Original articleThe effect of unilateral muscle pain on recruitment of the lumbar multifidus during automatic contraction. An experimental pain study
Introduction
Recent research has established an important role for muscle impairment within the pathology of low back pain (LBP). Changes in structure and function have been found in different trunk muscles and in all stages of LBP: acute, recurrent as well as chronic LBP. Many trunk muscles are required for control and stability of the spine, however, deep intrinsic muscles are critical for segmental stabilization of the spine (Panjabi, 1992).
It has been proven that the lumbar multifidus is a crucial stabilisator of the spine as the muscle is responsible for more than two-thirds of the lumbar segmental stiffness (Wilke et al., 1995). It is assumed that impairment of this muscle is a contributing factor in the development and maintenance of LBP. Many studies provide evidence for altered neuromotor control (i.e. delayed or reduced activity) as well as changed structure (i.e. reduced CSA (cross sectional area) and increased fat) of the multifidus in patients (Lindgren et al., 1993, Hides et al., 1994, Leinonen et al., 2001, Hodges and Moseley, 2003, van Dieen et al., 2003, Barker et al., 2004).
In 1994, Hides et al. published a study which demonstrated unilateral reduction in CSA of the multifidus in patients with acute unilateral LBP (Hides et al., 1994). Moreover, the muscle was affected only at the lumbar level of symptoms. Since then, other studies support the finding that changes in the multifidus are selective and therefore restricted to the side and level of symptoms (Barker et al., 2004, Macdonald et al., 2009). However, other LBP studies provide evidence of bilateral and more generalized changes, even in unilateral LBP (Kader et al., 2000, Dickx et al., 2008). The underlying mechanisms of these changes are not well understood. The question rises if acute unilateral pain only influences the multifidus at the level and side of the pain or has a more generalized effect. Studying this issue can gain more insight in the crucial role of pain within the complex pathology of LBP.
A method to target this question is investigation of the isolated contribution of pain on motor control. Different experimental models are available, of which the saline-model is commonly used in motor control studies (Arendt-Nielsen et al., 1996, Zedka et al., 1999, Hodges and Moseley, 2003, Hodges et al., 2003a). Using this model, the cause–effect relationship of pain on motor control can be examined. In addition, this set-up provides information of the non-pain and the pain condition within one subject, which is very important regarding the high variability in neuromotor control between subjects.
Usually electromyography (EMG) is used to investigate muscle activity, however, the deep multifidus muscles require intramuscular recording (Stokes et al., 2003). This invasive technique may provoke pain and thus interaction with our experimental unilateral pain model. An alternative non-invasive approach, is the indirect measurement of activity with dynamic ultrasound (US) during muscle contraction. It has been shown that increase in muscle thickness and more specific increase in multifidus thickness during low load contraction correlates with EMG activity (Hodges et al., 2003b, Kiesel et al., 2007). Recent studies have demonstrated that this technique is reliable and valid (Hides et al., 1995, Kiesel et al., 2007, Stokes et al., 2007, Wallwork et al., 2007).
The present experiment is based on a study of Kiesel et al. who investigated the effect of experimentally induced pain on lumbar multifidus activation during an arm lifting task (Kiesel et al., 2008). During this task, the multifidus muscle is recruited automatically, which is important for stabilization of the spine. They found that activation of the multifidus was reduced, at the level and the side of the pain induction. However, an important aspect of the study is that the multifidus was investigated only at one vertebral level and at one side (the level and the side of the pain induction), providing no information about the pattern of changes in the multifidus muscle. Therefore, the purpose of the current study is to investigate whether acute unilateral muscle pain alters the automatic activation of the multifidus selectively (at the level and side of the pain) or more generalized (at both sides and throughout different lumbar levels).
Section snippets
Subjects
Fifteen healthy subjects (6 male–9 female) volunteered for this study. Their mean age, height, and weight was 24 (±2) years, 172 (±9) cm, and 68 (±15) kg, respectively. Potential subjects were excluded from participation if they had any past or current back pain. All procedures were approved by the Ghent University Ethics Committee and each volunteer signed a written informed consent.
Ultrasound imaging
Subjects were positioned prone on the examination table with pillows under the abdomen to minimize the lumbar
Results
Inter- and intra-rater reliability.
The ICC is 0.93 for the inter-rater agreement and 0.98 for the intra-rater agreement, indicating excellent reliability.
Fear of pain and pain intensity.
Mean VAS score for ‘fear of injection’ was 2.9 (± 2.1) and was 3.5 (±2.1) for ‘fear of pain’. After injection ‘pain intensity’ was 6,0 (± 1.5). During the exercise, after completion of one body side, ‘pain intensity’ was still 6.0 (±1.5). At the end of the exercise ‘pain intensity’ was 4.4 (±2.3).
Descriptive
Discussion
Changes in structure and activity of the multifidus have been consistently found in LBP, although, two different patterns of changes have been demonstrated: selective changes in acute LBP (Hides et al., 1994, Barker et al., 2004, Hodges et al., 2006, Macdonald et al., 2009) versus more generalized changes in chronic LBP (Cooper et al., 1992, Parkkola et al., 1993, Kader et al., 2000).
The effect of pain on these changes in the multifidus muscle is not well known. A method to investigate the
Conclusion
The results of this experimental study have to be interpreted with care but can provide new insights within the pain mechanisms playing a role in LBP. To date, it remains uncertain if pain has a selective effect on the multifidus muscle, in line with the observations of the rapid onset of selective changes in structure in acute LBP, or a more generalized effect in line with the observations in chronic LBP. Our results demonstrate that experimental, unilateral pain has a more widespread effect
Acknowledgements
The authors wish to thank Ann Lavens for her assistance with this research and Esaote-Pie Medical Benelux for the technical support. This research was funded by BOF-Ghent University.
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