Textile-Integrated Magnetoinductive Waveguides for Low-Complexity, Single-Channel Quantitative Estimation of Isometric Elbow Torque.

Abstract

Wearable monitoring of isometric elbow torque is of great interest for rehabilitation monitoring and muscular disease management, yet remains limited by obtrusiveness, drift, and complexity. We present a wearable sensor that measures transient circumferential deformation of the mid-upper arm as a low-complexity bio-signal for joint torque monitoring. The proposed sensor operates on the basis of magnetoinductive waveguide principles. Participants (n=20) completed three sets of three contractions with sensor replacement between sets. A two-stage analysis extracted contraction-level calibration slopes and characterized variability using a three-level nested mixed effects model. Systematic bias was assessed and leave-one-out cross validation quantified prediction error at the subject, set, and contraction levels. The sensor data had a strong, linear relationship with elbow torque (mean $R^{2}=0.925\pm 0.063$) across the test population. Within-contraction modeling error was $2.5\;Nm$ (5.9%); leave-one-out calibration grouped by set, subject, and population increased error to $5.84\;Nm$ (14.0%), $7.70\;Nm$ (18.4%), and $9.76\;Nm$ (23.4%), respectively, with an opposite trend in calibration burden. No systematic bias was detected across set and contraction order, nor with respect to anthropometric data, supporting the sensor's generalizability. The herein described magnetoinductive waveguide sensor provides a strong, linear predictor of isometric elbow torque with calibration variability emerging primarily due to inter-subject differences that warrant further study. Extension to complex contractions remains critical for future work. The single-channel, low-complexity, textile-integrated modality enabled by transient circumferential deformation sensing creates a promising path towards monitoring of at-home rehabilitation and muscular disease management.