A novel robot for imposing perturbations during overground walking: mechanism, control and normative stepping responses

Abstract

Background: The most common approach to studying dynamic balance during walking is by applying perturbations. Previous studies that investigated dynamic balance responses predominantly focused on applying perturbations in frontal plane while walking on treadmill. The goal of our work was to develop balance assessment robot (BAR) that can be used during overground walking and to assess normative balance responses to perturbations in transversal plane in a group of neurologically healthy individuals.

Methods: BAR provides three passive degrees of freedom (DoF) and three actuated DoF in pelvis that are admittance-controlled in such a way that the natural movement of pelvis is not significantly affected. In this study BAR was used to assess normative balance responses in neurologically healthy individuals by applying linear perturbations in frontal and sagittal planes and angular perturbations in transversal plane of pelvis. One way repeated measure ANOVA was used to statistically evaluate the effect of selected perturbations on stepping responses.

Results: Standard deviations of assessed responses were similar in unperturbed and perturbed walking. Perturbations in frontal direction evoked substantial pelvis displacement and caused statistically significant effect on step length, step width and step time. Likewise, perturbations in sagittal plane also caused statistically significant effect on step length, step width and step time but with less explicit impact on pelvis movement in frontal plane. On the other hand, except from substantial pelvis rotation angular perturbations did not have substantial effect on pelvis movement in frontal and sagittal planes while statistically significant effect was noted only in step length and step width after perturbation in clockwise direction.

Conclusions: Results indicate that the proposed device can repeatedly reproduce similar experimental conditions. Results also suggest that "stepping strategy" is the dominant strategy for coping with perturbations in frontal plane, perturbations in sagittal plane are to greater extent handled by "ankle strategy" while angular perturbations in transversal plane do not pose substantial challenge for balance. Results also show that specific perturbation in general elicits responses that extend also to other planes of movement that are not directly associated with plane of perturbation as well as to spatio temporal parameters of gait.

Keywords: Balance; Balance assessment robot; Overground walking; Pelvic manipulator; Pelvis perturbations; Rehabilitation robotics.

Fig. 1

BAR - mechanical design and the actual system. Left: Detailed mechanical design of balance assessment robot. Mobile platform (MP): 1 - mobile platform frame, 2 - castor wheels, 3 - drive motors, 4 - batteries, 5 - bumper, 6 - control unit (Beckhoff PLC CX5020, Beckhoff Automation GmbH & Co. KG). Pelvis manipulator (PM): 7 - universal joint, 8 - vertical rod, 9 - spherical joint, 10 - pair of angularly displaced force sensors, 11 - pelvis element (PE), 12 - pelvis brace (PB), 13 - servo motor (Beckhoff AM122-F020, Beckhoff Automation GmbH & Co. KG), 14 - linear bearing (CASM-40-BS-0300AA-000, SKF Actuation Systems). Right: the actual system

Fig. 2

Pelvis DoFs. Schematic representation of available DoFs in pelvis when walking within BAR. From left to right: actuated forward(above)/backward(below) displacement, actuated left(above)/right(below) displacement, actuated CW(above)/CCW(below) rotation, passive anterior(above)/posterior(below) tilt, passive up (above)/down(below) obliquity, passive down(above)/up(below) displacement

Fig. 3

Kinematic framework of BAR. Schematic representation of selected locations on BAS that were used to develop kinematic model of BAR

Fig. 4

Perturbation directions. Schematic representation of perturbation directions with respect to human body

Fig. 5

Perturbation to left direction. Left - pelvis movement and interaction forces/moment associated with actuated DoF in transversal plane for unperturbed and perturbed walking over selected observation interval for single typical subject was averaged across five single responses. Right - graphical illustration of foot placement at left (approximately at 0, 100 and 200 %) and right (approximately at −50, 50, 150 and 250 %) foot strikes for unperturbed and perturbed walking over selected observation interval for a group of subjects was generated from averaged group step lengths and step widths and time-aligned at the onset of perturbation at 0 %

Fig. 6

Stepping responses. Step length, step width and step time responses in unperturbed walking and after selected perturbations in transversal plane. Step length and step width responses correspond to distances between anterior and posterior ankle marker at the time of left (approximately at 0, 100 and 200 %) and right (approximately at 50, 150 and 250 %) foot strikes. Step time responses correspond to time intervals between consecutive foot off and foot strike of the same leg, i.e. left step times (approximately from −50 to 0 %, from 50 to 100 % and from 150 to 200 %) or right step times (approximately from 0 to 50 %, from 100 to 150 % and from 200 to 250 %) feet respectively

Fig. 7

Perturbation to right direction. Left - pelvis movement and interaction forces/moment associated with actuated DoF in transversal plane for unperturbed and perturbed walking over selected observation interval for single typical subject was averaged across five single responses. Right - graphical illustration of foot placement at left (approximately at 0, 100 and 200 %) and right (approximately at −50, 50, 150 and 250 %) foot strikes for unperturbed and perturbed walking over selected observation interval for a group of subjects was generated from averaged group step lengths and step widths and time-aligned at the onset of perturbation at 0 %

Fig. 8

Perturbation to forward direction. Left - pelvis movement and interaction forces/moment associated with actuated DoF in transversal plane for unperturbed and perturbed walking over selected observation interval for single typical subject was averaged across five single responses. Right - graphical illustration of foot placement at left (approximately at 0, 100 and 200 %) and right (approximately at −50, 50, 150 and 250 %) foot strikes for unperturbed and perturbed walking over selected observation interval for a group of subjects was generated from averaged group step lengths and step widths and time-aligned at the onset of perturbation at 0 %

Fig. 9

Perturbation to backward direction. Left - pelvis movement and interaction forces/moment associated with actuated DoF in transversal plane for unperturbed and perturbed walking over selected observation interval for single typical subject was averaged across five single responses. Right - graphical illustration of foot placement at left (approximately at 0, 100 and 200 %) and right (approximately at −50, 50, 150 and 250 %) foot strikes for unperturbed and perturbed walking over selected observation interval for a group of subjects was generated from averaged group step lengths and step widths and time-aligned at the onset of perturbation at 0 %

Fig. 10

Perturbation to clockwise direction. Left - pelvis movement and interaction forces/moment associated with actuated DoF in transversal plane for unperturbed and perturbed walking over selected observation interval for single typical subject was averaged across five single responses. Right - graphical illustration of foot placement at left (approximately at 0, 100 and 200 %) and right (approximately at −50, 50, 150 and 250 %) foot strikes for unperturbed and perturbed walking over selected observation interval for a group of subjects was generated from averaged group step lengths and step widths and time-aligned at the onset of perturbation at 0 %

Fig. 11

Perturbation to counter clockwise direction. Left - pelvis movement and interaction forces/moment associated with actuated DoF in transversal plane for unperturbed and perturbed walking over selected observation interval for single typical subject was averaged across five single responses. Right - graphical illustration of foot placement at left (approximately at 0, 100 and 200 %) and right (approximately at −50, 50, 150 and 250 %) foot strikes for unperturbed and perturbed walking over selected observation interval for a group of subjects was generated from averaged group step lengths and step widths and time-aligned at the onset of perturbation at 0 %