Influence of Treadmill Speed and Perturbation Intensity on Selection of Balancing Strategies during Slow Walking Perturbed in the Frontal Plane

Abstract

Background. Common understanding is that adequate foot placement (stepping strategy) is crucial in maintaining stability during walking at normal speed. The aim of this study was to investigate strategies that humans use to cope with lateral perturbations during very slow walking. Methods. Ten healthy individuals underwent an experimental protocol whereby a set of perturbations directed inward (medially to a stance leg) and outward (laterally to a stance leg) of three intensities (F1=5%, F2=10%, and F3=15% of body weight), applied at three instances of a stance phase, were delivered in random order to the pelvis using a balance assessment robot while walking on a treadmill at three walking speeds (S1=0.4, S2=0.6, and S3=0.8 m/s). We analyzed the peak center of mass displacements; step length, step width, and step times; and the lateral component of ground reaction force for perturbations that were delivered at the beginning of the gait cycle. Results. Responses after inward perturbations were similar at all tested speeds and consistently employed stepping strategy that was further facilitated by a shortened stance. Wider and shorter steps were applied with increased perturbation intensity. Responses following outward perturbations were more complex. At S1, hip strategy (impulse-like increase of mediolateral ground reaction force) augmented with ankle strategy (mediolateral shift of the center of pressure) mainly contributed to responses already during the stance phase. The stance duration was significantly longer for all perturbation intensities. At S2, the relative share of hip strategy was reduced while with increased perturbation intensity, stepping strategy was gradually added. The stance duration was significantly longer for F1 and F2. At S3, stepping strategy was mainly used while the duration of stance was similar to the one in unperturbed walking. Responses following both inward and outward perturbations at all speeds were characterized by temporary slowing down movement in a sagittal plane that was more pronounced with increased perturbation intensity. Conclusions. This study provides novel insights into balancing strategies used at slower walking speeds which may be more relevant to understand the challenges of gait stability following perturbations in the frontal plane in clinical populations.

Dynamic Balancing Responses in Unilateral Transtibial Amputees Following Transversal Plane Perturbations During Slow Treadmill Walking Differ Considerably for Amputated and Nonamputated Side

“…Delaying the corrective action resulted in higher COM displacement, which is in agreement with ndings from other perturbation studies conducted in healthy and post-stroke subjects (Haarman et al, 2017;Matjačić et al, 2018. However, when perturbations were directed inward both control and UTA subjects reacted primarily with a stepping response by making the next step wider, regardless whether healthy, nonamputated or amputated leg was in stance phase (Vlutters et al 2018;Bruijn and Dieen, 2018;Matjačić et al, 2019).…”

“…This re ected in modulation of COP and GRF under the stance leg which has already been addressed in the literature (Hof 2007; Matjačić et al, 2019). Acting against perturbation during the stance phase is e cient as it quickly minimizes deviation of COM and restores stability (Hof et al, 2010, Matjačić et al, 2019. Likewise, when perturbed at the beginning of the stance phase on nonamputated side UTA subjects were also able to appropriately modulate COP and GRF under their nonamputated leg.…”

“…Considerably different situation occurs during walking where, depending on when in gait cycle perturbation occurs, balancing responses may depend on the ability of the stance leg to appropriately modulate COP and GRF under the leg in stance -"in-stance" balancing strategy. In particular, this is the case for very slow walking whereas during faster walking appropriate placement of the ensuing stance is dominant -"stepping" balancing strategy (Matjačić et al, 2019).…”

“…Walking speed plays a major role in selection of suitable/appropriate strategy when faced with unexpected loss of balance. Several studies showed that when the walking speed is relatively high (above 0.8 m/s) responses to unexpected perturbation consist primarily in adequate placement of ensuing step(s) (Bruijn and Dieen, 2018;Matjačić et al, 2019). However, when counteracting perturbations at lower walking speeds in-stance strategy plays dominant role (Matjačić et al, 2019).…”

“…However, when counteracting perturbations at lower walking speeds in-stance strategy plays dominant role (Matjačić et al, 2019). In particular, perturbations to outward direction induce movement of COM to lateral direction with respect to the leg in stance which at very slow walking requires well-organized activity of the stance leg muscles to appropriately modulate COP in sagittal and frontal planes and ground reaction forces (GRF) in all three planes of movement (Matjačić et al, 2019;Matjačić et al, 2020). Clearly, loss of ankle efferent and afferent function must have a pronounced effect on the abilities of UTA subjects to negotiate unexpected loss of balance when perturbation commences while being in the stance phase on the amputated side.…”