Head stabilization during standing in people with persisting symptoms after mild traumatic brain injury

Abstract

Increased postural sway is often observed in people with mild traumatic brain injury (mTBI), but our understanding of how individuals with mTBI control their head during stance is limited. The purpose of this study was to determine if people with mTBI exhibit increased sway at the head compared with healthy controls. People with persisting symptoms after mTBI (n = 59, 41 women) and control participants (n = 63, 38 women) stood quietly for one minute in four conditions: eyes open on a firm surface (EO-firm), eyes closed on a firm surface (EC-firm), eyes open on a foam pad (EO-foam), and eyes closed on foam (EC-foam). Inertial sensors at the head, sternum, and lumbar region collected tri-axial accelerations. Root-mean-square (RMS) accelerations in anteroposterior (AP) and mediolateral (ML) directions and sway ratios between the head and sternum, head and lumbar, and sternum and lumbar region were compared between groups. Temporal coupling of anti-phase motion between the upper and lower body angular accelerations was assessed with magnitude squared coherence and cross-spectral phase angles. People with mTBI demonstrated greater sway than controls across conditions and directions. During foam-surface conditions, the control group, but not the mTBI group, reduced ML sway at their head and trunk relative to their lumbar by increasing the expression of an anti-phase hip strategy within the frontal plane. These results are consistent with suggestions of inflexible or inappropriate postural control in people with mTBI.

Keywords: Balance; Concussion; Posture; Sensory Integration; Stability; Sway.

Conflict of interest statement

Declaration of Competing Interest The authors declared that there is no conflict of interest.

Copyright © 2020 Elsevier Ltd. All rights reserved.

Figures

Fig. 1.

Representation of acceleration sway within the transverse plane at the head, sternum, and lumbar sensor locations for a single subject during EO-Firm (left) and EC-Foam (right) conditions.

Fig. 2.

Sway ratios for each direction and condition. A-C) Sway ratios for the head-to-sternum, sternum-to-lumbar, and head-to-lumbar sway ratios in the anteroposterior direction. D-F) Sway ratios for the head-to-sternum, sternum-to-lumbar, and head-to-lumbar sway ratios in the mediolateral direction. Across all figures, the horizontal green line indicates a sway ratio equal to one. In E and F, * indicates significant between-group differences based on post-hoc pairwise t-tests. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Fig. 3.

Mean and standard deviation envelopes (±one SD) of magnitude-squared coherence (top) and cross-spectral phase (bottom) between frontal plane αUB and αLB for both mTBI (red) and Control (black) groups in each condition. T-statistics from the 1D statistical parametric mapping (SPM) are depicted in blue below each curve, with horizontal dashed red lines denoting the critical t-value for p < 0.01. Frequencies where the t-statistic is outside the upper or lower critical t-value represent statistically significant differences between mTBI and control groups. In the firm conditions, there were no between group differences in coherence or phase at any frequency. In the EO-Foam condition, the mTBI group exhibited less coherence between 0.3 and 0.5 Hz and smaller phase angles between 0.43 and 0.63 Hz compared to the control group. In the EC-Foam condition, the mTBI group had less coherence between 0.12 and 0.50 Hz and smaller phase angles between 0.19 and 0.44 Hz and 0.63 Hz, indicative of a weaker anti-phase relationship between αUB and αLB compared to the control group. The stronger anti-phase relationship in controls are consistent with increased head-in-space stabilization in the foam conditions. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Fig. 4.

Mean and standard deviation envelopes (±one SD) of magnitude-squared coherence (top) and cross-spectral phase (bottom) between sagittal plane αUB and αLB for both mTBI (red) and Control (black) groups in each condition. T-statistics from the 1D statistical parametric mapping (SPM) are depicted in blue below each curve, with horizontal dashed red lines denoting the critical t-value for p < 0.01. No group differences were found at any frequency in any condition. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Fig. 5.

Schematic of two-link postural control state estimation. Motion in the sagittal or frontal plane can be determined from the combined height of the hip joint, hhip, height of the center of mass, hcoM, and two of the three angles: the angle of the ankle with respect to vertical, θ, encoded through ankle proprioceptors; the angle of the hip with respect to the lower body, β, encoded through hip proprioceptors; and the angle of the upper body with respect to vertical, γ, encoded through visual and vestibular sensory systems. Using Equations (5), (6), or (7), the horizontal position of the CoM and the angle of a single-link inverted pendulum model, Φ, (shown in red) can be determined. All angles are defined as positive for a counterclockwise rotation with respect to the reference. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)