The influence of low back pain on somatosensory nervous system, muscle activity and movement behaviour during repetitive dynamic lifting

This study investigated changes in mechanical pain sensitivity (MPS) and pressure pain thresholds (PPT) across the lower back as well as changes in the distribution of lumbar erector spinae muscle activity and biomechanical disturbances in individuals with low back pain (LBP) with mild to moderate functional impairment and healthy controls. 19 people with chronic nonspecific LBP and 17 control subjects performed a repetitive lifting task while surface electromyographic (EMG) signals were recorded from multiple loca-tions over the lumbar erector spinae muscle with a grid of 64 electrodes while tridimen-sional tracking of body movement was achieved by means of an 8-camera stereo-photogrammetry system (motion capture system). The EMG root mean square (RMS) was computed for each location of the grid to form a map of the EMG amplitude distribution. Stimulus/response-functions (S/R-function) for MPS and dynamic mechanical allodynia (DMA) as well as PPT were recorded before and after the lifting task. Pain intensity was recorded before, during and after the lifting task. The individuals with LBP reported an increase in pain intensity of 38% during the repeti-tive lifting task. PPT was lower in the LBP group after completion of the repetitive task compared to baseline, whereas no change in PPT over time was observed for the control group. There were no changes observed for MPS in the LBP group compared to controls and baseline. None of the individuals with LBP showed DMA. The EMG RMS progressively increased more in the caudal region of the lumbar erector spinae muscle during the re-petitive task for the control subjects, resulting in a shift in the distribution of muscle activ-ity. In contrast, the distribution of muscle activity remained unaltered in the LBP group despite an overall increase in EMG amplitude. In comparison to the control group, the motion analysis did display reduced range of spinal motion and greater range of hip angle during the lifting task for individuals with LBP. The results demonstrate that LBP alters the normal adaptation of lumbar erector spinae muscle activity to exercise, which occurs in the presence of biomechanical disturbances and exercise-induced hyperalgesia. Reduced variability of muscle activity may have im-portant implications for the recurrence or chronicity of LBP due to repetitive tasks. How-ever, this changes where evident only in a subgroup of patients. Future studies should use longitudinal designs and patients with greater impairment due to LBP to validate this po-tentially prognostic factor.2017-03-0