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 (F ₁ = 5%, F ₂ = 10%, and F ₃ = 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 (S ₁ = 0.4, S ₂ = 0.6, and S ₃ = 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 S ₁, 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 S ₂, 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 F ₁ and F ₂. At S ₃, 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.

Figure 1

Photo of a subject walking on an instrumented treadmill while being embraced by the BAR-TM perturbing device; projection on the wall shows the middle of the BAR-TM working space as well as the current position and orientation of the pelvis in a transverse plane—the subjects were instructed to return to the middle of the BAR-TM working space after they rejected perturbation (a). Top view illustration of perturbation directions: outward RR: perturbation to the right triggered at right-foot contact; inward RL: perturbation to the left triggered at right-foot contact; outward LL: perturbation to the left triggered at left-foot contact; inward LR: perturbation to the right triggered at left-foot contact (b).

Figure 2

Schematic diagram of the experimental protocol.

Figure 3

Kinematics and kinetics of balancing responses following outward RR perturbation assessed in a representative subject. The first row shows the trajectories of COP_ML (solid lines) and COM_ML (dotted lines), while the second row shows GRF_ML trajectories. The third row shows COP_AP (solid lines) and COM_AP (dotted lines) trajectories, while the fourth row shows GRF_AP trajectories. Each graph contains responses to all three perturbation intensities along with the trajectories assessed during the unperturbed walking sessions. The left, middle, and right columns show the balancing responses at speeds S₁, S₂, and S₃, respectively. Half a stride prior to and one and a half strides following the perturbation commencement are shown. A stride is defined as the period between two consecutive right-foot contacts. The trajectories displayed show mean values of seven balancing responses.

Figure 4

Kinematics and kinetics of balancing responses following inward RL perturbation assessed in a representative subject. The first row shows the trajectories of COP_ML (solid lines) and COM_ML (dotted lines), while the second row shows GRF_ML trajectories. The third row shows COP_AP (solid lines) and COM_AP (dotted lines) trajectories, while the fourth row shows GRF_AP trajectories. Each graph contains responses to all three perturbation intensities along with the trajectories assessed during the unperturbed walking sessions. The left, middle, and right columns show the balancing responses at speeds S₁, S₂, and S₃, respectively. Half a stride prior to and one and a half strides following the perturbation commencement are shown. A stride is defined as the period between two consecutive right-foot contacts. The trajectories displayed show mean values of seven balancing responses.

Figure 5

Group average (±standard deviation) of peak COM_ML and COM_AP excursions across the three walking speeds during unperturbed walking and perturbed walking is shown for outward RR (a) and inward RL (b) perturbations along with the p values of 2-way rmANOVA. Asterisks (^∗) indicate significant difference from unperturbed walking in Bonferroni post hoc pairwise comparisons (p < 0.016).

Figure 6

Group average (±standard deviation) of step lengths, step widths, and step times across the three walking speeds during unperturbed walking in unperturbed walking session (UWS) and during unperturbed walking in perturbed walking sessions (PWS) is shown along with the p values of 2-way rmANOVA. Asterisks (^∗) indicate significant difference from unperturbed walking in post hoc pairwise comparisons (p < 0.05).

Figure 7

Group average (±standard deviation) of step lengths, step widths, and step times across the three walking speeds during unperturbed walking and perturbed walking is shown for outward RR (a) and inward RL (b) perturbations along with the p values of 2-way rmANOVA. Asterisks (^∗) indicate significant difference from unperturbed walking in Bonferroni post hoc pairwise comparisons (p < 0.016).

Figure 8

Group average (±standard deviation) of integrals of GRF_ML over the “in-stance” and “stepping” periods across the three walking speeds during unperturbed walking and perturbed walking is shown for outward RR (a) and inward RL (b) perturbations along with the p values of 2-way rmANOVA. Asterisks (^∗) indicate significant difference from unperturbed walking in Bonferroni post hoc pairwise comparisons (p < 0.016).