 1918 and has been accelerating in recent years. Considerable information has been gathered through the scientific research process. Specific series of postures and other techniques have been indicated as being beneficial for specific health conditions and populations.
Yoga therapy is a relatively novel and emerging clinical discipline within the broad category of mind-body medicine, whose growth is consistent with the burgeoning popularity of yoga in the West and the increasing worldwide use of alternative medicine. Many studies have examined the effectiveness of specific Yoga practices. Scientifically significant results have been found for: Chronic pain, Asthma, Arthritis, Depression, Anxiety, Irritable Bowel Syndrome, Premature ejaculation, prenatal health, Menopause, Cardiovascular diseases, Diabetes …and many more!
Health benefits of Yoga
The relative health benefits of yoga in relation to disease risk and its role in the management of chronic diseases is less clearly established. Studies have investigated physiological responses evoked by yoga practice in comparison to those evoked by more conventional forms of exercise. The heart rate response to typical yoga sessions in healthy adults at normal ambient temperatures has been shown to be equivalent to low intensity walking exercise in some studies (12, 13). Exercise at this intensity does not meet the currently recommended level of physical activity needed to promote health and cardiovascular fitness. However, other studies have provided conflicting evidence for healthy adults, with higher levels of cardiopulmonary stress being recorded during yoga sessions (14). Additionally, improvements in indices of cardiometabolic health have been observed in some (but not all) studies in healthy adults following programmes of yoga practice. A number of single group (uncontrolled) studies have reported improvements in maximum oxygen capacity (15-18), muscular strength (17,19), flexibility (18) and blood cholesterol profile (15), as well as reduced physiological effort at sub-maximal exercise intensities (20) and a lower level of perceived exertion at maximal exercise capacity (17). Such cardiometabolic adaptations suggest that yoga can provide a level of cardiopulmonary stress that is sufficient to achieve health benefits. Other benefits from yoga practice in healthy participants have been reported to be improved respiratory inspiratory and expiratory pressures and visual and auditory reaction times (19) and attenuated weight gain in overweight individuals (21). While some studies have found no improvement in cardiopulmonary variables after programmes of yoga practice (e.g. Blumenthal and others (22)), the actual level of physical exertion experienced during a session, and thus the stimulus for cardiometabolic adaptations, is likely to be strongly influenced by the type of yoga, the level of experience of the practitioner and the ambient temperature during the session. Yoga practice also involves a spiritual dimension and specific breathing exercises, not common to conventional forms of exercise, which may evoke other health benefits.
A review of the literature by Ross and Thomas (23) identified 12 studies that had compared the effects of yoga with more conventional forms of exercise in adults and older adults. A total of 597 of the 873 volunteers who participated in the studies were women and most of the studies compared yoga practice with aerobic exercise, such as walking, running, dancing, stationary cycling, etc. Three of the studies involved stretching and/or range of motion exercises. Four of the studies were in clinical 9 populations (type 2 diabetes, multiple sclerosis, haemodialysis patients, and schizophrenics). Eight of the 12 studies were RCTs and study durations ranged from a single session to 6 months. These studies assessed a wide range of physical and psychosocial outcomes, with evidence that conventional forms of exercise are more beneficial for increasing energy expenditure and improving in maximum aerobic capacity in the healthy volunteers. However, there was evidence that many other health outcomes were improved more by yoga practice than by conventional exercise in the healthy volunteers. These included indices of autonomic nervous system, perceptions of fatigue, flexibility, menopausal symptoms, psychological stress and metabolic markers.
In the clinical populations, one study showed that in patients with multiple sclerosis, yoga and conventional forms of exercise had a similar impact on perceptions of fatigue and quality of life. However, another study showed that yoga was more beneficial for schizophrenic patients in relation to control of psychotic symptoms, social and occupational functioning and improvements in quality of life. For haemodialysis patients, one of the trials showed that yoga was more beneficial than conventional exercise for promoting improvements in kidney function (urea, creatinine excretion), perceptions of fatigue, pain, sleep disturbance, grip strength and total cholesterol levels. Finally, for patients with type 2 diabetes, one of the included studies showed similar benefits for yoga practice in comparison with conventional exercise on fasting blood glucose, total cholesterol and other metabolic markers.
Hot yoga is performed in a room that is preheated to approximately 105o F. This is likely to increase the stimulus for cardiovascular adaptations, as the need for blood to supply oxygen to the exercising muscles is challenged by the need to supply blood to the peripheral vessels of the skin for thermoregulation. This means that for any given exercise intensity in warm/hot environments, heart rate and cardiac output must be higher to satisfy the competing metabolic demands of exercising muscles and body heat dissipation demands of the skin. The greater increase in body temperature encountered during a hot yoga class may have an effect on other physiological processes, including increased dissociation of oxygen from haemoglobin and myoglobin to the exercising muscles, a lowering of the activation energy rates for metabolic reactions, improved skeletal muscle blood flow (allowing improved oxygenation and removal of waste products) and an increase in the speed of nervous impulses (24). Additionally, the muscle-tendon unit is a visco-elastic structure, having both viscous and elastic properties and is capable of both plastic and elastic changes. The viscous properties of muscle connective tissue enable it undergo a permanent change in structure and the amount of viscous and elastic deformation is strongly influenced by the tissue temperature (25). Warm or hot ambient temperatures in combination with active muscular contractions could promote a greater reduction in the viscosity of muscle connective tissues, thereby enabling greater extensibility, and allowing deeper stretches to be achieved and potentially reducing the risk of injuries. Despite these potential physiological advantages over yoga practiced at normal ambient temperatures, higher sweat rates in warm or hot conditions necessitates proper attention to fluid replacement to prevent heat stress and a decline in physiological function.
To date, only one study in the published scientific literature has investigated the potential health benefits of hot yoga practice (26). This study investigated the effects of Bikram yoga on skeletal muscle strength and neuromuscular control in young adults. Individuals assigned to the yoga group participated in a 90 minute Bikram yoga class, three times a week for eight weeks (24 sessions in total). Sessions were performed in a heated (95-105o F) and humidified (60% relative humidity) studio. Participants in the yoga group were compared with a control group who were instructed to maintain their usual level of activity for the same time period. Knee extensor (thigh) strength was improved after the yoga intervention but there was no change in elbow flexor (biceps) strength. An improvement in knee extensor steadiness (indicating better lower-limb neuromuscular control) and timed one-leg balance was also observed in the yoga group after eight weeks. Interesting, an improvement in knee extensor steadiness was also observed in the control group which might indicated a learning (practice) effect. The modest strength improvements observed for the knee extensors were attributed to the greater focus of the exercise training stimulus on the lower limbs. The improvements in lower-limb force and balance evoked by the Bikram yoga programme could have significant impact on individuals with impaired leg steadiness and balance, suggesting that further research in elderly populations is warranted.
Yoga and Chronic Disease Risk Factors
Two systematic reviews of the literature on the ‘protective’ effect of yoga in relation to insulin resistance, type 2 diabetes and cardiovascular disease were undertaken by Innes and others (27,28). These two systematic reviews identified many of the same studies. The first review (27), which was much broader in scope, identified 70 studies, including 22 RCTs, 21 non-randomized controlled trials, 26 uncontrolled trials and one observational study. The broad conclusion from this review was that considered together, yoga practice was associated with beneficial changes in a range of outcomes that are indicative of insulin resistance and predictive of cardiovascular disease, including glucose tolerance, insulin sensitivity, lipid lipoprotein levels, anthropometric markers, blood pressure, oxidative stress and blood coagulation markers, autonomic function and clinical end-points. The majority of reviewed studies were conducted in healthy young to middle-aged adults (50%) or adults with or at risk of cardiovascular
disease (30%). The second review (28) was more focused on adults with type 2 diabetes and identified 25 studies, including four RCTs, six non-randomized RCTs and 15 uncontrolled trials. The conclusion from this review was almost identical to the first review (27) and the evidence suggested that yoga practice can improve cardiovascular risk profiles in patients with type 2 diabetes, and could have a role in the prevention and management of cardiovascular complications in this clinical group. However, severe methodological limitations was a feature of many studies included in both these systematic reviews (e.g. non-randomized or uncontrolled designs, small participant numbers, poor description of the yoga intervention, etc.), which prevents firm conclusions from being drawn.
A systematic review conducted by Yang and others (29) identified 32 studies that had investigated the impact of yoga practice on four well established biological markers and risk factors for chronic disease, namely excess body weight, hypertension, elevated fasting glucose level and high cholesterol. Poor methodological quality was not a reason for exclusion of studies from the review, but studies were excluded if they were primarily focused on meditation or relaxation or if they were case studies. As well as intervention studies, observational studies were also included. The reviewed studies revealed evidence of beneficial effects of regular yoga exercise in relation to reductions in body weight in overweight participants and in patients with coronary heart disease, fasting blood glucose levels in patients with type 2 diabetes and coronary heart disease and blood pressure in healthy individuals and clinical populations, such as those with hypertension, cardiovascular disease and type 2 diabetes. In addition to the impact of yoga on these biological markers, these studies show that yoga is a feasible exercise intervention for these clinical populations to engage in.
A more rigorous systematic review of studies that had investigated yoga practice for the management of type 2 diabetes undertaken by Aljasir and others (30) yielded only five studies that had recruited a total of 363 participants. The methodological quality of these studies was moderate to low. Yoga practice alone was studied in three of these studies, and in combination with other lifestyle modifications in the other two. All studies compared the effects of yoga practice with those of a control group receiving standard medical treatment only. Three of the included trials reported a beneficial effect of yoga practice on circulating levels of total and good ‘HDL’ cholesterol and triglycerides. Inconsistent results were observed for circulating levels of harmful ‘LDL’ cholesterol. An improvement in nerve conduction velocity was also observed in one of the studies but this was not statistically significant. The results of these studies suggest favorable effects of yoga practice on clinical diabetes outcomes, but only in the short term as results for long-term outcomes were generally not significant. Methodological limitations prevented a definitive recommendation for physicians to encourage their patients to engage in yoga practice.
Yoga during Cancer Rehabilitation
There is a growing interest in the role of yoga and other exercise interventions for ameliorating the physical and psychological side-effects of cancer and its treatments. This has resulted from increasing rates of cancer survivorship and the well-documented lingering symptoms such as fatigue, sleep disturbances and joint pain that are frequently encountered but often inadequately treated with conventional approaches (31).
At least two reviews of the published literature have been focused on yoga as a complementary therapy for cancer patients and/or survivors. A narrative review of mind-body therapies for cancer by Elkins and others (32) identified nine small-scale studies that had investigated the effects of yoga practice on clinically important outcomes in cancer patients. Patients with lymphoma who participated in a seven week Tibetan yoga intervention reported lower sleep disturbance than a waiting list control group but the yoga intervention had no effect on measures of anxiety, depression or fatigue (33). In contrast, short-term yoga interventions evoked improvements in psychosocial outcomes such as anxiety, depression (and mood), mental health, quality of life, psychosocial functioning, cancer-related fatigue and spirituality (and spiritual wellbeing) and hot flashes in breast cancer patients (34-39). Another single group design study reported reduced levels of pain and fatigue in a sample of 13 metastatic breast cancer patients after participation in an eight week programme of gentle yoga (40). Two other studies involved larger samples: 128 breast cancer survivors and a large multi-centre randomized controlled trial involved 410 early-stage cancer survivors. In the breast cancer trial, women allocated to a 12-week Hatha yoga intervention experienced moderate improvements in fatigue, overall mood, spiritual wellbeing and quality of life in comparison to a waiting list control group (41). In the multicentre study (42), participants who were randomized to a twice-weekly 75 minute yoga class for 4 weeks experienced a 22% improvement in sleep quality compared with a 12% improvement in the control group. The yoga group showed these sleep improvements despite a 12% reduction in their sleep medication, in comparison to a 5% increase in sleep medication in the control group. In addition, the yoga group reported a 42% reduction in cancer-related fatigue and a 6% improvement in quality of life, versus a reduction of 12% in cancer-related fatigue and no change in quality of life in the controls. These results show some promising data for clinically important cancer outcomes but most studies to date have involved very small numbers of patients and more larger-scale, well controlled prospective intervention studies are needed in a broader spectrum of cancer patients and survivors.
An evidence-based review of studies that investigated the efficacy of yoga as a complementary intervention for patients for cancer by Smith and Pukall (43) identified a further four studies that were not considered by the previous review. Of these, two studies were RCTs (44,45) and two were uncontrolled trials (46,47). One of the RCTs reported improvements in emotional functioning and quality of life in breast cancer patients not currently engaged in any other form of treatment in comparison to a
waiting list control group after a 7 week programme of yoga therapy (45). The other RCT reported a decrease in post-chemotherapy-induced nausea frequency and nausea intensity, intensity of anticipatory nausea and anticipatory vomiting in breast cancer patients who engaged in a yoga therapy programme compared with a supportive therapy control group (44). Outcomes were assessed before and after their fourth cycle of treatment. The two uncontrolled trials reported improvements in psychosocial variables, including feelings of peace, spiritual wellbeing, positive mind state and decreased stress levels in cancer patients following programmes of yoga therapy (46,47).
Yoga for Stroke Rehabilitation
There are very few studies on the efficacy of yoga for stroke rehabilitation. A systematic review of the literature undertaken by Lynton and others (48) revealed no controlled studies of yoga for stroke rehabilitation. Only one small-scale study involving an eight-week programme of Hatha yoga (2 x 1.5 hour sessions per week) in four stroke patients was identified. Statistical analysis of the data was not possible from such a small number of patients, but at the individual level, two patients experienced an improvement in balance and three of the patients achieved improvements on a timed battery of movement tests. As all patients were at least 9 months post-stroke and had reached a plateau of recovery, these results are promising and are perhaps unlikely to have resulted from natural recovery of function over time.
A small pilot study involved a 12-week course of twice-weekly 1.5 hour Kundalini yoga classes in three stroke patients (48). The patients were six-months post-stroke and all had a diagnosis of aphasia. The yoga postures were modified to accommodate the capabilities of the patients. All three patients showed an improvement in their speech impediment and dexterity after the yoga intervention. The authors felt that the results of this small scale study were very promising and justified further investigations of yoga for chronic stroke rehabilitation.
Yoga and Asthma
A review of the literature by Steurer-Stey and others (49) identified four randomized controlled trials that had investigated the effects of yoga practice in asthmatics. One study (50) investigated the effects of yoga practice in 53 pairs of asthmatic patients that were matched for age, sex and severity of asthmatic symptoms. Patients were randomized to receive yoga therapy or standard treatment (no yoga therapy). Over 54 months of follow-up, the yoga group experienced an improvement in the number of weekly asthma attacks and peak flow rate, as well as a decrease in drug treatment score. Another smaller-scale study of asthmatic students reported non-significant improvements in medication use, lung function and symptom score in the group randomly assigned to yoga practice. The other two trials (51, 52) compared the effects of yoga breathing exercises (Pranayama) with other breathing exercises or no intervention. These reported positive changes in mental state (coping with stress, subjective perception of symptoms, etc.) and at least trends for improvement in lung function parameters in the intervention groups. The latter three trials involved only 75 patients in total, and lack of statistical power most probably explains why changes in some of the outcomes were inconclusive.
Yoga for Depression and Anxiety
A critical review of the evidence by Uebelacker and others (53) identified seven randomised controlled trials and one non-randomised controlled trial that had investigated the effect of yoga practice on symptoms of depression in individuals with a diagnosis of depression or elevated depression symptoms (Uebelacker et al. 2010). Of the five studies that compared yoga to no treatment or a minimal-treatment control group, four showed evidence of improvement in depressive symptoms in comparison to the controls (54-57). In three of these trials, the participants had a clinical diagnosis of major depressive disorder and the fourth trial involved students with mild depressive symptoms but no psychiatric diagnosis. Of the other trials, one showed more favourable effects for electroconvulsive therapy over Sudarshan Kriya yoga (SKY) in clinically depressed patients (58), a second trial showed that SKY was not more effective than partial SKY (pranayama only) in alleviating depressive symptoms in clinically depressed patients (59), the third trial reported that yoga exercise was no more effective than aerobic exercise for reducing depressive symptoms in individuals with elevated depressive symptoms and the final trial involving students with severe depressive symptoms did not undertake a statistical comparison between groups randomised to Shavasana yoga or no treatment. There was considerable methodological heterogeneity between the studies which varied in duration (few days to 12 weeks), intensity (daily to once per week), yoga type (e.g. SKY, Iyengar and other meditation-based interventions) and control group. Furthermore, five trials had considerable methodological weaknesses and inconsistencies in results between the trials means that it is difficult to draw any generalised conclusions.
Anxiety disorders include phobias, panic disorder, obsessive compulsive disorder, post-traumatic stress disorder and generalized anxiety disorder. A few studies have reported positive effects of yoga practice on measures of anxiety in non-clinical populations (60-64). Two of these studies in particular are worth further consideration. In the first of these studies (57), the effects of twice weekly yoga classes for 2 months were investigated in women experiencing symptoms of depression and anxiety. In comparison to the waiting list control patients, women in the yoga group experienced a significant reduction in state and trait anxiety levels. In the second of these studies (65), 130 volunteers with mild to moderate levels of stress were randomized to a weekly 1 hour session of hatha yoga or relaxation for 10 weeks. After 10 weeks, yoga and relaxation were found to be equally effective for reducing stress and anxiety but yoga was more effective for improving mental health status. However, at the longer term follow-up time point of 16 weeks, more positive effects were observed in the relaxation group, which is likely to be a reflection of the higher proportion of participants in the relaxation group who continued to practice.
Kirkwood and others (66) undertook a systematic review of the research evidence on the effectiveness of yoga for the treatment of anxiety and anxiety disorders. Eight studies were identified, six RCTs and two non-randomized controlled trials. In five studies, the participants had been diagnosed with an anxiety disorder (anxiety neurosis, obsessive compulsive disorder and psychoneurosis), in two studies the volunteers were suffering from examination anxiety and snake phobia and in the remaining study, the participants were about to undergo an anxiety-provoking examination. One of the studies (67), which compared yoga with a meditative control group in patients with a diagnosis of OCD, showed significantly greater improvements on the Yale-Brown OCD Scale and other scales in the yoga group after 3 months. Two studies investigated the effects of yoga as a treatment for anxiety neurosis in hospital outpatients (68, 69). One (68) reported a clinically significant reduction in Hamilton Anxiety Scale score in the yoga group versus controls after 3 weeks and the other (69) showed evidence of greater reductions in anxiety levels and overall improvement of symptoms in yoga versus diazepam treatment group after 3 months. Both these trials were non-randomized designs. A further two studies undertaken by the same research group (70, 71) investigated the effects of yoga in psychoneurosis (mixed anxiety and depression disorders) or psychosomatic disorder. In the first of these studies, significantly lower anxiety scores were recorded in a yoga verses ‘placebo yoga’ group after 4 weeks. In the other study, patients who engaged in yoga practice experienced a greater reduction in Taylor’s Anxiety Scale scores after 6 weeks than patients allocated to a drug treatment group. Other studies provided some evidence that relaxation techniques derived from yoga practice could be more effective than progressive relaxation and no intervention (control) for reducing pre-examination anxiety levels in university students (54,72).
Yoga for Sleeping Disorders
A systematic review of complementary medicine approaches for insomnia by Kozasa and others (73) identified only one study that had investigated the potential health benefits of yoga practice in this respect. This study was a RCT involving 120 older people with sleep complaints who were randomized to one of three groups: yoga, Ayurvedic herbal medicine or waiting list control. After six months of practice, the yoga group showed a decrease in the time taken to fall asleep, an increase in the number of hours slept per night and an improved feeling of being rested in the morning in comparison to the other two groups. The sample of older people recruited for this study reported have sleeping problems and did not have a clinical diagnosis of chronic insomnia. A cross-sectional study published in 2009 provided some further support for the role of yoga in promoting sleep quality (74). In this study 16 volunteers with a minimum of 3 years of yoga practice were compared with a control group. Long-term yoga practice was associated with better sleep quality and a modulatory action on circulating levels of cortisol.
Yoga for Children and young adults
A systematic review of the literature undertaken by Birdee and others (1) identified 19 randomized controlled trials and a further 15 non-randomized controlled trials involving children and young adults. A large majority of the studies reviewed reported positive effects of yoga practice on physical fitness and cardio respiratory health. Two nonrandomized studies suggested that yoga, as well as more conventional exercise such as swimming and dance, promoted mental health benefits in young adults older than 18 years of age. As a method of building discipline and self-awareness in children, intuitively, yoga has potential benefits for children with attention problems. Initial evidence also suggests that yoga has potential to reduce ADHD symptoms in children (3) but no conclusive studies exist at the present time. Studies that have investigated yoga as an intervention for reducing anxiety and stress and improvement in coping abilities/mood states have yielded mixed results (3). Although the large majority of reviewed studies reported health benefits for children and young adults who participate in yoga classes, methodological limitations prevent strong conclusions from being drawn and the evidence can only be considered preliminary. Further studies with improved experimental rigour are required before firm conclusions can be drawn.
Interest in yoga is growing amongst older adults and could be an ideal exercise modality for promoting strength, flexibility and improved balance in elderly populations. A recent critical review of the literature by Roland and others (5) identified 10 studies (involving 544 participants) that had investigated the impact of yoga practice on physical fitness and function in older adults. Five of the studies were RCTs and the other five were uncontrolled trials. The length of the yoga intervention ranged from 4-24 weeks. The three larger-scale RCTs involving over 100 participants (22, 75, and 76) reported improvements in lower-body flexibility, endurance, shoulder and hip range of motion, balance measures and diastolic blood pressure following 4-6 months of yoga practice. Two small-scale RCTs also reported improvements in lower body strength, postural control, steadiness and balance confidence following programmes of yoga practice (77, 78) and these data were generally supported by the uncontrolled studies. The authors concluded that while the scientific evidence supporting yoga for improved fitness in older adults shows promise, methodological issues in the currently available literature means that further studies are needed.
A large majority of postmenopausal women experience menopausal symptoms, including hot flashes and night sweats (vasomotor symptoms), excessive fatigue and impaired sleep, mood disturbances and headaches. Innes and others (79) undertook a systematic review of the literature on mind-body therapies for menopausal symptoms. The systematic review identified three randomized controlled trials and five uncontrolled trials, ranging from 7-16 weeks in duration, which had investigated the effects of yoga and/or yogic meditation alone or in combination with education or other co-interventions on menopausal symptoms. Four studies investigated breast cancer patients and/or survivors. The majority of these studies reported an attenuation of symptoms (including overall symptoms, vasomotor symptoms, musculoskeletal pain, psychological distress and sleep disturbance) in women who had engaged in yoga and yogic-meditation practice. There was an overall reduction in menopausal symptoms of 36-80%, depending on the study design, with improvements being maintained in many cases for at least 3 months after completion of the programme. One of the reviewed studies showed that yoga and walking were equally effective for enhancing positive affect and menopause-related quality of life, as well as for reducing negative affect in middle-aged women (80). This RCT involved 164 women randomized to one of three groups, yoga, walking, or control group and the duration of the intervention was 4 months. The women who experienced improvements in menopausal symptoms also experienced improvements in all positive mental health and quality of life outcomes and reductions in negative mental health outcomes. Interestingly, the yoga intervention was less intensive (2 x 90 min classes per week) than the walking intervention (3 x 1 hour sessions per week, plus individualized home exercise prescription 1-2 days per week). Compliance to the sessions was also lower in the yoga group (63% versus 70%). Considered together, the weight of evidence from these reviewed studies suggests that yoga and yogic-meditation therapies can have a positive impact on menopausal symptoms in otherwise healthy women and breast cancer patients/survivors. However, the authors concluded that methodological limitations inherent in most of the studies hinder interpretation of the findings and mean that firm conclusions cannot be drawn until more rigorous RCTs with larger cohorts of women have been undertaken.
Summary and Conclusions
The weight of available evidence suggests that yoga practice is safe and can bring many health benefits to practitioners, whether they are young, old, healthy, recovering from illness or looking for a therapeutic option to help them to manage a chronic condition. It is important to be mindful of the fact that most positive evidence to date has emanated from studies that are considered to have only poor to moderate methodological quality, e.g. non-randomized controlled trials and uncontrolled, single group studies. In addition, many of the available studies in the scientific literature have been conducted in India and there is a relative shortage of good quality studies involving Western populations. However, these methodological weaknesses should be weighed against the inherent limitations in RCT design. Additionally, although RCTs offer the highest level of evidence, it is not always possible to discern the full range of health benefits that might be gleaned from this type of study. RCTs usually have a preselected set of outcome measures that are assessed before and after the intervention in both experimental and control groups. It is possible that this way of assessing efficacy might miss some important physiological or psychosocial variable that has a key impact on health or quality of life. The inclusion of qualitative outcomes in future studies, such as focus groups and structured interviews, and a greater involvement of yoga practitioners, teachers and patient groups in the design of studies, could help to overcome this.
For future research, a number of other issues need to be considered. Firstly, the studies that are currently available in the published literature involve a wide range of yoga ‘interventions’ in which the health benefits of different yoga styles have been investigated. For example, the yoga interventions in some studies have placed much more emphasis on breathing exercises (pranayama), whereas others have focused more on the physical components (asanas). This means that it is very difficult to compare across different studies or to develop a clear evidence-base for a particular type of yoga in a specific population. This problem is compounded by the wide variety of yoga intervention ‘doses’ that have been investigated across different studies. For example, most reviews of the literature (including some systematic reviews) have attempted to make broad conclusions about the health benefits of yoga from intervention studies that have not only differed in style but also programme duration. More high quality controlled studies, with larger sample populations, and with the purpose of investigating the health benefits of balanced yoga systems (e.g. involving pranayama, asana and meditation delivered by qualified teachers), are urgently needed. Ideally, both quantitative and qualitative physiological/psychosocial outcomes should be assessed over short-term and longer-term periods of follow-up. For optimal impact, the inclusion of diverse populations at risk of lifestyle-related chronic diseases would also be highly desirable.
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