Advancing Physical Therapist Interventions by Investigating Causal Mechanisms

Training parameters and longitudinal adaptations that most strongly mediate walking capacity gains from high-intensity interval training post-stroke

Background

Locomotor high-intensity interval training (HIIT) has been shown to improve walking capacity more than moderate-intensity aerobic training (MAT) after stroke, but it is unclear which training parameter(s) should be prioritized (e.g. speed, heart rate, blood lactate, step count) and to what extent walking capacity gains are the result of neuromotor versus cardiorespiratory adaptations.

Objective

Assess which training parameters and longitudinal adaptations most strongly mediate 6-minute walk distance (6MWD) gains from post-stroke HIIT.

Methods

The HIT-Stroke Trial randomized 55 persons with chronic stroke and persistent walking limitations to HIIT or MAT and collected detailed training data. Blinded outcomes included 6MWD, plus measures of neuromotor gait function (e.g. fastest 10-meter gait speed) and aerobic capacity (e.g. ventilatory threshold). This ancillary analysis used structural equation models to compare mediating effects of different training parameters and longitudinal adaptations on 6MWD.

Results

Net gains in 6MWD from HIIT versus MAT were primarily mediated by faster training speeds and longitudinal adaptations in neuromotor gait function. Training step count was also positively associated with 6MWD gains, but was lower with HIIT versus MAT, which decreased the net 6MWD gain. HIIT generated higher training heart rate and lactate than MAT, but aerobic capacity gains were similar between groups, and 6MWD changes were not associated with training heart rate, training lactate, or aerobic adaptations.

Conclusions

To increase walking capacity with post-stroke HIIT, training speed and step count appear to be the most important parameters to prioritize.

What Are the Mechanisms of Action of Cognitive-Behavioral, Mind-Body, and Exercise-based Interventions for Pain and Disability in People With Chronic Primary Musculoskeletal Pain?: A Systematic Review of Mediation Studies From Randomized Controlled Trials

OBJECTIVES

This systematic review examined studies that used mediation analysis to investigate the mechanisms of action of cognitive-behavioral, mind-body, and exercise-based interventions for pain and disability in people with chronic primary musculoskeletal pain.

MATERIALS AND METHODS

We searched 5 electronic databases for articles that conducted mediation analyses of randomized controlled trials to either test or estimate indirect effects.

RESULTS

We found 17 studies (n=4423), including 90 mediation models examining the role of 22 putative mediators on pain or disability, of which 4 had partially mediated treatment effect; 8 had mixed results, and 10 did not mediate treatment effect. The conditions studied were chronic whiplash-associated pain, chronic low back pain, chronic knee pain, and mixed group of chronic primary musculoskeletal pain.

DISCUSSION

We observed that several of the studies included in our systematic review identified similar mechanisms of action, even between different interventions and conditions. However, methodological limitations were common. In conclusion, there are still substantial gaps with respect to understanding how cognitive-behavioral, mind-body, and exercise-based interventions work to reduce pain and disability in people with chronic primary musculoskeletal pain.

Causal mechanisms of a scapular stabilization intervention for patients with subacromial pain syndrome: a secondary analysis of a randomized controlled trial

Background

Causal mediation analysis is one way to bridge this gap by exploring the causal pathways of a given intervention. The aim of this study was to assess whether scapular motion, position, and periscapular muscle strength are mediators for pain and shoulder disability outcomes following a scapular stabilization intervention for patients with subacromial pain syndrome.

Methods

Sixty patients were randomized into two groups: scapular stabilization or periscapular strengthening exercises. The intervention consisted of three sessions per week for 8 weeks. The primary outcome measures were pain and disability and the following outcome measures were considered as potential mediators: scapular motion, scapular position, periscapular muscle strength, age, duration of symptoms, and side of the complaint. A model-based inference approach with bootstrap simulations was used to estimate the average causal mediation effect, average direct effect, and the average total effect from the data of a randomized clinical trial that evaluated the effect of adding scapular stabilization exercises to a scapulothoracic strengthening program in people with subacromial pain syndrome.

Results

The results demonstrated that none of the putative mediators were influenced by the intervention. However, muscle strength of serratus anterior, upper, middle, and lower trapezius muscles was associated with shoulder disability.

Conclusion

Scapular kinematic and periscapular muscle strength did not mediate the effect of scapular stabilization exercises on shoulder pain or disability scores in subjects with subacromial pain syndrome. Muscle strength of serratus anterior, upper, middle and lower trapezius were associated with shoulder disability scores at 8-weeks follow-up.

Supplementary Information

The online version contains supplementary material available at 10.1186/s40945-022-00138-1.

Making exercise count: Considerations for the role of exercise in back pain treatment

INTRODUCTION

Chronic low back pain (CLBP) is pain that has persisted for greater than three months. It is common and burdensome and represents a significant proportion of primary health presentations. For the majority of people with CLBP, a specific nociceptive contributor cannot be reliably identified, and the pain is categorised as 'non-specific'. Exercise therapy is recommended by international clinical guidelines as a first-line treatment for non-specific CLBP.

AIM

This comprehensive review aims to synthesise and appraise the current research to provide practical, evidence-based guidance concerning exercise prescription for non-specific CLBP. We discuss detailed initial assessment and being informed by patient preferences, values, expectations, competencies and goals.

METHODS

We searched the Cochrane Database of Systematic Reviews, PubMed and the Physiotherapy Evidence Database (PEDro) using broad search terms from January 2000 to March 2021, to identify the most recent clinical practice guidelines, systematic reviews and randomised controlled trials on exercise for CLBP.

RESULTS/DISCUSSION

Systematic reviews show exercise is effective for small, short-term reductions in pain and disability, when compared with placebo, usual care, or waiting list control, and serious adverse events are rare. A range of individualised or group-based exercise modalities have been demonstrated as effective in reducing pain and disability. Despite this consensus, controversies and significant challenges remain.

CONCLUSION

To promote recovery, sustainable outcomes and self-management, exercise can be coupled with education strategies, as well as interventions that enhance adherence, motivation and patient self-efficacy. Clinicians might benefit from intentionally considering their own knowledge and competencies, potential harms of exercise and costs to the patient. This comprehensive review provides evidence-based practical guidance to health professionals who prescribe exercise for people with non-specific CLBP.

Producing Clinically Meaningful Reductions in Disability: A Causal Mediation Analysis of a Patient Education Intervention

Patient education is recommended as first-line care for low back pain (LBP), although its efficacy for providing clinically meaningful reductions in disability has been questioned. One way to improve treatment effects is to identify and improve targeting of treatment mechanisms. We conducted a pre-planned causal mediation analysis of a randomized, placebo-controlled trial investigating the effectiveness of patient education for patients with acute LBP. 202 patients who had fewer than six-weeks' duration of LBP and were at high-risk of developing chronic LBP completed two, one-hour treatment sessions of either intensive patient education, or placebo patient education. 189 participants provided data for the outcome self-reported disability at three-months and the mediators, pain self-efficacy, pain catastrophizing, and back beliefs at one-week post treatment. This causal mediation analysis found that pain catastrophizing (mediated effect, -0.64; 95% Confidence Interval [CI], -1.31 to -0.15) and back beliefs (mediated effect, -0.51; 95% CI, -1.15 to -0.02) partly explained the effect of patient education on disability but pain self-efficacy did not (mediated effect, -0.40; 95% CI -1.13 to 0.28). Considering the mediator-outcome relationship, patient education would need to induce an 8 point difference on the pain self-efficacy questionnaire (0-60); an 11 point difference on the back beliefs questionnaire (9-45); and a 21 point difference on the pain catastrophizing scale (0-52) to achieve a minimally clinically important difference of 2 points on the Roland Morris Disability Questionnaire (0-24). PERSPECTIVE: Understanding the mechanisms of patient education can inform how this treatment can be adapted to provide clinically meaningful reductions in disability. Our findings suggest that adapting patient education to better target back beliefs and pain self-efficacy could result in clinically meaningful reductions in disability whereas the role of pain catastrophizing in acute LBP is less clear.

Items for consideration in a reporting guideline for mediation analyses: a Delphi study

OBJECTIVES

Mediation analysis is a widely used quantitative method for investigating how interventions and exposures in randomised controlled trials and observational studies have an effect on healthcare outcomes. This study aimed to assess the importance of items that should be considered in a consensus meeting aimed at developing a guideline for reporting mediation analyses.

DESIGN

International online Delphi study.

PARTICIPANTS

International experts in the development and application of mediation analysis.

MAIN OUTCOME MEASURES

The Delphi panel were asked to rate the importance of a list of items for inclusion in a guideline for reporting mediation analyses. Thresholds for disagreement and consensus on importance for inclusion were specified a priori. We used the Research ANd Development/University of California Los Angeles appropriateness method to quantitatively assess the importance for inclusion and panel agreement.

RESULTS

Nineteen expert panellists (10 female) from seven countries agreed to participate. All panellists contributed to all three rounds conducted between 10 June 2019 and 6 November 2019. The panel reached consensus on 34 unique reporting items for study design, analytic procedures and effect estimates, with three items rated 'optional'. Panellists added one extra item and provided 60 qualitative comments for item refinement and prioritisation.

CONCLUSION

This Delphi study used a rigorous consensus process to reach consensus on 34 reporting items for studies that use mediation analysis. These results will inform a consensus meeting that will consolidate a core set of recommended items for reporting mediation analyses.

Using Mediation Analysis to Understand How Treatments for Paediatric Pain Work: A Systematic Review and Recommendations for Future Research

Clinicians have an increasing number of evidence-based interventions to treat pain in youth. Mediation analysis offers a way of investigating how interventions work, by examining the extent to which an intermediate variable, or mediator, explains the effect of an intervention. This systematic review examined studies that used mediation analysis to investigate mechanisms of interventions on pain-relevant outcomes for youth (3–18 years) with acute or chronic pain, and provides recommendations for future mediation research in this field. We searched five electronic databases for clinical trials or observational longitudinal studies that included a comparison group and conducted mediation analyses of interventions on youth and assessed pain outcomes. We found six studies (N = 635), which included a total of 53 mediation models examining how interventions affect pain-relevant outcomes for youth. Five studies were secondary analyses of randomized controlled trials of psychological interventions for chronic pain; one was a longitudinal observational study of morphine for acute pain. The pain conditions studied were irritable bowel syndrome, functional abdominal pain, juvenile fibromyalgia, mixed chronic pain, and post-operative pain. Fourteen putative mediators were tested, of which three partially mediated treatment effect; seven did not significantly mediate treatment effect and four had mixed results. Methodological and reporting limitations were common. There are substantial gaps in the field with respect to investigating, and therefore understanding, how paediatric interventions work.

How much of the effect of exercise and advice for subacute low back pain is mediated by depressive symptoms?

BACKGROUND

Exercise and advice can reduce pain and disability in patients with subacute low back pain. However, the mechanisms by which this combined intervention works are unclear. Our objective was to estimate how much of the effect of a physiotherapist-directed exercise and advice intervention on pain and disability is mediated via changes in depressive symptoms.

METHODS

Causal mediation analysis of a randomized controlled trial. We measured our hypothesized mediator - depressive symptoms (Depression, Anxiety, Stress Scale-21) at 6 weeks, and the outcomes - pain (numerical rating scale) and disability (Roland Morris Disability Questionnaire) at 3 months, estimating the average causal mediation effect (ACME), average direct effect (ADE), and total effect. We pre-specified a causal model to identify potential confounders of the mediator-outcome effect and conducted sensitivity analyses to assess the robustness of the ACME under varying levels of unknown confounding.

RESULTS

Data from 240 patients were analyzed (average age 50.5 (SD 15.6) years, 52% male, median depressive symptoms score 4). The effect of exercise combined with advice was not mediated via depressive symptoms: ACME on pain (0.05, 95%CI -0.24 to 0.15), ACME on disability (-0.10, 95%CI -0.59 to 0.38). However depressive symptoms were associated with pain (regression coefficient 0.06; SE 0.03)) and disability (regression coefficient 0.17; SE 0.05).) CONCLUSIONS: Depressive symptoms did not mediate the effect of exercise and/or advice in this sample. However, depressive symptoms were associated with pain and disability.

Causal mechanisms of a healthy lifestyle intervention for patients with musculoskeletal pain who are overweight or obese

PURPOSE

To assess the causal mechanisms of a healthy lifestyle intervention for patients with chronic low back pain and knee osteoarthritis, who are overweight or obese.

METHODS

We conducted causal mediation analyses of aggregated data from two randomized controlled trials (RCTs); which included 160 patients with chronic low back pain, and 120 patients with knee osteoarthritis. The intervention consisted of brief advice and referral to a six-month telephone-based healthy lifestyle coaching service. We used causal mediation to estimate the indirect, direct and path-specific effects of hypothesized mediators including: self-reported weight, diet, physical activity, and pain beliefs. Outcomes were pain intensity, disability, and quality of life (QoL).

RESULTS

The intervention did not reduce weight, improve diet or physical activity or change pain beliefs, and these mediators were not associated with the outcomes. Sensitivity analyses showed that our estimates were robust to the possible effects of unknown and unmeasured confounding.

CONCLUSIONS

Our findings show that the intervention did not cause a meaningful change in the hypothesized mediators, and these mediators were not associated with patient-reported outcomes.

Exercise intensity affects acute neurotrophic and neurophysiological responses poststroke

Aerobic exercise may acutely prime the brain to be more responsive to rehabilitation, thus facilitating neurologic recovery from conditions like stroke. This aerobic priming effect could occur through multiple mechanisms, including upregulation of circulating brain-derived neurotrophic factor (BDNF), increased corticospinal excitability, and decreased intracortical inhibition. However, optimal exercise parameters for targeting these mechanisms are poorly understood. This study tested the effects of exercise intensity on acute BDNF and neurophysiological responses. Sixteen ambulatory persons >6 mo poststroke performed three different 20-min exercise protocols in random order, approximately 1 wk apart, including the following: 1) treadmill high-intensity interval training (HIT-treadmill); 2) seated-stepper HIT (HIT-stepper); and 3) treadmill moderate-intensity continuous exercise (MCT-treadmill). Serum BDNF and transcranial magnetic stimulation measures of paretic lower limb excitability and inhibition were assessed at multiple time points during each session. Compared with MCT-treadmill, HIT-treadmill elicited significantly greater acute increases in circulating BDNF and corticospinal excitability. HIT-stepper initially showed BDNF responses similar to HIT-treadmill but was no longer significantly different from MCT-treadmill after decreasing the intensity in reaction to two hypotensive events. Additional regression analyses showed that an intensity sufficient to accumulate blood lactate appeared to be important for eliciting BDNF responses, that the interval training approach may have facilitated the corticospinal excitability increases, and that the circulating BDNF response was (negatively) related to intracortical inhibition. These findings further elucidate neurologic mechanisms of aerobic exercise and inform selection of optimal exercise-dosing parameters for enhancing acute neurologic effects. NEW & NOTEWORTHY Acute exercise-related increases in circulating BDNF and corticospinal excitability are thought to prime the brain for learning. Our data suggest that these responses can be obtained among persons with stroke using short-interval treadmill high-intensity interval training, that a vigorous aerobic intensity sufficient to generate lactate accumulation is needed to increase BDNF, that interval training facilitates increases in paretic quadriceps corticospinal excitability, and that greater BDNF response is associated with lesser intracortical inhibition response.