Interpretable Force Perturbations Promote Gait Variability without Affecting Perceived Exoskeleton Transparency

Abstract

Force perturbations during gait training can increase movement variability, which may support motor exploration and learning. However, when such perturbations are delivered through a robotic exoskeleton, they can also reduce perceived exoskeleton transparency, potentially hindering user acceptance. We tested whether visualizing continuous upper leg-level perturbations in immersive virtual reality (VR) could preserve their variability-enhancing effect while mitigating the cost to perceived transparency in a pelvis-centered walking task. Thirty healthy adults walked on a treadmill while wearing a robotic exoskeleton and performed a ball-in-cup task, requiring continuous mediolateral control of the pelvis. Participants trained under one of three conditions: Control (no perturbations), Perturbation (continuous noise-like multisine forces applied at the thighs), or Perturbation + Visual (same forces with a real-time, body-referenced force-beam visualization). Step-width variability was evaluated during Training. Task performance, intrinsic motivation, and perceived transparency were assessed across Baseline, Training, Retention, and a faster-speed Transfer test (120% of preferred speed). Both perturbation conditions significantly increased step-width variability during Training relative to Control, with no detectable difference between Perturbation and Perturbation + Visual. Task performance improved from Baseline to Retention and Transfer across all conditions, with no significant differences across conditions. Motivation did not differ between conditions either. Critically, perceived transparency decreased only in the non-visualized Perturbation condition and remained stable in both Control and Perturbation + Visual. Our results show that continuous leg-level perturbations reliably enrich lateral gait variability and that simple visual force cues can prevent a perceived transparency cost without compromising the variability manipulation. Future work should test adaptive dosing, multi-session training, and clinical cohorts with impaired lateral stability.