Design of a wearable perturbator for human knee impedance estimation during gait

Abstract

Abstract-Mechanical impedance modulation is the key to natural, stable and efficient human locomotion. An improved understanding of this mechanism is necessary for the development of the next generation of intelligent prosthetic and orthotic devices. This paper documents the design methodologies that were employed to realize a knee perturbator that can experimentally estimate human knee impedance during gait through the application of angular velocity perturbations. The proposed experiment requires a light, transparent, wearable, and remotely actuated device that closely follows the movement of the biological joint. A genetic algorithm was used to design a polycentric hinge whose instantaneous center of rotation is optimized to be kinematically compatible with the human knee. A wafer disc clutch was designed to switch between a high transparency passive mode and a high impedance actuated mode. A remote actuation and transmission scheme was designed to enable high power output perturbations while minimizing the device's mass. Position and torque sensors were designed for device control and to provide data for post-processing and joint impedance estimation. Pending the fabrication and mechanical testing of the device, we expect this knee perturbator to be a valuable tool for experimental investigation of locomotive joint impedance modulation.

A Perturbation Mechanism for Investigations of Phase-Dependent Behavior in Human Locomotion

“…Perturbation studies often consist of unexpectedly disturbing one or more leg joints during walking, thus disrupting specific periods of the gait cycle. There are different biomechanical properties that can be studied from such experiments (e.g., joint impedances [21]–[25], gait cycle phase [6], [26], etc.). The type of perturbation used in a particular experiment is highly dependent on the biomechanical property that needs to be studied.…”

“…As an example, if the impedance of a particular joint is to be studied experimentally then joint-level perturbations are carefully designed for the joint [22]–[25]. However, if a hypothesis regarding the overall gait cycle is being studied (e.g., behavior of humans to slipping conditions), then a multi-joint perturbation might be needed [3], [4], [27], [28].…”

An adaptive and robust online method to predict gait events

“…In combination, stride duration and toe off estimation yield swing and stance phase duration. This was employed in the gait exploration study [3] to target swing phase specifically.…”

“…However, abnormally long or short strides could drastically affect the estimates, since they are typically not rejected. Although they are generally rare, it is especially important to consider this, when they are induced as part of an experimental protocol, e. g., in experiments involving perturbations to the subject's gait [3]. In the course of such experiments, irregularities are expected to be present during perturbations, and while the subject recovers.…”

“…Exact timing is also needed for the exploration of human walking by applying well-timed perturbations to the leg [3]. For these applications, it is common that a constant estimate of stride duration -derived by averaging the stride duration over a period of steadystate walking [4], [5] -is used as a reference to control intervention timing.…”