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Changes in recruitment of transversus abdominis correlate with disability in people with chronic low back pain
  1. P H Ferreira1,2,
  2. M L Ferreira1,
  3. C G Maher3,
  4. K Refshauge1,
  5. R D Herbert3,
  6. P W Hodges4
  1. 1Discipline of Physiotherapy, The University of Sydney, Sydney, Australia
  2. 2Departamento de Fisioterapia, Universidade Federal de Minas Gerais, Belo Horizonte, Brazil
  3. 3The George Institute for International Health, The University of Sydney, Sydney, Australia
  4. 4Centre for Clinical Research Excellence in Spinal Pain, Injury and Health, School of Health and Rehabilitation Sciences, The University of Queensland, Brisbane, Australia
  1. Correspondence to Dr P Ferreira, Discipline of Physiotherapy, Faculty of Health Sciences, University of Sydney, PO Box 170, Lidcombe 1825, Australia; p.ferreira{at}usyd.edu.au

Abstract

Objectives Although motor control exercises have been shown to be effective in the management of low back pain (LBP) the mechanism of action is unclear. The current study investigated the relationship between the ability to recruit transversus abdominis and clinical outcomes of participants in a clinical trial.

Methods Ultrasonography was used to assess the ability to recruit transversus abdominis in a nested design: a sample of 34 participants with chronic LBP was recruited from participants in a randomised controlled trial comparing the efficacy of motor control exercise, general exercise and spinal manipulative therapy. Perceived recovery, function, disability and pain were also assessed.

Results Participants with chronic LBP receiving motor control exercise had a greater improvement in recruitment of transversus abdominis (7.8%) than participants receiving general exercise (4.9% reduction) or spinal manipulative therapy (3.7% reduction). The effect of motor control exercise on pain reduction was greater in participants who had a poor ability to recruit transversus abdominis at baseline. There was a significant, moderate correlation between improved recruitment of transversus abdominis and a reduction in disability (r = −0.35; 95% CI 0.02 to 0.62).

Conclusion These data provide some support for the hypothesised mechanism of action of motor control exercise and suggest that the treatment may be more effective in those with a poor ability to recruit transversus abdominis.

https://doi.org/10.1136/bjsm.2009.061515

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    Changes in recruitment of both the superficial and deep trunk muscles are common in people with low back and pelvic pain. The activity of the large superficial muscles is often increased, although the nature of the increase varies.1 Evidence suggests increased superficial muscle activity such as increased co-contraction of the flexor and extensor muscles,2 increased erector spinae activity during gait3 and during a sit-up task,4 and increased bracing of the abdominal muscles during an active straight leg raise.5 Conversely, activity of the deep trunk muscle, the transversus abdominis, is delayed6,–,8 or reduced9 10 during movements of the limbs and trunk that challenge the stability of the spine. In the absence of low back pain (LBP), the transversus abdominis is generally activated before movement of the limbs or trunk and this activity appears to be independent of the direction of limb movement.11,–,13 Healthy individuals also activate the transversus abdominis in response to loading and force application to the trunk.14 It has been argued that this pattern of activation of the transversus abdominis is important for the control of intervertebral movement,15 particularly shear forces,16 and for the control of stability of the sacroiliac joints of the pelvis.17 In LBP, changes in control of the trunk muscles, including the transversus abdominis, are therefore thought to compromise control of the lumbar spine and pelvis. As activity of the transversus abdominis can be impaired in LBP, one of the aims of contemporary exercise interventions (such as the motor control exercise programme) for patients with LBP is to retrain the coordination of this muscle, as a component of the intervention.18

    Interventions aimed at training the control and coordination of the trunk muscles, including the transversus abdominis, have been shown to be effective in the management of low back19,–,23 and pelvic pain.24 However, we do not yet know whether clinical improvements are associated with changes in recruitment of this muscle. Because the transversus abdominis is situated deep to the more superficial abdominal muscles, intramuscular finewire electromyography has been required to evaluate its activity.6,–,8 13 25,–,29 Recent data using invasive recording methods in small groups of subjects have provided initial evidence that temporal aspects of the activity of the transversus abdominis can be modified with training.30 31 More recently, ultrasound imaging has been used to evaluate the morphology or recruitment of deep muscles of the trunk, in an attempt to use less invasive tools.18 32,–,36 During contraction, muscles change shape (eg, thickness) in association with shortening of the muscle with sliding of actin and myosin filaments (even during isometric contractions because of tendon stretch).36 There is a curvilinear relationship between changes in transversus abdominis thickness and electromyographic activity, but this is almost linear when activity increases from a relaxed state up to approximately 20% of maximum contraction.36

    A protocol developed to assess the activity of the transversus abdominis with ultrasound imaging has been shown to be able to distinguish between people with and without LBP.10 This protocol showed that when subjects perform an isometric leg flexion and extension task, and changes in the thickness of the transversus abdominis are averaged across the two directions, LBP subjects have approximately 72% less increase in the thickness of the transversus abdominis, approximately 53% less in obliquus internus abdominis and approximately 2% less in obliquus externus abdominis than controls. That study10 also demonstrated moderate to substantial correlations between ultrasound and fine-wire electromyography measurements of muscle recruitment for the transversus abdominis and obliquus internus (Pearson's r = 0.74−0.85), but not the obliquus externus (Pearson's r = 0.19).

    Motor control exercise aims to train coordination of the muscles of the trunk to meet the demands for optimal spinal function. The exercise includes training recruitment of the deeper muscles of the trunk such as the transversus abdominis.18 It could be hypothesised that a greater change in the thickness of the transversus abdominis muscle would be observed following this intervention than after other treatments, such as general exercise or spinal manipulative therapy, which do not specifically train activation of the deep muscles of the spine. To be of clinical importance, a change in ultrasound thickness should be accompanied by improvements in clinical outcomes.

    Changes in the ability to recruit transversus abdominis were measured in a sample drawn from chronic LBP patients participating in a randomised controlled trial of motor control exercises, general exercises and spinal manipulative therapy.37 The specific aims of this study were to investigate whether: (1) the ability to recruit transversus abdominis improves following an 8-week programme of motor control exercise, general exercise, or spinal manipulative therapy; (2) changes in recruitment of transversus abdominis are greater in patients receiving motor control exercise than patients receiving general exercise or spinal manipulative therapy; (3) changes in the ability to recruit transversus abdominis correlate with improvements in clinical outcomes of perceived recovery, function, disability and pain; and (4) the effect of motor control exercise (compared with general exercise) on the clinical outcomes of perceived recovery, function, disability and pain depends on the subjects' ability to recruit transversus abdominis measured at baseline.

    Methods

    A sample of non-specific chronic LBP patients was taken from a randomised controlled trial37 that compared the efficacy of motor control exercise, general exercise and spinal manipulative therapy. The final 45 subjects to be enrolled in the randomised controlled trial were invited to participate in this study, of whom 34 were eligible to participate. This sample size provided 80% power to detect a Pearson's correlation coefficient between transversus abdominis recruitment and clinical outcome measurements of at least 0.4 (fair)38 with 95% CI of 0.2 to 0.6. The study was approved by the ethics committees of the University of Sydney and the South Western and Western Sydney Area Health Services. Recruitment of transversus abdominis was measured using a published ultrasonography protocol.10 The ultrasound measurement was made before participants were randomly assigned to a motor control exercise group, a general exercise group, or a spinal manipulative therapy group, and again after the application of 12 sessions of treatment over an 8- week period. The clinical outcomes of perceived recovery, function, disability and pain were also collected at the time of the ultrasound measurement.37

    Subjects

    Patients aged between 18 and 80 years with chronic LBP (symptoms for at least 3 months) with or without pain referral to the leg, but without neurological deficit were recruited for the study. To be included in the trial, patients needed to have persistent pain or disability for at least 3 months, and they had to score at least 3 points on the Roland Morris disability questionnaire and at least 2 units on the 0–10 pain scale at the screening consultation. Exclusion criteria were: spinal surgery in the past 12 months; pregnancy at the first assessment; suspected or diagnosed serious spine pathology (inflammatory spondyloarthropathy, fracture, malignancy, cauda equina syndrome or infection); nerve root compromise; contraindications to exercise or poor English comprehension.

    Intervention

    Based on the randomisation procedure, participants received motor control exercise, general exercise, or spinal manipulative therapy. Participants allocated to the motor control exercise group were prescribed exercises aimed at improving control of lumbopelvic movement and stability. Exercises included training the function of specific deep muscles of the low back region, coordination of deep trunk muscles with a diaphragmatic respiration pattern, control of a neutral lumbar posture and reduction of any excessive superficial trunk muscle activation.39 Participants allocated to the general exercise group received the programme described by Klaber Moffet and Frost,40 which is based on a biopsychosocial model and aims to overcome a fear of movement and to improve physical function in both the short and long term. For subjects allocated to the spinal manipulative therapy group, joint mobilisation techniques, but not thrust manipulation techniques, were applied to the participant's spine or pelvis using grades and techniques that were at the discretion of the treating physiotherapist.41

    Clinical outcomes

    Clinical outcomes were measured at baseline and after 8 weeks of treatment. Global impression of recovery was measured on an 11-point scale.42 Disability was measured using the 24-item version of the Roland Morris disability questionnaire.43 Average pain intensity over the past week was measured on a numerical rating scale.42 Function was measured with a modified patientspecific functional scale.42

    Ultrasonography

    Ultrasound images were made with a 5.5 cm, 5 MHz linear array ultrasound transducer (Logic 100 Pro; General Electric, Unit 4B, 21 South Street, Rydalmere NSW 2116, Australia). The transducer was placed transversely across the right abdominal wall along a line midway between the inferior angle of the rib cage and the iliac crest. The medial edge of the transducer was positioned so that the medial edge of the transversus abdominis was aligned in the right-hand one-third of the ultrasound image when the subject was relaxed. The location of the transducer was recorded for standardisation of placement across measurement sessions. All ultrasound measurements were made blinded to the subject's treatment group.

    Procedure

    A previously published ultrasonography protocol was used to measure the change in thickness of the transversus abdominis as an indirect measure of recruitment during a task that involved generation of flexion and extension torque at the knee.10 Unlike other measurements of voluntary transversus abdominis recruitment, such as the abdominal drawing-in manoeuvre, this protocol involved measurement of the automatic activation of trunk muscles during the leg task with no conscious attention to the abdominal muscles. Participants were positioned supine on a bed with arms crossed over the chest, hips flexed to 50° and knees flexed to 90°. Knee flexion and extension force was monitored with a spring scale attached to a belt strapped around the ankles. Patients were instructed to remain relaxed before testing and then to perform isometric knee flexion or extension contractions to target forces of 7.5% of body weight. The order of testing movement direction was randomised, and patients were provided with verbal feedback about force by the examiner reading the spring scale. Two repetitions of each task were performed and static transversus abdominis ultrasound images were made both at rest and once the target isometric knee flexion or extension force had been reached. Reliability analysis for this ultrasonography protocol has been shown to be excellent with an intraclass correlation[3, 1] coefficient of 0.85 and a minimal detectable change score of 1.16%.44

    Data extraction

    Transversus abdominis thickness was measured using ultrasonography with custom-designed software. A grid was placed over the image and measurements of muscle thickness of the transversus abdominis were made at three sites: the middle of the image and 1 cm (calibrated to the image scale) on either side of the midline. The average of the three measurements from each image was recorded for analysis and the thickness for each direction of movement was expressed as a proportion of the thickness at rest and averaged over the two repetitions of the task. Change in thickness of the transversus abdominis was obtained by averaging the values for both directions of movement.

    Statistical analysis

    Means and standard deviations were employed to describe demographic data, recruitment of transversus abdominis recorded as a change in thickness measured with ultrasonography and clinical outcomes (perceived recovery, function, disability and pain).

    Changes in recruitment of transversus abdominis and clinical outcomes within groups were analysed with paired t tests. Differences between treatment groups in the ability to recruit transversus abdominis were analysed using a one-way analysis of variance and Tukey post-hoc tests on the change scores. Pearson's r was used to analyse the relationship between changes in transversus abdominis recruitment and changes in clinical outcomes. Linear regression was used to analyse whether the effect of motor control exercise (in contrast to general exercise) on final clinical outcomes was influenced by the subject's ability to recruit transversus abdominis at baseline after adjusting for baseline clinical outcomes. A significance level of 5% was chosen a priori.

    Results

    Forty-five participants in the randomised controlled trial, within which the present study was nested, were invited to participate. Of these, four refused to participate, three did not tolerate the test procedure because of pain in the knee or hip joints, and in foour it was not possible to obtain a clear image of the transversus abdominis as a result of excessive adipose tissue. A total of 34 participants was therefore recruited into this study. The final sample included 11 participants in the motor control exercise group, 10 patients in the general exercise group and 13 patients in the spinal manipulative therapy group (fig 1).

    Figure 1
    Figure 1

    Flowchart of progress of patients. TrA, transversus abdominis.

    Baseline demographic data, recruitment of transversus abdominis and clinical outcomes are presented in table 1. Patients attended a mean (SD) of 8.7 (2.6) motor control exercise sessions, 11.2 (1.5) general exercise sessions and 9.2 (2.4) spinal manipulative therapy sessions. All 34 patients were re-assessed after treatment. There were no significant between-group differences in the baseline clinical outcomes or transversus abdominis recruitment (one-way analysis of variance, p≥0.05 for all comparisons). Between-group comparisons were conducted on change scores or analysis of covariance-adjusted scores using the baseline as a covariate so that any discrepancies in baseline scores would not cause bias.

    View this table:
    Table 1

    Baseline characteristics of the study participants

    The clinical and ultrasound measures are shown in table 2. All three groups improved in the clinical outcomes of perceived recovery, function, disability and pain at the 8-week follow-up. Transversus abdominis recruitment improved by 7.8% in the motor control exercise group and deteriorated by 4.9% in the general exercise and 3.7% in the spinal manipulative therapy groups. Paired t tests revealed that none of these changes were statistically significant. However, the change in recruitment observed with motor control exercise approached significance (t10 = 2.02; p=0.07).

    View this table:
    Table 2

    Baseline, final and improvement scores for transversus abdominis recruitment and clinical outcomes, for all groups

    There were significant differences between groups with respect to the improvement in transversus abdominis recruitment (F2,31 = 4.09; p=0.026). Motor control exercise produced a 12.7% greater improvement in transversus abdominis recruitment than general exercise (p=0.043) and a 11.4% greater improvement than spinal manipulative therapy (p=0.053). No difference in improvement was found between the spinal manipulative therapy and general exercise groups (p=0.963).

    When data from the three groups were pooled (fig 2), there was a correlation between improvements in recruitment of transversus abdominis and improvements in the clinical outcomes of perceived recovery (r = 0.27; 95% CI −0.08 to 0.55), Roland Morris disability scores (r = −0.35; 95% CI 0.02 to 0.62), patient-specific functional scores (r = 0.19; 95% CI −0.16 to 0.50) and pain (r = −0.28; 95% CI 0.07 to −0.56). Only the correlation with the Roland Morris disability score was statistically significant.

    Figure 2
    Figure 2

    Overall correlation between changes in recruitment of transversus abdominis measured with ultrasonography and changes in clinical outcomes. Lines represent the r2 line of best fit. PSFS, patient-specific functional scale; RM, Roland Morris; TrA, transversus abdominis.

    The effect of motor control exercise (vs general exercise) was greater in subjects who had a poorer ability to recruit transversus abdominis at baseline; however, the estimates of this interaction effect are imprecise and only statistically significant for the pain outcome. The interaction effect for pain was 18.1 (1.2 to 34.9) p=0.037. The interpretation of this finding is that a subject who had a baseline transversus abdominis activation score of 0.00 would have 3.6 units less pain at the conclusion of treatment than a subject whose baseline transversus abdominis activation score was 0.20, ie, (0.00−0.20) × 18.1 = −3.6. The interaction effects for the other outcomes were Roland Morris disability (effect 16.1; −34.0 to 66.2; p=0.506), patient-specific function (effect −13.7; −67.2 to 39.9; p=0.597) and for perceived recovery (effect −19.9; −45.3 to 5.5; p=0.116).

    Discussion

    This study shows that, in chronic LBP, the improvement in recruitment of the trunk muscle transversus abdominis, measured by changes in thickness with ultrasonography, was greater in those who performed motor control exercise than in those who undertook a programme of general exercise or spinal manipulative therapy. The motor control exercise group showed an absolute increase in recruitment of transversus abdominis of 7.8% compared with a slight decrease in recruitment of transversus abdominis in the general exercise group (−4.9%) and in the spinal manipulative therapy group (−3.7%). All these values exceeded the previously reported minimal detectable change score of 1.16% for the ultrasonographic measurement of the transversus abdominis. This magnitude of change demonstrates that changes in recruitment with the implementation of treatments are above the potential error associated with the measurement.

    Our results suggest that relieving pain with spinal manipulative therapy or encouraging general activity with a general exercise programme is not sufficient to improve maximally the ability to recruit transversus abdominis. We found that the application of a motor control exercise programme was the most effective method for improving recruitment of transversus abdominis in people with chronic LBP. Improvements in transversus abdominis recruitment associated with motor control training have also been identified in the short term (4 weeks) in LBP patients45 as well as in asymptomatic individuals.46 Other studies of fine-wire electromyography recordings of transversus abdominis activity have reported changes in the timing of muscle activation after motor control training, both immediately31 and at 6 months.30 Similar results have been noted for other deep muscles of the spine in another trial evaluating physical treatment of acute LBP.47 Hides et al47 found in patients with acute LBP, that motor control exercise, but not usual medical care, reduced multifidus asymmetry between the symptomatic and asymptomatic sides. Similar to the present study, the participants in the study by Hides and colleagues47 in both groups exhibited similarly large improvements in pain and disability at the end of 4 weeks of treatment; however, the group that did not receive multifidus exercise did not restore the symmetry of multifidus. Although the shortterm outcomes were similar for pain and disability, differences were apparent in the long term, the group who had restored symmetry of multifidus experienced a significantly reduced rate of recurrence of episodes.47

    What is already known on this topic

    • ▶ Recruitment of transversus abdominis is impaired in patients with LBP.

    • ▶ Motor control exercises aimed at training the control and coordination of the trunk muscles, including the transversus abdominis, are effective in the management of LBP.

    What this study adds

    • ▶ Changes in recruitment of transversus abdominis appear to be specific to the implementation of motor control exercises and are moderately associated with improvements in disability.

    • ▶ The treatment effects of motor control exercise are greater in those with a poorer ability to recruit transversus abdominis.

    When data from the three treatment groups in our trial were pooled there was a moderate correlation between change in recruitment of transversus abdominis and perceived recovery. This correlation was positive, which means that an increase in recruitment of transversus abdominis correlated moderately well with improvements in perceived recovery. There was also a moderate, negative correlation between increase in recruitment of transversus abdominis and disability measured with the Roland Morris questionnaire, which means that an increase in recruitment of transversus abdominis was associated with reductions in disability. Although neither pain nor function was statistically significantly correlated with transversus abdominis recruitment, the effects were in the anticipated direction.

    An important finding of the study was the interaction between subject's ability to recruit transversus abdominis at baseline and the effect of motor control exercise (vs general exercise) on pain outcomes. The effect we found was in the direction suggested by clinical theories. In the main trial37 we demonstrated that motor control exercise produced better short-term outcomes than general exercise, and so on average motor control exercise is superior. However, the interaction effect means that motor control exercise worked best for participants who had a poor ability to recruit transversus abdominis and conversely for participants who have a good ability to activate this muscle general exercise may be a better treatment option. We are aware that it is possible to generate spurious findings when looking for treatment interaction effects in clinical trials.48 49 To reduce the risk of this we specified our analysis a priori and confined our analysis to one predictor that was biologically plausible. We used the preferred approach of a statistical test of an interaction and confined the analysis to primary outcomes.

    Although we have demonstrated that the baseline ability to recruit transversus abdominis modifies the treatment effect of motor control exercise (vs general exercise) it would be premature to attempt to apply this research finding to the routine clinicalmanagement of LBP. We first need to replicate the result in a larger, independent sample so that we can generate a more precise estimate of themagnitude of effectmodification and also generate cut-off scores for the ability to recruit transversus abdominis. Such data could be used to develop a clinical prediction rule. Finally, the rule would have to be validated in a clinical trial and then demonstrate the impact of implementation of the rule on the outcomes of care in subsequent research.50 We expect this process to take some years.

    Conclusion

    It has been uncertain whether motor control exercises lead to changes in the activation of the transversus abdominis and whether these changes are associated with clinical improvements. Our findings show that, after adjusting for baseline values, a greater change in the automatic activation of the transversus abdominis, measured by ultrasonography, occurs after a motor control exercise programme than after other interventions, and this change in muscle activity is associated with improvements in disability. The study also demonstrated that the pain-relieving effect of motor control exercise is greater in subjects who have a poor ability to recruit this muscle at baseline.

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    View Abstract

    Footnotes

    • Funding The trial was funded by the Motor Accident Authority of NSW; CGM, RDH and PWH are supported by the National Health and Medical Research Council of Australia.

    • Competing interests None.

    • Ethics approval The study was approved by the ethics committees of the University of Sydney and the South Western and Western Sydney Area Health Services.

    • Patient consent Obtained.

    • Provenance and peer review Not commissioned; externally peer reviewed.