The Relationships Between Measures of Stature Recovery, Muscle Activity and Psychological Factors in Patients with Chronic Low Back Pain
SOURCE: Manual Therapy 2012 (Feb); 17 (1): 27-33
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.
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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.