Manual Therapy
Volume 17, Issue 1 , Pages 27-33, February 2012

The relationships between measures of stature recovery, muscle activity and psychological factors in patients with chronic low back pain

  • Sandra Lewis

      Affiliations

    • Institute for Performance Research, Manchester Metropolitan University, Crewe CW1 5DU, United Kingdom
    • Corresponding Author InformationCorresponding author. Tel.: +44 161 247 5765; fax: +44 161 247 6375.
    • ,
  • Paul Holmes

      Affiliations

    • Institute for Performance Research, Manchester Metropolitan University, Crewe CW1 5DU, United Kingdom
    • ,
  • Steve Woby

      Affiliations

    • Research & Development, Pennine Acute Hospitals NHS Trust, North Manchester General Hospital, Manchester M8 5RB, United Kingdom
    • ,
  • Jackie Hindle

      Affiliations

    • Research Institute for Health and Social Change, Manchester Metropolitan University, Manchester M13 0JA, United Kingdom
    • ,
  • Neil Fowler

      Affiliations

    • Institute for Performance Research, Manchester Metropolitan University, Crewe CW1 5DU, United Kingdom

Received 7 January 2011; received in revised form 29 July 2011; accepted 2 August 2011. published online 07 September 2011.

Article Outline

Abstract 

Individuals with low back pain (LBP) often exhibit elevated paraspinal muscle activity compared to asymptomatic controls during static postures such as standing. This hyperactivity has been associated with a delayed rate of stature recovery in individuals with mild LBP. This study aimed to explore this association further in a more clinically relevant population of NHS patients with LBP and to investigate if relationships exist with a number of psychological factors. Forty seven patients were recruited from waiting lists for physiotherapist-led rehabilitation programmes. Paraspinal muscle activity while standing was assessed via surface electromyogram (EMG) and stature recovery over a 40-min unloading period was measured on a precision stadiometer. Self-report of pain, disability, anxiety, depression, pain-related anxiety, fear of movement, self-efficacy and catastrophising were recorded.

Correlations were found between muscle activity and both pain (r=0.48) and disability (r=0.43). Muscle activity was also correlated with self-efficacy (r=−0.45), depression (r=0.33), anxiety (r=0.31), pain-related anxiety (r=0.29) and catastrophising (r=0.29) and was a mediator between self-efficacy and pain. Pain was a mediator in the relationship between muscle activity and disability. Stature recovery was not found to be related to pain, disability, muscle activity or any of the psychological factors. The findings confirm the importance of muscle activity within LBP, in particular as a pathway by which psychological factors may impact on clinical outcome. The mediating role of muscle activity between psychological factors and pain suggests that interventions that are able to reduce muscle tension may be of particular benefit to patients demonstrating such characteristics, which may help in the targeting of treatment for LBP.

Keywords: Low back pain, Psychology, Electromyography

 

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1. Introduction 

Patients with low back pain (LBP) often demonstrate altered muscle function compared to asymptomatic controls. In particular, individuals with LBP have been found to exhibit hyperactivity of the superficial paraspinal muscles during static postures such as standing (e.g. Ambroz et al., 2000).

The height of intervertebral discs changes in response to compressive forces (due to a combination of fluid flow and elastic deformation) and this is reflected in changes in stature. Stature change is therefore used as a proxy measure of the load on the spine and measurements have been shown to correlate with more direct measurements of changes in lumbar spine length assessed via Magnetic Resonance Imaging (MRI) (Lewis and Fowler, 2009). It has also been shown that both chronic low back pain (CLBP) patients and asymptomatic individuals are able to produce stature measurements with a good level of repeatability (Healey et al., 2005b). Healey et al. (2005a) found significantly reduced stature recovery in individuals with mild LBP compared to controls, with stature recovery negatively correlated with paraspinal muscle activity. The authors hypothesized that the elevated muscle activity observed in the LBP group resulted in greater compressive loads on the spine that, in turn, prevented the intervertebral discs from regaining their initial height and consequently prolonged stature recovery. Reduced stature recovery may increase the risk of future back pain and increase loading on spinal structures such as the facet joints (Adams et al., 2002). Significant negative correlations between stature recovery and both pain and disability appear to support the clinical relevance of this relationship (Healey et al., 2005a).

Psychological factors are known to play an important role in LBP and are sometimes viewed as ‘obstacles to recovery’ (e.g. Foster et al., 2010). It has been suggested that one of the ways psychological factors may affect the condition is via increased spinal loading resulting from altered paraspinal muscle activity. Furthermore, LBP patients with high levels of pain-related fear generally exhibit elevated paraspinal muscle activity compared to low fearful patients (Vlaeyen et al., 1999), especially when confronted with movements which they believe to be harmful (Vlaeyen and Linton, 2000). It is proposed that pain-related fear may perpetuate pain and disability via this muscle guarding. Muscle activity may therefore be a contributory factor in the link between psychological factors and clinical outcome.

The study carried out by Healey et al. (2005a) involved individuals who all self-managed their pain. The aim of this study was to extend those findings by analysing the relationship between stature recovery, muscle activity, pain and disability in a more clinically relevant population of NHS patients with LBP, including individuals with more severe back pain than previously examined. In addition, this study sought to establish whether a range of self-report psychological factors are associated with muscle activity or stature change. An asymptomatic control group was included in the design to enable comparison between the two groups. It was hypothesized that, consistent with the findings of Healey et al. (2005a), the patients with LBP would have higher muscle activity and reduced stature recovery compared to the asymptomatic group and that stature recovery would be negatively related to each of muscle activity, pain and disability. Based on the limited previous research in the area, it was expected that muscle activity would be correlated with the psychological factors considered. It was expected that these psychological factors may impact on stature recovery via their influence on muscle activity and hence also lead to observed correlations with stature change.

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2. Methods 

2.1. Participants 

Data were collected from 47 patients with LBP (age, 46.2±11.1yr; height, 166.4±7.5cm; body mass, 79.3±18.8kg) who had been referred to a physiotherapist-led rehabilitation programme in North Manchester and 18 asymptomatic controls (age, 44.6±13.3yr; height, 169.0±10.3cm; body mass, 70.8±12.7kg). The LBP group was mixed (18 men, 29 women), between the ages of 23 and 70 years.

Patients on the waiting list for the rehabilitation programme were sent information regarding the study through the post and asked to return a reply slip if they wished to participate. The exclusion criteria considered were: nerve root compression; central nervous system impairment; progressive motor deficit; sphincter impairment from neurologic cause; and presence of ‘red flags’ (e.g. unexplained weight loss, recent urinary tract infection, history of intravenous drug use). Many of the patients were taking analgesics for their back pain; it was not considered practical to exclude those on medication. Participants were offered £12.50 for each session they attended to cover travel and parking expenses. Ethical approval was granted by the North Manchester NHS ethics committee and local NHS permission was granted by Pennine Acute NHS Trust. All participants provided written informed consent.

The control group consisted of 9 men and 9 women aged between 25 and 64 years. Participants were excluded from the control group if they had, at any time, experienced recurring or persistent back pain, lost a working day because of back pain, or had consulted a physician about back pain within the last 15 years.

2.2. Muscle activity measurement 

Raw electromyographic signals were recorded using a DELSYS EMG system (Delsys Inc. Boston, MA, USA). Single differential surface electrodes consisting of two silver bars with an inter-electrode spacing of 10mm were used. Signals were band-pass filtered between 20 and 450Hz with a sampling frequency of 1000Hz.

As shown in Fig. 1, electrodes were placed over the erector spinae muscle at the level of the L1–2 and L4–5 interspaces, approximately 3cm from the midline on either side and the reference electrode was placed on the right iliac crest. Participants then assumed a standing posture for 10s while a recording was taken. The electromyogram (EMG) data were normalised relative to a reference voluntary contraction (RVC). This reference task required each participant to stand while holding up a specific mass (0.5kg) in each hand with arms bent (upper arms horizontal, lower arms vertical) for 10s (shown in Fig. 2). The signal mean value was removed from the raw EMGs, before rectifying and integrating over a period of 5s. The EMG reading and the RVC were both taken to be the average of the three readings recorded during the session.

2.3. The stadiometer 

Changes in stature were measured with a standing stadiometer, which consisted of a rigid frame, mounted at a right angle to a base plate and inclined backward 15° from the vertical (Fig. 3). Four anatomical points were identified (Lewis and Fowler, 2009) and then supported by the frame to maintain the natural contours of the head and spine. The position of the feet was marked and head position was controlled by the use of spectacle frames with attached lasers, which were aligned with two movable targets above the participant’s head. A high-resolution linear variable displacement transducer (LVDT) (Solartron Metrology, DC50) was used to detect changes in stature by measuring vertical displacement with an accuracy of approximately 0.1mm. The information was observed graphically on a laptop computer at the time of collection and stored digitally for later analysis at a sampling rate of 100Hz.

All participants initially undertook a brief familiarisation session on the stadiometer to enable them to practice the adoption of a repeatable and comfortable posture. This consisted of five recordings, between which the participant was asked to lean forward and break contact with the postural controls before resuming their position for the next measurement. A pilot study was carried out in which eight asymptomatic participants all performed this brief familiarisation of five readings. The results (average standard deviation (SD) 1.0mm, standard error of measurement (SEM) 0.8mm) demonstrated that this approach was sufficient to produce reliable stadiometer readings. Participants remained in position for a period of 20s and the stature value used was the mean reading over the final 10s.

2.4. Measures 

At the end of the testing session patients were asked about their pain intensity during the past 24h. They were then given a questionnaire booklet containing a series of self-report measures. Although some patients completed the booklet immediately, the majority completed it at home and returned it at a later date. Asymptomatic controls completed only the anxiety and depression (HADS) questionnaire.

2.5. Pain intensity 

A numerical rating scale (NRS) was employed to assess pain intensity. Participants were asked to rate their pain during the past 24h on a scale ranging from (0) ‘no pain’ to (10) ‘worst possible pain’. Research supports the reliability and validity of NRSs of pain intensity (Jensen, 2003) and the 11-point NRS was recently recommended by the Initiative on Methods, Measurement and Pain Assessment in Clinical Trials (IMMPACT) to assess chronic pain intensity (Dworkin et al., 2005).

2.6. Disability 

The Roland Disability Questionnaire (RDQ; Roland and Morris, 1983) is a 24-item self-report measure that assesses disability due to back pain. Patients are asked to select which statements, related to perceived limitations in typical daily activities, apply to them. The score is calculated by adding up the number of selected items, so that a higher number represents greater perceived disability. The RDQ has excellent reliability, validity and responsiveness (Roland and Fairbank, 2000, Turner et al., 2003) and is widely used to sample within the specific population for this study.

2.7. Anxiety and depression 

The Hospital Anxiety and Depression Scale (HADS; Zigmond and Snaith, 1983) is a widely used measure, designed for use with general medical outpatient populations. It consists of anxiety and depression subscales, both of which have seven items. Each item is rated on a four-point scale from 0 to 3, determining the extent to which the individual feels that each statement applies to them. Higher scores therefore represent higher levels of anxiety and depression. Both subscales of the HADS have established validity and reliability in a clinical population (Johnston et al., 2000, Bjelland et al., 2002, Pallant and Bailey, 2005).

2.8. Functional self-efficacy 

Functional self-efficacy refers to the confidence that an individual has in their ability to successfully accomplish functional activities. The functional subscale of the Chronic Pain Self-Efficacy Scale (CPSS-PF; Anderson et al., 1995) was used to assess functional self-efficacy. The original CPSS-PF, which has been shown to be reliable and valid (Anderson et al., 1995), is scored on a ten-point Likert scale, with higher scores reflecting higher levels of self-efficacy. This study used a nine-point Likert scale because it provides patients with a mid-point option and has also been shown to have excellent internal consistency and test–retest reliability with CLBP patients (Woby et al., 2007).

2.9. Fear of movement/(re)injury 

The Tampa Scale of Kinesiophobia (TSK; Kori et al., 1990) aims to measure fear of movement/(re)injury in individuals with pain. It consists of 17 items, for which patients rate themselves on a four-point Likert scale ranging from ‘strongly disagree’ to ‘strongly agree’. Four of the items are inversely scored. Scores range from 17 to 68, with higher scores representing greater fear of movement/(re)injury. The English version of the TSK has been found to be valid and reliable (Woby et al., 2005, French et al., 2007).

2.10. Catastrophising 

The Pain Catastrophising Scale (PCS; Sullivan et al., 1995) consists of 13 items describing thoughts or feelings that may be experienced when in pain. For each statement, participants rate the extent to which they have the feeling when experiencing pain on a five-point Likert scale ranging from (0) ‘not at all’ to (4) ‘all the time’. A higher score indicates greater levels of pain catastrophising. The PCS has been shown to have good reliability and validity (Sullivan et al., 1995, Osman et al., 1997, Osman et al., 2000).

2.11. Pain-related anxiety 

The Pain Anxiety Symptoms Scale-20 (PASS-20) is a shortened 20-item version of the original Pain Anxiety Symptoms Scale (PASS: McCracken et al., 1992), a 40-item measure developed to assess pain-related anxiety in individuals with chronic pain. Higher scores represent higher levels of pain-related anxiety. The PASS-20 has been shown to have good factorial validity, internal consistency and close association to the original 40-item version (Coons et al., 2004, Roelofs et al., 2004).

2.12. Procedure 

Following the brief familiarisation session on the stadiometer, a baseline stature measurement and initial EMG readings (at rest and during the RVC) were taken. Participants then assumed an unloading position on a physiotherapy bed for 20min. This was either a side-lying or a prone position, whichever the participant felt was the most comfortable. After 20min, the participants stood up and performed the same EMG and stadiometer measurements, before again assuming an unloading position for a further 20min. The same measurements were then taken for a final time. Stature change was calculated as the difference between the final and the initial stadiometer readings.

2.13. Analysis 

Parametric tests were used, based on the results of Kolmogorov–Smirnov and Shapiro–Wilk tests of normality. One-tailed t-tests were calculated to examine for differences in levels of muscle activity, stature recovery, anxiety and depression between the LBP patients and the asymptomatic controls. For the LBP patients, Pearson’s correlation coefficient was calculated to investigate the relationships between the measures.

Mediational analysis (Baron and Kenny, 1986), including Sobel’s test of mediation, was carried out to investigate whether muscle activity acted as a partial mediator between a range of psychological factors and either pain or disability, and also whether pain mediated the effect between muscle activity and disability. It was verified that there were no cases with a Cook’s distance of greater than 1, or leverage values that were 3 times the average value and, to control for multicollinearity, variance inflation factors had to be below 10.

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3. Results 

The mean duration of pain was 7.2 years (range: 3 months to 40 years). Forty two patients and 15 asymptomatic participants completed the questionnaires. Of these, 19 patients (45%) were classed as moderately disabled (RDQ: 9–16) and 11 (26%) as severely disabled (RDQ: 17–24), with classifications based on the work by Stratford et al. (1998). Technical problems with the EMG system at the start of the study meant that EMG data were not recorded for three patients. In addition, one patient was excluded from the EMG data due to high noise levels in the signal. The EMG data of two further patients were excluded from the analysis as they were materially different (>2 SDs) to the values of all other patients and they were therefore considered to be outliers.

3.1. Group characteristics 

The characteristics of the patient and control groups (where applicable) are given in Table 1. The muscle activity figures presented are the average of the four electrode sites used. As shown in Table 1, the control group had significantly reduced muscle tension (as a % of the RVC) (p=0.01) and a trend for greater stature recovery (p=0.06) compared to the patient group. Variability in stature measurements is often used as an exclusion criterion. Analysis was therefore also carried out excluding the stature recovery data for those participants who found it difficult to maintain a consistent posture in the stadiometer, as demonstrated by a high SD over the five familiarisation readings. For this purpose a SD of 1.7mm was taken as the cut-off point as it marked a clear gap between the majority of participants and the relatively small number with large SD values, and values exceeding this figure were considered to represent unacceptably high variation. This resulted in the data for 11 patients and one asymptomatic participant being excluded. The remaining patients had an average SD of 1.0mm over the five readings, which equalled that for the control group. With these participants excluded, the control group had significantly greater stature recovery than the patient group (p=0.03). It was noted that the control group had significantly lower body mass than the patient group (p=0.04), which is known to affect both stature change (Rodacki et al., 2005) and EMG readings (Burden, 2008). When the control group was compared with a matched (age, sex, height, weight) subset of the patient group, there was a trend for reduced stature recovery (p=0.06) and elevated muscle activity (p=0.09) in the patient group compared to the control group, but it no longer reached significance. This was also true when the stature recovery data for those with a SD of over 1.7mm was excluded.

Table 1. Group characteristics (n=47 patients, 18 controls).
VariablePatient groupControl groupEffect sizePossible range
Mean (±SD)Mean (±SD)
Body mass (kg)79.3±18.870.8±12.7*0.45
Stature change (mm)2.8 (±2.6)3.9 (±1.5)0.42
Muscle activity (% of RVC)75.5 (±12.2)a66.8 (±15.8)*0.71
Disability11.5 (±5.9) 0–24
Pain NRS4.8 (±2.2) 0–10
Anxiety8.5 (±3.7)5.2 (±2.8)**0.880–21
Depression6.3 (±3.4)3.5 (±2.1)**0.800–21
Functional self-efficacy45.5 (±18.3) 0–72
Pain-related anxiety42.4 (±23.9) 0–100
Catastrophising21.1 (±12.8) 0–52
Fear of movement36.7 (±9.3) 17–68

SD: standard deviation; RVC: reference voluntary contraction; NRS: numerical rating scale.

*p<0.05, **p<0.01.

aThe muscle activity details for the patient group exclude two outliers. If these two patients’ details are included, the mean (±SD) would be 73.1% (±16.3%), p=0.09, effect size=0.38.

3.2. Correlational analysis 

The correlations between all markers (excluding EMG data for three patients as discussed above) are shown in Table 2.

Table 2. Correlation coefficients between outcome measures (n=47).
1.2.3.4.5.6.7.8.9.10.
1. Stature change–
2. Muscle activitya−0.15–
3. Disability−0.160.43**–
4. Pain−0.160.48**0.57**–
5. Anxiety−0.130.31*0.55**0.42**–
6. Depression0.030.33*0.57**0.34*0.68**–
7. Self-efficacy0.02−0.45**−0.73**−0.49**−0.47**−0.57**–
8. Pain-related anxiety−0.080.29*0.54**0.37*0.67**0.57**−0.56**–
9. Catastrophising0.070.29*0.41**0.38**0.59**0.63**−0.45**0.81**–
10. Fear of movement−0.140.200.39**0.27*0.57**0.44**−0.250.63**0.65**–

*p<0.05, **p<0.01.

aThe muscle activity data for the patient group exclude two outliers.

3.3. Correlational analysis: muscle activity 

Significant correlations were found between muscle activity and both pain (p<0.01) (Fig. 4) and disability (p<0.01) (Fig. 5). Of the psychological factors considered, muscle activity demonstrated links with functional self-efficacy (p<0.01) (Fig. 6), depression (p=0.03), anxiety (p=0.03), pain-related anxiety (p=0.05) and catastrophising (p=0.04).

  • View full-size image.
  • Fig. 6 

    Relationship between self-efficacy (CPSS-PF) and muscle activity (as a % of the RVC). CPSS-PF: functional subscale of the Chronic Pain Self-Efficacy Scale; RVC: reference voluntary contraction.

3.4. Correlational analysis: stature recovery 

Stature recovery was not significantly related to any of the other factors. In particular, the relationship between muscle activity and stature recovery was not found to be significant (p=0.17). However, when the 11 patients who demonstrated high variability in stature measurements were excluded, there were trends for links between stature recovery and pain (r=−0.23, p=0.09) and depression (r=0.25, p=0.08).

3.5. Muscle activity as a mediator in the relationship between psychological factors and clinical outcome 

Mediational analysis was performed to investigate whether muscle activity acted as a partial mediator between any of the psychological factors and either pain or disability. The results (shown in Table 3) demonstrated that muscle activity was a significant mediator in the relationship between self-efficacy and pain (z=−1.92, p=0.028). Similar analysis showed that there was a trend for muscle activity to also be a partial mediator in the relationship between anxiety (p=0.056), depression (p=0.051), pain-related anxiety (p=0.061) and catastrophising (p=0.061) and pain. Muscle activity was not found to be a mediator between any of the psychological factors and disability.

Table 3. Muscle activity as a mediator in the relationship between self-efficacy and pain.
Dependent variableIndependent variableR2Significance
Pain (analysis 1)Self-efficacy0.240.001
Muscle activity (analysis 2)Self-efficacy0.210.005
Pain (analysis 3)Muscle activity0.290.019
Self-efficacy 0.169

3.6. Pain as a mediator in the relationship between muscle activity and disability 

Mediational analysis (shown in Table 4) demonstrated that pain was a significant mediator in the relationship between muscle activity and disability (z=2.13, p=0.017).

Table 4. Pain as a mediator in the relationship between muscle activity and disability.
Dependent variableIndependent variableR2Significance
Disability (analysis 1)Muscle activity0.180.007
Pain (analysis 2)Muscle activity0.230.001
Disability (analysis 3)Pain0.320.011
Muscle activity 0.198

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4. Discussion 

In line with previous research, there was a trend for patients with LBP to have higher muscle activity and delayed stature recovery compared to asymptomatic individuals, although this was not significant when comparing to a matched control group, and the effect size of 0.42 for the comparison of muscle activity (0.71 for the comparison with the total, unmatched, patient group) was less than the average effect size of 1.14 during standing reported in a recent meta-analysis of 20 studies (Geisser et al., 2005). The patient group also scored significantly higher on anxiety and depression than the asymptomatic individuals.

The results confirm that patients with greater pain and disability exhibit elevated paraspinal muscle activity compared to those with lower levels. Muscle activity was significantly correlated with self-efficacy, depression, anxiety, pain-related anxiety and catastrophising and was found to be a partial mediator in the relationship between self-efficacy and pain. This is an important finding which verifies the link between psychological and biomechanical factors in CLBP and appears to confirm the role of muscle activity as a pathway by which psychological factors may affect clinical outcome. Although it is widely accepted that psychological factors such as self-efficacy have an impact on clinical outcome and there is also a limited body of research which has reported correlations between such factors and muscle activity, this is one of the first studies to show that muscle activity acts as a partial mediator in this way. The role of muscle activity as a partial mediator between back pain and self-efficacy (with a trend for a similar role in the link with depression, anxiety, pain-related anxiety and catastrophising), suggests that interventions that are able to reduce muscle tension may be of particular benefit to patients demonstrating these characteristics, which may help in the targeting of treatment for LBP. For example, the presence of elevated muscle tension might indicate that a patient should be screened for the presence of psychological factors as a priority and conversely, high scores on certain psychological questionnaires might act as triggers to indicate that a patient is likely to demonstrate elevated muscle tension.

Although muscle activity was significantly associated with a range of psychological factors, it was not found to be related to fear of movement, which appears contrary to current literature relating to the fear-avoidance model and muscle guarding. This may be because muscle activity was only measured during relaxed standing whereas hyperactivity due to muscle guarding, for example, may become more apparent in certain postures or during movements perceived as threatening/harmful.

The results of the mediational analysis suggest that muscle activity affects disability via its influence on pain, further confirming the importance of muscle activity in LBP. However, the relationship between these three variables is likely to be more complicated than a single pathway and, within their impact on pain, both muscle activity and disability may separately play a mediating role.

The data did not support the hypothesized relationships between stature recovery and the other factors considered, including muscle activity. This is in contrast to the findings of Healey et al. (2005a), who did establish such a relationship in individuals with mild disability (RDQ 5.6±2.9). The current study had the advantage of deriving data from a clinical sample with moderate levels of pain and disability and the results suggest that the relationship between muscle activity and stature recovery within this patient population may be more complex than originally thought. However, the results may also reflect the heterogeneity typical of such a clinical population and the existence of sub-groups within the patient group. In addition, the stature change measurements may have been influenced by patients attending at different times of day, whereas Healey et al. (2005a) restricted the testing sessions to approximately one hour after rising.

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5. Conclusions 

Patients who demonstrated higher paraspinal muscle activity were those with more severe CLBP and the mediational analysis also indicated that muscle activity may affect disability via its influence on pain. The results therefore support the clinical relevance of this measure and suggest that treatments that reduce muscle activity may improve outcome. In addition, muscle activity was significantly correlated with a number of psychological factors and was found to act as a partial mediator between self-efficacy and pain, confirming the link between psychological and biomechanical factors in CLBP. Furthermore, it suggests that there may be particular benefit in reducing muscle activity in those with low self-efficacy.

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Acknowledgements 

The authors wish to thank both the patients and the staff at the physiotherapy department at North Manchester General Hospital where the data collection took place.

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PII: S1356-689X(11)00123-8

doi:10.1016/j.math.2011.08.001

Manual Therapy
Volume 17, Issue 1 , Pages 27-33, February 2012

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