Background
Chronic pain, or persistent pain, is a significant problem globally with substantial impacts to individuals, their support networks, and society. The International Association for the Study of Pain (IASP) defines chronic pain as “pain that persists beyond the normal tissue healing time, usually ≥3 months, in the absence of an obvious underlying biological cause” [
1]. Yet there is also a great deal of evidence associating chronic pain with precipitating events beyond that of specific tissue damage, specifically in association with interpersonal trauma. A systematic review and meta-analysis examining chronic widespread pain, in association with a range of traumatic events found participants exposed to traumatic events were 3.35 times more likely to suffer from chronic widespread pain [
2]. Another study (
N = 1152) reported the adjusted relative-risk of chronic pain and chronic pelvic pain in women exposed to psychological inter-partner violence to be 1.91 and 1.62 respectively [
3]. Further reviews have reported that individuals with a history of sexual abuse were 2.2 times more likely to be diagnosed with non-specific chronic pain, and 2.73 times more likely to be diagnosed with chronic pelvic pain [
4] and that such effects appear to be additional to those of physical injury-related pain [
5].
It is widely accepted within the current pain literature that chronic pain is closely associated with traumatic stress outcomes [
6], particularly post-traumatic stress disorder (PTSD) [
7‐
9]. PTSD is a complex ‘Trauma and Stressor Related Disorder’ [
10]. Current complex trauma theory emphasizes a psychobiological mechanistic model, that is, intense traumatic experience dysregulates the functioning of the autonomic nervous system and increases allostatic load [
11,
12]; where allostatic load refers to the added burden (cost, and wear and tear) to the autonomically dysregulated system working to restore homeostatisis [
13]. Certainly there is considerable evidence of associations of a dysregulated stress response and traumatic life experience [
14‐
16] and in particular early life stress [
17,
18]. However, autonomic dysregulation has also been implicated in the development and maintenance of chronic pain, said to “set up a feedback loop between pain and stress reactivity” [
19]. Which suggests that when the persistent pain patient is autonomically dysregulated, they have increased allostatic load (biopsychosocial burden), which in turn negatively impacts persistent pain levels [
20]. Yet there is very little research investigating the efficacy of psychobiological mechanistic modelled therapies for chronic pain (hence forth referred to as persistent pain).
Current psychobiological complex trauma therapies emphasize a bottom-up, right brain focus [
21]. Such approaches include body-oriented therapies [
22] and sensorimotor psychotherapy [
23], where emotional regulation and mind-body integration is targeted through focused breath, movement, posture, touch, balance, and mindfulness. Such components have also been recognised to be present in yoga, and as a consequence, trauma sensitive yoga treatments and programs have emerged [
24‐
27]. There have also been a number of RCTs investigating the efficacy of yoga for pain, specifically with regard to safety [
28] and pain improvements [
29]. However, such research is very much in its infancy and is hindered by methodological limitations, in particular, the lack of consistent, comparable forms of exercise [
30]. Regardless, a number of yoga for pain programs have been developed [
31]. The current trauma sensitive yoga and yoga for pain programs emphasize a gentle approach to the practice in an effort to promote relaxation and movement confidence in a minimally confronting way.
An alternative approach to target autonomic dysregulation may be bottom-up sensorimotor retraining/exposure therapy vigorous activity. Although sensorimotor retraining [
32], and exposure based treatments [
33] have previously been suggested to improve persistent pain, previous strategies have emphasized top-down processing and utilized exteroceptive stimuli. In this context, vigorous exercise, through bottom-up processing, would expose the pain patient to a high level of physical discomfort (interoceptive stimuli: rapid heart rate and respiration, muscle tension, perspiration), in a safe supportive environment. Exposure to safe and controlled physical discomfort will allow the pain patient to practice how they emotionally respond to physical and emotional sensations. Such practice can train the pain patient to better self-regulate, reducing their biopsychosocial burden, and by association, reducing persistent pain levels.
The primary aim of this feasibility study was to provide preliminary evidence of efficacy and assess feasibility of two types of vigorous exercise (Bikram yoga and high intensity interval training [HIIT]) as bottom-up sensorimotor retraining/exposure therapies to improve persistent pain severity and interference in women with persistent pain and a history of trauma. It was hypothesized that Bikram yoga would be the superior exercise as it possesses more core features of, and the focused mindfulness of, other psychobiological trauma therapies, while the HIIT does not. Second, the study aimed to explore the variability of impacts of Bikram yoga compared with HIIT on a range of persistent pain associated biopsychosocial factors. The outcomes of the study can be used to determine power and sample size for a full scale trial of vigorous exercise for persistent pain; and also provides evidence of the feasibility of vigorous exercise interventions for women with persistent pain and history of trauma.
Discussion
The present study found no difference in BPI TOT scores between women participating in a course of Bikram yoga compared with those completing a course of HIIT. Consequently, these results can not inform power calculations of future pain studies. However, as a whole, the women in this study did achieve statistically significant improvement in persistent pain levels, although it is impossible to determine if these improvements were specific to vigorous exercise. While no difference was seen between the two types of exercise in relation to pain, the Bikram yoga group achieved significant improvements in self-reported measures of physical functioning, mental health, and a physiological measure of heart rate variability, compared with the HIIT group. Furthermore, overall pain levels were diminished, only 3 women (9%) stopped attending classes after finding them too intense to continue, and the intention-to-treat completion rates were high. The results of the present study suggest that vigorous exercise interventions might be a feasible undertaking for people with a persistent pain condition.
Across the entire sample, the primary outcome measure (BPI TOT) was seen to significantly decrease from t0 to t1 but the level of decrease seen between the two types of vigorous exercise was not significant. As a result, it is not appropriate to conclude that the improvement in pain levels was due to the vigorous exercise as there are other influences that might also positively impact pain levels. For example: the Hawthorne effect [
69]; physical activity and exercise (not vigorous) [
70]; and the social benefits of participating in group exercise classes [
71] may have all contributed to the overall positive pain outcomes. Future research might involve a waitlist control arm and a non-vigorous exercise intervention arm to help differentiate whether any improvements in persistent pain levels can be attributed to the undertaking of a vigorous exercise intervention, and the waitlist control would differentiate from any social group-exercise and Hawthorne related trial effects.
The Bikram yoga group did see significant increases in the SF-36 physical functioning subscale scores, SF-36 mental health subscale scores, and the SDNN measure of heart rate variability, compared with the HIIT group. SF-36 physical functioning and mental health represent a physical and a mental assessment of quality of life. Improved quality of life outcomes have been frequently associated with a reduced burden of disease, most commonly arthritis, back pain, depression, diabetes, and hypertension [
72]. Increased SDNN is associated with increased heart rate variability which is considered a physiological indicator of better health [
73]. Specifically, increased heart rate variability is an indicator of improved parasympathetic functioning [
74] which has been associated with a wide range of positive medical [
75,
76] and psychological [
77,
78] health outcomes. Conversely, decreased heart rate variability has been associated with increased mortality [
79]. With regards to pain specifically, a meta-analysis of 11 studies investigating associations between persistent pain and heart rate variability, reported a large significant effect of decreased SDNN with increased persistent pain (− 5.43 (95% CI [− 8.54 to − 2.32]) [
64].
While our hypothesis that Bikram yoga would be the superior vigorous exercise intervention for improving pain was not supported, the above results provide limited evidence that Bikram yoga may have potential for targeting psychobiological mechanisms such as improved autonomic dysregulation and decreased allostatic load. For example, increased SDNN heart rate variability has been associated with improved sympatho-vagal balance [
80]. And lower burden of disease has been associated with less allostatic load [
81], specifically with regard to physical functioning [
82] and mental health [
83]. Therefore, Bikram yoga may have potential as a bottom-up, sensorimotor exposure therapy to improve autonomic dysregulation, and decrease allostatic load, but more evidence is needed.
The effect sizes were consistent with the between group differences observed, therefore, the outcome of the present study can be used to estimate required sample size for future studies investigating the efficacy of Bikram yoga for improving quality of life, physical functioning, mental health, and physiological indicators of better health such as heart rate variability. Clearly any intervention related change in scores must still be viewed cautiously due to the potential bias of placebo and Hawthorn effects. For example, it has previously been reported that the placebo effect accounts for approximately half of any improvements in the self-report psychological measures in exercise interventions [
84]. However, the SDNN measure of heart rate variability is a physiological measures obtained via an ECG assessment. Although this was the only physiological measure to see such a change, a previous yoga study has also reported a significant change in HRV, with no change in other ECG measures [
85]. Consequently, arguments that the positive gains of the study are bias due to the placebo effect may not be entirely valid. Furthermore, bias in the differences between the groups due to the Hawthorne effect [
86] might also be considered minimal as reportedly, the Hawthorne effect varies depending on the level of participation [
87]. In the present study, both the experimental and control arms had similar levels of participation.
Our findings suggest that vigorous exercise interventions may be safely undertaken by selected individuals with persistent pain. Firstly, on average, persistent pain levels were not seen to worsen. Secondly, the retention rates, overall and per group, were acceptable for a persistent pain exercise intervention [
88,
89]. However, the prescribed number of classes per week was three but the average number of classes attended each week was 1.4 for the Bikram yoga group and 1.2 for the HIIT group. This suggests that 3 classes per week was too much for the participants, perhaps due to such things as physical and muscle recovery time [
90]. Future vigorous exercise interventions involving persistent pain participants could decrease the prescribed exercise dose, to two classes per week for example, and they would still adhere to the Delphi recommendations [
48].
The study had several limitations. The recruited participants were women aged 20 to 50, self-selected volunteers from the community. They were informed at recruitment about the vigorous nature of the intervention and only those who felt they would be able to manage such intensive exercise proceeded. This is the main reason why one third of the 54 women who inquired about the study did not proceed. Therefore, the feasibility of a vigorous exercise intervention for persistent pain is biased to those who feel high functioning enough to manage it. Also, participants were not exposed gradually to the exercises however, both exercises lend themselves for the participants to be able to go at their own pace. Instead, participants were encouraged to attend three classes from the beginning. Intensity was increased with each class by the participant at their pace. Another limitation was the 8-week length of the intervention as this is the minimum recommended Delphi dose [
48]. The better quality studies (rated using the Oxford Level of Evidence) identified in a number of systematic reviews of yoga for pain [
30,
91‐
93], all had intervention lengths of between 12 weeks [
94‐
96] and 24 weeks [
97]. Eight weeks may not be a sufficient length of time for yoga to impact pain and any future Bikram yoga for pain studies should be at least 12 weeks long. Furthermore, although the Bikram yoga group saw significant improvements in mental health, physical functioning and heart rate variability, as compared to the HIIT group, this study was not powered on such measures. Additionally, there was no adjustment for multiple analyses within the secondary outcome measures. Therefore the significance of these results should be viewed with caution.
Lastly, even though no one participating in this study suffered any injuries due to taking part, some adverse effects were seen. However, the rates of adverse events were not greater than those previously reported by less vigorous yoga intervention studies [
91]. Also, a few participants who didn’t complete didn’t ever start their exercise classes. These were all assigned to the HIIT group and the process of getting the participants enrolled into the gyms conducting the HIIT classes was longer and more complicated that it was for the Bikram yoga studios. It is believed this may have impacted the momentum of participation and we suggest that future studies need to make access to interventions as streamlined as possible.
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