The Behaviour of the Lumbar Paraspinal Muscles in People with Recurrent Low Back Pain During Symptom Remission

Low back pain is a prevalent and costly musculoskeletal condition. It has been estimated that between 60 and 90% of people will experience an episode of low back pain in their lifetime. At the moment three million Australian adults are suffering with low back pain and one million of those people will continue to have recurrent episodes. Changes in the behaviour of the trunk muscles have been argued to contribute to these recurrent episodes and rehabilitation strategies have been developed to optimise, enhance or restore the function of those muscles in this population. Some rehabilitation strategies targeting the paraspinal muscles have focused on the lumbar multifidus based on biomechanical models that argue the lumbar multifidus can contribute to the control of motion at the lumbar spine and limited electromyographic and imaging data that suggest that the morphology and behaviour of the lumbar multifidus are altered in people with low back pain. Based on morphological data, it is believed that changes in multifidus behaviour are specific to the components of the multifidus that cross only two lumbar segments. However, specific recording of the deep and superficial fibres of the lumbar multifidus had not been undertaken to test this belief. Furthermore, determining whether changes in the behaviour of the lumbar multifidus are present in people with recurrent low back pain during a period of symptom remission had not been investigated and could offer a candidate mechanism to explain symptom recurrence. The overall aim of this thesis was to investigate the control of the deep and superficial fibres of the lumbar multifidus muscles in people with recurrent unilateral low back pain during a period of symptom remission and in healthy pain-free individuals. Specific recordings of deep and superficial fibres of the lumbar multifidus activity were made using intramuscular electromyography during internal (Studies I: Chapter 4 and III: Chapter 6; rapid upper extremity movements) and external postural perturbations (Study II: Chapter 5; predictable and unpredictable trunk loading) with varying levels of cognitive demand (Study III: Chapter 6) and preparation for trunk loading (Study II: Chapter 5). The behaviour of the lumbar multifidus was also investigated using ultrasound imaging during voluntary lower limb movements commonly used to challenge trunk control in low back pain populations (Study IV: Chapter 7). These studies showed temporal and spatial differences in the control of the deep and superficial fibres of the lumbar multifidus muscles in people with recurrent unilateral low back pain during a period of symptom remission compared to healthy pain-free individuals. Temporal differences (Study I: delayed onset of deep multifidus activity relative to the onset of deltoid activity) are present bilaterally but greater on the previously symptomatic side. Spatial differences (Study II: decreased amplitude of deep multifidus activity and earlier onset of peak superficial multifidus amplitude prior to predictable trunk loading) are also present bilaterally but differed between the previously symptomatic and non-symptomatic sides. However, spatial differences were not affected by increased cognitive demand (Study III: no difference in the amplitude of the deep or superficial fibres of the lumbar multifidus during internal postural perturbations with increased cognitive demand). Ultrasound imaging also showed differences in lumbar multifidus behaviour between these two groups during performance of a lower limb movement (Study IV: greater increase in multifidus percentage thickness change during the prone straight leg raise in people with a recurrent low back pain during symptom remission compared to healthy pain-free individuals). Our findings show that differences are present in the control of the lumbar multifidus between people with recurrent low back pain during symptom remission and healthy pain-free individuals. These findings support the notion that there are differences in the strategy used by the nervous system to control spinal motion in people with recurrent low back pain. These findings are consistent with therapeutic exercise programmes attempting to restore the control of specific components of the paraspinal muscle group. Finally, these findings suggest that despite the resolution of symptoms paraspinal muscle function is not restored and could offer a candidate mechanism to explain symptom recurrence