Effect of low back pain and pain-related fear on human movement

Thesis summary In Chapter 2, we found that our measures to assess motor control of the trunk during seated repetitive reaching, variability (temporal and spatial) and local dynamic stability, were sufficiently reliable in single-session experiments, in a mixed group of people with and without chronic/recurrent low back pain (CLBP). In addition, within-session, a plateau in reliability appeared to be reached at 40 repetitions for spatial and temporal variability. The reliability of local dynamic stability seemed to continue to increase beyond the 40 repetitions. Between-session, reliability was lower although still sufficiently reliable for multi-session experiments. We used this information to design the subsequent studies. In Chapter 3, we found no differences in trunk motor control between people with and without CLBP, during seated repetitive reaching. However, we did find an interaction effect of task-specific pain-related fear (high/low) and group (CLBP/healthy controls) on trunk motor control. People with CLBP and high ‘Expected Back Strain’ (EBS), moved with significantly larger trunk spatial variability compared to healthy controls and the CLBP-group with low EBS. The larger trunk spatial variability was opposite of what we expected based on the association between CLBP and guarded movement, which would be expected to coincide with low variability. We found no associations between general questionnaires of pain-related fear and trunk motor control. In Chapter 4, instead of changes expected in association with guarded movement, we found in line with Chapter 3, that perceived postural threat increased trunk movement variability (temporal and spatial). This effect was statistically independent of the presence of CLBP, or taskspecific pain-related fear (EBS). In addition, among participants with CLBP, those with a higher EBS showed higher trunk spatial movement variability regardless of postural threat. These results suggest that perceived postural threat may underlie changes in motor behavior due to CLBP. To explain the increased trunk movement variability described in chapters 3 and 4, we formulated a new hypothesis based on results of Ribot-Cicsar et al. (2000). They showed that both the mean and variance of firing of muscle spindle afferents increase with arousal. We proposed that arousal caused the decrease in movement precision, due to high EBS or postural threat resulting from higher variance of spindle firing. At the same time, an increased resistance against external perturbations results from a higher mean spindle firing rate. In Chapter 5, we report results supporting our new hypothesis that one neurophysiological process, activated by arousal, can both increase resistance against external perturbations and simultaneously decrease movement precision. We found that higher EBS, was associated with increased resistance to external mechanical perturbations, reflected in a decreased perturbation effect. In the higher EBS-group, the perturbation effect was significantly and negatively associated with movement precision measured with a trunk movement tracking task. These results demonstrated that pain-related fear influenced trunk motor control and support the idea that subjects with more pain-related fear show higher arousal during performance leading to an increased resistance against perturbations, at the expense of movement precision. In Chapter 6 we demonstrated that the trunk movement tracking error was more sensitive to CLBP and more responsive to treatment compared to trunk movement variability. Therefore, the tracking error is a promising marker of motor control impairments in people with CLBP.