Systematic Evaluation of Hip Exoskeleton Assistance Parameters for Enhancing Gait Stability During Ground Slip Perturbations

TL;DR

Falls are a major risk for older adults, and active hip exoskeletons offer a path to enhance gait stability beyond purely energetic optimization. This study systematically varied hip exoskeleton torque magnitude $T_{ ext{max}}$ and duration $D$ during sagittal-plane ground-slip perturbations in eight adults, using whole-body angular momentum $WBAM$ as the primary stability metric and comparing to an energetics-based baseline controller. A significant $T_{ ext{max}} imes D$ interaction emerged, with longer-duration torque generally stabilizing and high-magnitude, short-duration inputs potentially destabilizing; the best parameters achieved a $WBAM$ range reduction of 27.4±9.8% versus no-exoskeleton and 25.7±11.4% versus the baseline controller, albeit with substantial inter-subject variability. The results demonstrate that stability-focused exoskeleton control must emphasize temporal parameterization and user-specific personalization, advancing toward adaptive, real-time fall-prevention devices for real-world locomotion.

Abstract

Falls are the leading cause of injury related hospitalization and mortality among older adults. Consequently, mitigating age-related declines in gait stability and reducing fall risk during walking is a critical goal for assistive devices. Lower-limb exoskeletons have the potential to support users in maintaining stability during walking. However, most exoskeleton controllers are optimized to reduce the energetic cost of walking rather than to improve stability. While some studies report stability benefits with assistance, the effects of specific parameters, such as assistance magnitude and duration, remain unexplored. To address this gap, we systematically modulated the magnitude and duration of torque provided by a bilateral hip exoskeleton during slip perturbations in eight healthy adults, quantifying stability using whole-body angular momentum (WBAM). WBAM responses were governed by a significant interaction between assistance magnitude and duration, with duration determining whether exoskeleton assistance was stabilizing or destabilizing relative to not wearing the exoskeleton device. Compared to an existing energy-optimized controller, experimentally identified stability-optimal parameters reduced WBAM range by 25.7% on average. Notably, substantial inter-subject variability was observed in the parameter combinations that minimized WBAM during perturbations. We found that optimizing exoskeleton assistance for energetic outcomes alone is insufficient for improving reactive stability during gait perturbations. Stability-focused exoskeleton control should prioritize temporal assistance parameters and include user-specific personalization. This study represents an important step toward personalized, stability-focused exoskeleton control, with direct implications for improving stability and reducing fall risk in older adults.

Figures (6)

• Figure 1: Robotic hip exoskeleton designed to assist the user’s hip flexion and extension during locomotion. (A) Actuators located at each hip joint are controlled by an onboard microprocessor. Adjustable orthotic shells and pelvic strapping secure the device to the user. (B) A trapezoidal hip extension torque profile was applied at the hip. Five magnitude and five duration values were chosen to evaluate the assistance parameter space. Informed consent for publication of this image was obtained from the participant.
• Figure 2: Experimental setup, protocol, and representative data. (A) Subjects experienced anteroposterior slip perturbations during treadmill walking while wearing the robotic exoskeleton. Anticipatory cues were minimized using noise-canceling headphones and visual-obscuring glasses. (B) The experimental design comprised 28 different assistance conditions, repeated across four sessions. (C) Treadmill belt velocity profile used to elicit the slip response. (D) Time-series data from a representative trial illustrating biological hip torque (blue), exoskeleton assistance (red), and the resulting WBAM (green), relative to the perturbation duration (gray area). Informed consent for publication of this image was obtained from the participants.
• Figure 3: WBAM across protocol repetitions. The repeated-measures ANOVA indicated that no significant effect of session repetition on the percent change in WBAM range. Black dots indicate the the group mean while gray dots represent individual subject data points.
• Figure 4: Influence of assistance duration and magnitude on gait stability. Decreases in WBAM range indicate improved stability. (A) Effect of assistance duration at fixed assistance magnitudes. (B) Effect of assistance duration aggregated across all magnitudes. Large gray dots represent group means and small dots indicate subject-specific averages. All data are normalized to no-exoskeleton condition. The dashed red line shows the mean WBAM range for the baseline controller condition, and shaded regions indicate 95% confidence intervals. $\beta_{1}$ represents the slope of the line. Significance levels are indicated as: *$p$$\leq$0.01.
• Figure 5: Average percent change in WBAM compared across conditions using a within-subject design. The best-performing parameter set represents the parameter set that produced the lowest percent change in WBAM range for each subject, averaged across subjects. Significance levels are denoted as: * $p$$\leq$0.05, ** $p$$\leq$0.01.