RELATIONSHIP BETWEEN TRUNK POSTURAL CONTROL AND LOW BACK PAIN

Abstract

Suboptimal trunk postural control would affect the quality of coordinated movement and postural adjustments. This is likely to have consequences for the development and/or persistence of low back pain (LBP). The overall aim of this thesis was to understand trunk postural control in individuals with and without acute LBP. This was achieved by investigating postural control in an unstable sitting paradigm where individuals sit on a seat that is mounted on a hemisphere. Postural control in sitting was studied to enable evaluation of lumbar spine control with reduced contribution of the limbs. This thesis involved Five Studies. Study One was a systematic review with an individual participant data (IPD) meta-analysis that included 40 studies (1,821 participants) that assessed overall balance performance during unstable sitting. Outcome measures were extracted from either the force plate or motion capture systems to quantify seat movements. In comparison to pain-free controls, IPD meta-analysis revealed that individuals with LBP had greater movement that was less corrective and lower sway frequency. IPD meta-analysis also revealed that overall balance performance was impacted more in individuals with LBP than in pain-free controls when visual input was eliminated and by higher age and body mass index (BMI). Studies Two and Three were cross-sectional studies. Study Two investigated hip and spine coordination to maintain balance in the unstable sitting paradigm in pain-free individuals (n=72), and Study Three investigated whether this differed from individuals with acute LBP (n=130). Hip and spine kinematics via optical motion capture systems and centre of pressure (CoP) via force plate were extracted. Coordination of movement between the hip and spinal regions with the seat was quantified using cross-spectral analyses to measure amplitude spectrum (amount of movement), phase angle (movement relation between two segments: in-phase [segments move together]; out-of-phase [segments move opposite]) and coherence (consistency of phase relation). Study Two showed that in pain-free individuals, seat movements were coherently counteracted (out-of-phase) by hip and lower lumbar spine movements in the sagittal plane and lumbar spine movements in the frontal plane. Furthermore, eye closure and higher BMI were associated with greater movement amplitude, but they did not compromise movement coherence. Instead, movement coherence increased. Study Three showed that seat movements were greater in both planes and sagittal coherence between the hip or lumbar spine and that seat movements was lower in acute LBP than in controls. Furthermore, three different coordination strategies to maintain balance were identified, and these strategies demonstrated different success in overall balance performance. Study Four was a longitudinal study that investigated whether the quality of hip and spine coordination in individuals with acute LBP at baseline predicts recovery at six months (n=94), using the same methods as Studies Two and Three. This study showed that individuals with LBP who did not recover at six months exhibited lower coherence between the lumbar spine and seat movements in both planes at baseline, and these seat-lumbar movements were more in-phase in the sagittal plane and less out-of-phase in the frontal plane when compared to those who recovered. Additionally, coherence between hip and seat movements in both planes was higher in individuals with LBP who partially recovered than recovered, indicating a compensation strategy. Study Five was a cross-sectional study aimed to evaluate whether sitting postures adopted in the laboratory (including unstable sitting) relate to sitting postures used in the real-world in pain-free individuals (n=25). Kinematic data were recorded by wearable motion sensors to evaluate lumbar spine curvature via mean lumbar flexion angle (laboratory and real-world), time spent (real-world: 48 hours) during sitting and walking, and overall balance performance and lumbar spine coherence during unstable sitting (laboratory). This study showed that the posture adopted in unstable sitting was related to the overall and most common postures adopted in the real-world, whereas the posture adopted when instructed to sit upright was related to the second most common posture adopted in the real-world. This thesis makes several important contributions to the understanding of LBP. First, whereas previous works have focused on the evaluation of CoP as the net outcome of postural control, this thesis has provided substantial new evidence of how this outcome is achieved by the evaluation of kinematics in the frequency domain. This enabled the interpretation of how the strategy for trunk postural control is achieved differently between groups. It provided a sophisticated interpretation of how individuals with acute LBP differ from healthy controls and identified features that are related to long-term outcomes. Second, the thesis provided evidence of the potential to undertake IPD analysis using biomechanical data to generate more convincing estimates of overall effect sizes and interaction effects. Third, the final study took advantage of new advances in wearable technologies to provide the first evidence of which measures made in the laboratory have relevance for function in the real-world. Critically, the task that was most relevant was unstable sitting, which was a fundamental concept studied throughout this thesis. These contributions provide a strong foundation to propose potential new directions for LBP research and new opportunities for the prevention of recurrence/persistence of LBP.