Amplitude effects of medio-lateral mechanical and visual perturbations on gait
Introduction
Each year there are over 700,000 new non-fatal fall injuries, which will eventually lead to nearly $35 billion in hospitalization and lost productivity costs (CDC, 2011). The third-leading cause of these falls was a slip, trip, or stumble while walking (CDC, 2011). Therefore, a better understanding of walking balance is vital to reducing physical, emotional, and economic costs associated with these accidents. Because walking balance relies on proprioceptive and visual feedback, gait perturbations increase demand on these sensory systems in a way that may provide better insight into the maintenance of walking balance. However, each of these perturbation types affects balance in unique ways which have not been extensively studied in the same environment. Also, the scaling effects of perturbation amplitude on kinematic and temporospatial variability during gait have received little attention. The combination of limited knowledge and significant personal and financial burdens of fall-related injuries presents an urgent need to better understand these responses.
To enhance our understanding of falls caused by visual and mechanical perturbations, recent investigations have examined changes in gait patterns during applied perturbations. Sinusoidal support surface perturbations during walking in a static visual environment induced increased step width variability (Brady et al., 2009). Visual environments that included changes in scene complexity, tilt angle, and optical flow caused shorter and wider steps and greater step width and stride velocity variability (Hollman et al., 2006, Nyberg et al., 2006, Hollman et al., 2007, Lamontagne et al., 2007). Additionally, exposure to a virtual curved corridor during treadmill walking caused significant increases in weight acceptance and push-off forces along with increased step width, reduced stride length, and larger step width and stride velocity variabilities (Hollman et al., 2006, Hollman et al., 2007). These effects were also found to be significantly greater when the perturbations were in the medio-lateral (M-L) direction (Warren et al., 1996, O'Connor and Kuo, 2009). These findings were confirmed as part of this study's precursor, which included optic flow matched to treadmill walking speed during platform or visual perturbations in the M-L or anterior–posterior (A–P) directions (McAndrew et al., 2010). However, because that study used only one perturbation amplitude for each perturbation type, it was not possible to quantify amplitude sensitivity, which could provide information about scaling effects, individual responsiveness, and balance limits.
Currently, knowledge of amplitude effects on visually and mechanically perturbed gait at multiple amplitudes is limited. One study examined gait sensitivity to discrete perturbations at two amplitudes and found that M-L sternum displacements and corrective step widths increased for the higher amplitude perturbations (Oddsson et al., 2004). Additionally, two visual perturbation studies presented sinusoidal perturbations at multiple amplitudes and revealed a correlation between perturbation amplitude and kinematic and temporospatial responses (Warren et al., 1996, O'Connor and Kuo, 2009). However, both visual studies used pure sinusoidal perturbations that may have allowed for adaptation and entrainment of responses to these predictable perturbations.
Intuitively, perturbation effects might be expected to increase with perturbation amplitude, but these effects are not necessarily consistent or linear. Furthermore, the responses to mechanical perturbations are likely to be inherently different from those for visual perturbations. Better knowledge of amplitude effects could be useful for protocol development and provide insight into the characteristic responses produced by different stimuli. There may also be limited responses at amplitude extremes when perturbations are either so small that they have negligible influence or so large that gait is either completely disrupted (with a stumble or fall). For instance, mechanical perturbations induce an immediate involuntary displacement and subsequent corrective movements to maintain balance, walking speed, and direction. However, responses to visual perturbations are driven only by a perceived need to correct posture and walking direction. Should these unnecessary corrections actually increase walking variability, they will drive additional corrections as the body attempts to recover normal walking patterns. This fundamental difference suggests that the sensitivity to perturbation amplitude must be established separately for each perturbation type.
Our goal was to quantify the amplitude-driven temporospatial and kinematic changes in responses to pseudo-random, continuous visual and mechanical perturbations. We anticipated that participants would exhibit: (1) variabilities proportional to perturbation amplitude and (2) amplitude responsiveness specific to each perturbation type.
Section snippets
Methods
Eleven young healthy individuals with no gait deficiencies provided written informed consent, as approved by the Institutional Review Boards at Brooke Army Medical Center, Ft. Sam Houston, TX, and The University of Texas at Austin. Participants walked at normalized speeds (1.21±0.03 m/s) in a Computer Assisted Rehabilitation ENvironment (CAREN) (Motek, Amsterdam, Netherlands).
The CAREN includes a 2 m×3 m instrumented treadmill embedded in a 4 m diameter movable platform surrounded by a 7 m diameter
Platform perturbations
Mechanically perturbed trials (P1–P3) produced significantly larger mean step widths (p<0.001) but post-hoc analyses showed that SWmn did not change with amplitude increases beyond P1 (Fig. 1A). Conversely, step width variability (SWsd) increased with each increase in amplitude (p<0.006; Fig. 1C). The between-trial difference (Trial 1 vs. Trial 2) in SWmn was just beyond significance (p=0.051), but this difference was only 1 mm for mean step width of 12.7 cm. Like the group means, individual SWmn
Discussion
This study was the first to establish the amplitude effects of visually and mechanically perturbed gait in the same environment. In doing so, we demonstrated that responses to mechanical perturbations differ from responses to visual perturbations in overall sensitivity to perturbation amplitude and consistency across individuals. There was little adaptation within each condition, as indicated by non-significant or negligible between-trial differences. Of six gait parameters, only measures of
Conflict of interest statement
No benefits in any form have been or will be received from a commercial party related directly or indirectly to the subject of this manuscript.
Acknowledgments
This research was supported by NIH Grant R01-HD059844. The views expressed herein are those of the authors and do not reflect the official policy or position of Brooke Army Medical Center, the US Army Medical Department, the US Army Office of the Surgeon General, the Department of the Army, Department of Defense or the US Government.
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