Speed-dependent changes in spatiotemporal gait parameters and margins of stability in response to optic flow perturbation in healthy young adults

Introduction: Mediolateral optic flow perturbations can be used to assess balance control, but it is unclear how the immediate effects of these perturbations, and the subsequent recalibration are modified by gait speed.
Research Question: Do the immediate and prolonged effects of mediolateral optic flow perturbations on step parameters and gait stability depend on gait speed?
Methods: 21 young adults (age 23.43±4.19, 11 females) were instructed to walk on an instrumented treadmill during 3 phases: (i) a 3-minute baseline phase, (ii) an 8-minute perturbation phase in which participants were exposed to an immersive, mediolateral optic flow perturbation presented on a large screen in front of the treadmill, and (iii) a 3-minute post-perturbation phase. The entire protocol was presented to each participant at a slow (0.6 m/s), intermediate (1.2 m/s) and fast (1.8 m/s) treadmill speed, with the optic flow speed matching the speed of the treadmill. The order of walking speeds was randomized among the participants. Ground reaction forces were recorded to calculate mediolateral margins of stability [1], and 3D locations of the sacrum and heels were captured to obtain the step length and step width.
Results: Across all three speeds, there was a decrease in step length and an increase in step width and margins of stability during early perturbation (first minute after exposure) compared to baseline. The increase in step width and margins of stability was more pronounced during the slowest and intermediate speeds (Figure 1). Furthermore, all parameters reverted toward their baseline levels with prolonged perturbation. In addition, the increase in step length was less pronounced during slow walking compared to intermediate or fast speeds with prolonged perturbation.
Discussion: The present findings show that the immediate, destabilizing effects of optic flow perturbations during walking, and the adaptation rate after prolonged exposure, depend on gait speed. At the onset of perturbation, the gait of individuals may be more unstable during slow walking [2], and the longer single support time during slow walking [3] makes step width and margins of stability more affected. With prolonged perturbation, the increase in step length at slow speeds is lowest. Lower gait speed is expected to result in shorter step length and lower step frequency [4], which reduces the flexibility and frequency of modifying step length. Furthermore, prolonged exposure to optic flow perturbations may cause sensory reweighting, reducing reliance on visual input [5]. However, it has also been observed that individuals rely more on visual feedback to maintain balance when walking at slow speeds [6]. This may cause gait recalibration to be more challenging at slow walking. Altogether, this should be considered when utilizing this type of perturbation for assessment or training.

References:
[1] T.J.W. Buurke, L. Van De Venis, R. Den Otter, J. Nonnekes, N. Keijsers, Comparison of ground reaction force and marker-based methods to estimate mediolateral center of mass displacement and margins of stability during walking, J. Biomech. 146 (2023) 111415. https://doi.org/10.1016/j.jbiomech.2022.111415.
[2] O. Beauchet, C. Annweiler, Y. Lecordroch, G. Allali, V. Dubost, F.R. Herrmann, R.W. Kressig, Walking speed-related changes in stride time variability: effects of decreased speed, J. NeuroEngineering Rehabil. 6 (2009) 32. https://doi.org/10.1186/1743-0003-6-32.
[3] T.J.W. Buurke, C.J.C. Lamoth, L.H.V. van der Woude, A.L. Hof, R. den Otter, Bilateral temporal control determines mediolateral margins of stability in symmetric and asymmetric human walking, Sci. Rep. 9 (2019) 12494. https://doi.org/10.1038/s41598-019-49033-z.
[4] D.W. Grieve, R.J. Gear, The Relationships Between Length of Stride, Step Frequency, Time of Swing and Speed of Walking for Children and Adults, Ergonomics 9 (1966) 379–399. https://doi.org/10.1080/00140136608964399.
[5] A. Mahboobin, P.J. Loughlin, M.S. Redfern, P.J. Sparto, Sensory re-weighting in human postural control during moving-scene perturbations, Exp. Brain Res. 167 (2005) 260–267. https://doi.org/10.1007/s00221-005-0053-7.
[6] R.F. Reynolds, B.L. Day, Visual guidance of the human foot during a step, J. Physiol. 569 (2005) 677–684. https://doi.org/10.1113/jphysiol.2005.095869.