Quantifying changes in individual-specific center-of-mass dynamics during walking with ankle exoskeletons using Hybrid-SINDy

TLDR: In this article , an equation-free data-driven method for inferring sparse hybrid dynamics from a library of candidate functional forms was proposed to identify optimal sets of physically interpretable mechanisms describing COM dynamics, termed template signatures.

Abstract: Ankle exoskeletons alter whole-body walking mechanics, energetics, and stability. Controlling the dynamics governing center-of-mass (COM) motion is critical for maintaining efficient and stable gait; these dynamics are altered following neurological injury. However, how COM dynamics change with ankle exoskeletons is unknown, and how to optimally model individual-specific COM dynamics is unclear. Here, we evaluated individual-specific changes in COM dynamics in unimpaired adults and a case study of one individual with post-stroke hemiparesis while walking in shoes-only and with zero-stiffness and high-stiffness passive ankle exoskeletons. To identify optimal sets of physically interpretable mechanisms describing COM dynamics, termed template signatures, we leveraged hybrid sparse identification of nonlinear dynamics (Hybrid-SINDy): an equation-free data-driven method for inferring sparse hybrid dynamics from a library of candidate functional forms. In unimpaired adults, Hybrid-SINDy automatically identified spring-loaded inverted pendulum-like template signatures, which did not change with zero- or high-stiffness exoskeletons (p>0.13). Conversely, post-stroke paretic leg and rotary stiffness mechanisms increased by 11% with zero- and high-stiffness exoskeletons, respectively. While unimpaired COM dynamics appear robust to passive ankle exoskeletons, how neurological injuries affect changes in COM dynamics with exoskeletons merits further investigation. Our findings also support Hybrid-SINDy’s potential to discover mechanisms describing individual-specific COM dynamics with assistive devices.