Short-term Performance-based Error-augmentation versus Error-reduction Robotic Gait Training for Individuals with Chronic Stroke: A Pilot Study

Abstract

The success of locomotion training with robotic exoskeletons requires identifying control algorithms that effectively retrain gait patterns in neurologically impaired individuals. Here we report how the two training paradigms, performance-based error-augmentation versus error-reduction, modified walking patterns in four chronic post-stroke individuals as a proof-of-concept for future locomotion training following stroke. Stroke subjects were instructed to match a prescribed walking pattern template derived from neurologically intact individuals. Target templates based on the spatial paths of lateral ankle malleolus positions during walking were created for each subject. Robotic forces were applied that either decreased (error-reduction) or increased (error-augmentation) the deviation between subjects' instantaneous malleolus positions and their target template. Subjects' performance was quantified by the amount of deviation between their actual and target malleolus paths. After the error-reduction training, S1 showed a malleolus path with reduced deviation from the target template by 16%. In contrast, S4 had a malleolus path further away from the template with increased deviation by 12%. After the error-augmentation training, S2 had a malleolus path greatly approximating the template with reduced deviation by 58% whereas S3 walked with higher steps than his baseline with increased deviation by 37%. These findings suggest that an error-reduction force field has minimal effects on modifying subject's gait patterns whereas an error-augmentation force field may promote a malleolus path either approximating or exceeding the target walking template. Future investigation will need to evaluate the long-term training effects on over-ground walking and functional capacity.

Keywords: Force field; Gait rehabilitation; Rehabilitation robotics; Stroke; Walking.

Figure 1

ALEX II (Second version of the Active Leg EXoskeleton).

Figure 2

Malleolus path template and the performance-based force fields. d is the distance between the subject's current lateral malleolus position (P) and the nearest point on the target template (N). D₀ is defined as the width of the virtual wall. The normal forces (F_n) were produced only when the distance between P and N exceeded D₀. The tangential forces (F_t) were minimal, designed to ensure that the subjects' leg produced continuous movement along the malleolus path. When the deviation of the subject's instantaneous malleolus position (P) from the target template exceeds D₀, the performance-based error-augmentation algorithm would take the subject's leg further away from the target (N) (see F_n in red dashed line) whereas the performance-based error-reduction algorithm would bring the subject's leg towards the target (N) (see F_n in solid black line).

Figure 3

The training protocol included four 6-bout training sessions on four consecutive days (Days 1-4) and a 1-bout training session two days later (Day 5). Real-time visual display of the subject's instantaneous malleolus positions and target malleolus path was provided for a total of five bouts. For all evaluation trials (not shown here), no force field or visual display of malleolus paths was provided to the subjects. Subjects walked with the exoskeleton in zero-torque mode for the evaluation bouts but walked with a performance-based force field for the training bouts.

Figure 4

(A) S1's baseline malleolus paths and prescribed target template, (B) hip and knee joint kinematics, and (C) area between actual and prescribed malleolus paths. (A) S1's baseline malleolus paths are shown for Day One (red solid line) and Day Five (red dashed line). The black dashed line represents the prescribed target template of S1. (B) S1's hip and knee joint kinematics over the gait cycle are shown for Day One (red solid line) and Day Five (red dashed line). (C) Mean data ± 1 standard deviation across strides are shown for the evaluation bout before (Baseline, B) and after the training (Post-test, P) on each day. The white region represents Area Above whereas the grey region represents Area Below. A smaller Total Area indicates less deviation from the target template, and vice versa.

Figure 5

(A) S4's baseline malleolus paths and prescribed target template, (B) hip and knee joint kinematics, and (C) area between actual and prescribed malleolus paths.

Figure 6

(A) S2's baseline malleolus paths and prescribed target template, (B) hip and knee joint kinematics, and (C) area between actual and prescribed malleolus paths.

Figure 7

(A) S3's baseline malleolus paths and prescribed target template, (B) hip and knee joint kinematics, and (C) area between actual and prescribed malleolus paths.