A Subject-Specific Kinematic Model to Predict Human Motion in Exoskeleton-Assisted Gait

Abstract

The relative motion between human and exoskeleton is a crucial factor that has remarkable consequences on the efficiency, reliability and safety of human-robot interaction. Unfortunately, its quantitative assessment has been largely overlooked in the literature. Here, we present a methodology that allows predicting the motion of the human joints from the knowledge of the angular motion of the exoskeleton frame. Our method combines a subject-specific skeletal model with a kinematic model of a lower limb exoskeleton (H2, Technaid), imposing specific kinematic constraints between them. To calibrate the model and validate its ability to predict the relative motion in a subject-specific way, we performed experiments on seven healthy subjects during treadmill walking tasks. We demonstrate a prediction accuracy lower than 3.5° globally, and around 1.5° at the hip level, which represent an improvement up to 66% compared to the traditional approach assuming no relative motion between the user and the exoskeleton.

Keywords: benchmarking; lower limb; rehabilitation; skeletal modeling; walking; wearable robot.

Figure 1

Marker placement and labeling (written informed consent was obtained from the individual for the publication of this image).

Figure 2

Schematic diagram of the Human-Exoskeleton model generation (left), and the resulting model scaled to one real test subject (right).

Figure 3

Schematic diagram of the human and exoskeleton ground-truth joint angles estimation (left) and detail of the markers used for the estimation of the human and exoskeleton ankle flexion (right).

Figure 4

Definition of the knee flexion-extension angles for the human (θ_h) and exoskeleton (θ_e) models.

Figure 5

Schematic diagram of the human and exoskeleton kinematic models and their fixations (left) and Inputs and Outputs of the EIKPE (right).

Figure 6

Schematic diagram of the human joint angles estimation using the EIKPE.

Figure 7

Ground-Truth (blue) and estimates (rigid model in green and the EIKPE in red) of the hip, knee, and ankle flexion-extension angles for representative gait cycles of a test subject.

Figure 8

Box plots of the RMSE and ROME metrics of the angle estimations provided by the rigid model (blue) and the EIKPE (orange).

Figure 9

Reconstructed poses of the human lower limb at a particular phase of the gait cycle with the joint angles of the MOCAP (left), the EIKPE (middle), and rigid model (right).