Lewis S, Holmes P, Woby S, Hindle J, Fowler N.
Institute for Performance Research,
Manchester Metropolitan University,
Crewe CW1 5DU, United Kingdom.
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.
From the FULL TEXT Article
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 (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’ (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.
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.
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.