Coordination of Axial Trunk Rotations During Gait in Low Back Pain. A Narrative Review

Abstract

Chronic low back pain patients have been observed to show a reduced shift of thorax-pelvis relative phase towards out-of-phase movement with increasing speed compared to healthy controls. Here, we review the literature on this phase shift in patients with low back pain and we analyze the results presented in literature in view of the theoretical motivations to assess this phenomenon. Initially, based on the dynamical systems approach to movement coordination, the shift in thorax-pelvis relative phase with speed was studied as a self-organizing transition. However, the phase shift is gradual, which does not match a self-organizing transition. Subsequent emphasis in the literature therefore shifted to a motivation based on biomechanics. The change in relative phase with low back pain was specifically linked to expected changes in trunk stiffness due to 'guarded behavior'. We found that thorax-pelvis relative phase is affected by several interacting factors, including active drive of thorax rotation through trunk muscle activity, stride frequency and the magnitude of pelvis rotations. Large pelvis rotations and high stride frequency observed in low back pain patients may contribute to the difference between patients and controls. This makes thorax-pelvis relative phase a poor proxy of trunk stiffness. In conclusion, thorax-pelvis relative phase cannot be considered as a collective variable reflecting the orderly behaviour of a complex underlying system, nor is it a marker of specific changes in trunk biomechanics. The fact that it is affected by multiple factors may explain the considerable between-subject variance of this measure in low back pain patients and healthy controls alike.

Keywords: coordination; gait; low back pain; relative phase; trunk.

Figure 1

Thorax-pelvis relative phase at relatively high walking speed The figures left from the midline represent realistic thorax-pelvis rotations at a walking speed of ± 1.5 m/s. The figures on the right display the harmonic frequencies of the time-series separated using a Fourier transform (time domain to frequency domain) and inverse Fourier Transform (harmonic frequency content to time domain). Upper panel: Time-series of thorax- and pelvis rotations of two complete periods for each signal. The black line with two circles represents an arbitrary point in time ‘t₁’, the black line with two diamonds ‘t₂’. Middle panel: Phase plots of the pelvis (blue) and thorax (red). The angles displayed in the upper panel are now plotted against the angular velocities of the same signals. The presented phase plots complete two orbits, one per stride cycle. t₁ and t₂ from the upper panel are displayed in this panel as well, representing the same points in time. Note that the left two figures are not perfectly round, which is mainly caused by the higher harmonic frequency content in the time series (causing a loop in the left and right side of the pelvis phase plot and a dent on the upper and lower side of the thorax phase plot). Lower panel: The angle between the phase of the thorax and pelvis at t₁ and t₂: thorax-pelvis relative phase. Note that thorax-pelvis relative phase is different between t1 and t2 in the left two figures, which is mainly the result of higher harmonics, and identical in the right two figures.

Figure 2

Thorax-pelvis relative phase as a function of gait speed in 5 healthy subjects. Solid lines represent normal walking, dashed lines represent walking with large steps, i.e. at a lower than normal stride frequency. Corresponding colors and symbols indicate the same subject. The data illustrate the between-subject variance in the speed-relative phase relationship and the gradual decrease in relative phase with speed. The fact that relative phase decreases with speed in spite of the concomitant increase in stride frequency and the consistency of the relationships between normal and large steps in the same subjects indicate that gait speed affects the relative-phase more than stride frequency. Data from (Liang et al., 2014).

Figure 3

Relative timing of thorax and pelvis rotations and arm and leg swing during slow and fast walking in low back pain patients and healthy controls. The first harmonic frequency content of each time series is displayed. The timing of most displayed signals is similar between low back pain patients and healthy controls. Hence, the pelvis, arms and legs are represented by single lines in both plots, as is the thorax in the upper plot. In the lower plot, the thorax of low back pain patients is represented by a dotted line. At low gait speed, thorax and pelvis rotate more-or-less out-of-phase with the pendular movements of the legs. At high gait speed the timing of the pelvis becomes more in-phase with the legs. In healthy controls, the timing of thorax rotations relative to the legs is relatively constant. In low back pain patients, the timing of thorax rotations shifts towards more in-phase with the legs with increasing gait speed, resulting in less out-of-phase thoraxpelvis coordination compared to healthy controls. HS = Heel Strike

Figure 4

Forward dynamic models of the trunk that have been used to gain insight in mechanisms that affect thorax-pelvis coordination during gait. Left: Two forward dynamic second order linear models that predict time series of thorax rotations from pelvis rotations, trunk stiffness and damping and shoulder reaction forces. Right: The frequency response function of each model when provided with actual gait data (black) compared to the actual frequency response function of the provided data (gray). Thorax-pelvis relative phase as defined in this review relates to the phase of the frequency response function at the first harmonic frequency (i.e., the stride frequency), indicated with an ‘×’ in each figure. The arms have a significant effect on thorax-pelvis relative phase, particularly at the first harmonic frequency. Hence, a second order linear model without the driving force of the arms does not appear a valid model of thorax-pelvis coordination during gait.

Figure 5

Effects of five mechanisms affecting thorax-pelvis relative phase during gait, predicted by a forward dynamic model. Each figure displays the mean thorax-pelvis relative phase (vertical y-axis) of 30 subjects for different values of each of the five control parameters. Error bars indicate standard deviations. In the left four figures, the values at 100% on the x-axis correspond to actual experimental values of that parameter. Increasing stride frequency with a constant amplitude of arm swing, would increase shoulder reaction forces, which increases the effect on thorax-pelvis timing (‘Realistic Arm Moment’) compared to a simulation where this effect is neglected (‘Constant Arm Moment’). In the right figure, the lag of 0 degrees indicates the average experimental timing of arm swing relative to the pelvis, negative phase shifts indicate a shift of arm swing timing relative to the experimental pelvis rotations, resulting in more in-phase movement relative to the pelvis. Details about the methods resulting in this figure are described in the Appendix.