Trunk neuromechanics during turning: a window into recurrent low back pain

During steady‐state locomotion there are characteristic speed‐dependent patterns of inter‐segmental coordination between the trunk and the pelvis in the axial plane. The ability to modulate this coordination, and the magnitude of stride‐to‐stride variability in inter‐segmental coordination, is reduced in symptomatic individuals with chronic low back pain. This has been primarily attributed to changes in trunk dynamics as a result of increased muscle activity during walking in these individuals. In addition, symptomatic individuals with chronic low back pain appear to devote greater attentional resources to locomotion than healthy individuals. As existing studies have only investigated treadmill walking, and have used surface electromyography to quantify the activity of the superficial paraspinal muscles, it is unclear how these aspects of trunk postural control are controlled and modulated during common locomotor perturbations such as walking turns. It is also unclear how the individual muscles in the paraspinal group contribute to trunk control during turning. In addition, it has not been established if changes in trunk neuromechanics during locomotion persist in individuals with persistent but recurrent low back pain between symptomatic episodes. ❧ The purpose of chapter III was to evaluate axial plane inter‐segmental coordination between the trunk and the pelvis, and the activity of the individual paraspinal muscles during ipsilateral pivot turns performed at a controlled speed, and to compare these aspects of trunk postural control in healthy individuals and asymptomatic individuals with a history of recurrent low back pain. Fourteen healthy individuals and fourteen asymptomatic individuals with a history of recurrent low back pain performed multiple ipsilateral walking turns at an average speed of 1.5 m/s. The frequency that inphase, antiphase, trunk phase, and pelvic phase patterns of inter‐segmental coordination occurred and the stride‐to‐stride variability in inter‐segmental coordination were quantified using the vector coding technique. Intramuscular EMG electrodes were inserted into the deep fibers of the lumbar multifidus, the lumbar longissimus and thoracic longissimus. The duration of activity in each muscle across the stride cycle of the turn was calculated. There was significantly greater inphase coordination during the stance phase of the turn compared with the swing phase. Coordination variability was greatest during swing phase in all participants. There was no relationship between duration of paraspinal muscle activity and the coordination kinematics. The lack of difference in trunk neuromechanics during turning between healthy individuals and individuals with a history of recurrent low back pain in this study indicates that previously demonstrated changes in these variables in symptomatic individuals may be due to concurrent pain rather than persistence of symptoms. ❧ The purpose of Chapter IV was to examine the modulation of inter‐segmental coordination in the axial plane between the trunk and pelvis, and duration of paraspinal muscle activity, in response to increased locomotor speed. The participants performed multiple repetitions of ipsilateral walking turns both at self‐selected speed and a faster average speed of 1.5 m/s. Separate mixed‐model ANOVA demonstrated that amplitude of thoraco‐pelvic motion and frequency of antiphase coordination increased significantly at the faster speed. Coordination variability was not affected. On the whole, the duration of muscle activity increased with speed, particularly during swing phase. The distribution of this increase across the three muscles was different between groups. Healthy individuals increased the duration of activity in the deep fibers of multifidus at the faster speed, whereas the individuals with a history of low back pain decreased the duration of activity. There was no relationship between changes in duration of muscle activity and changes in coordination kinematics. This study demonstrates that there are similar modulations in trunk postural control in response to increasing speed during turning as there are during steady‐state locomotion. It also suggests that even when they are asymptomatic, individuals with a history of recurrent low back pain utilize a different control strategy in the paraspinals to respond to increasing locomotor speed. ❧ The purpose of Chapter V was to investigate the effect of divided attention on inter‐segmental coordination and muscle activity during walking turns. Participants performed multiple repetitions of ipsilateral walking turns at a controlled speed both with and without a concurrent verbal working‐memory task. Mixed‐model ANOVA demonstrated that coordination variability was significantly decreased during the divided attention condition compared with the baseline condition. This reduction was the same in both groups and was most evident during swing phase. The other variables were not significantly affected by divided attention. There was a significant negative correlation between change in coordination variability and change in the frequency of inphase coordination, but no relationship between changes in the other variables. ❧ The purpose of Chapter VI was to establish if the insertion of intramuscular EMG electrodes into the paraspinal muscles results in altered locomotor kinematics. Participants performed ipsilateral walking turns at self‐selected and controlled speed, before and after instrumentation with intramuscular EMG electrodes in the deep fibers of the lumbar multifidus, the lumbar longissimus and the thoracic longissimus. Tests of equivalence were used to determine if self‐selected locomotor speed, peak‐to‐peak amplitude of thoraco‐pelvic motion, inter‐segmental coordination, or coordination variability were affected by electrode insertion. Average self‐selected locomotor increased following intramuscular EMG insertion but all of the other variables did not change from pre‐ to post‐insertion. ❧ The findings from these studies establish that inter‐segmental coordination between the trunk and the pelvis is modulated from the steady state during walking turns. Inphase coordination is used to achieve re‐orientation of the trunk during the stance phase of the turn, while there is a rapid return to more complex patterns of coordination during the swing phase. Increased coordination variability during swing phase may be evidence of feedback correction of errors in performance during the turn. The lack of relationship between duration of muscle activity and inter‐segmental coordination, and the significant effect of divided attention on coordination variability, suggest that inter‐segmental coordination is not solely a product of inherent trunk dynamics but is also dependent upon attentional resources. In addition, the association between reduced coordination variability and increased inphase coordination during the divided attention condition suggests that attentional focus may be a mechanism for the altered trunk postural control that is evident in symptomatic individuals with chronic low back pain during walking. ❧ In addition, this work demonstrates that changes in postural control of the trunk in association with low back pain may form a continuum. Despite a prolonged history of symptoms, individuals with recurrent low back pain who were symptom free did not demonstrate altered ability to modulate inter‐segmental coordination. This suggests that these kinematic changes may only be evident with concurrent pain or during more mechanically demanding tasks. However, they did demonstrate altered modulation of deep multifidus and thoracic longissimus muscle activity. These changes are consistent with those demonstrated in existing studies in individuals with acute and chronic pain. Further work is needed to establish if these changes in muscle activity contribute to symptom recurrence.