Non-invasive load measurement in the human tibia via spectral analysis of flexural waves

TL;DR

This paper presents a novel non-invasive method using spectral analysis of flexural waves in the tibia to measure compressive forces, validated with a wearable prototype and high correlation results.

Contribution

The study introduces a new spectral analysis technique for non-invasive tibial load measurement using a wearable system based on flexural wave spectra.

Findings

Spectral peak locations vary linearly with tibial compressive force.

High correlation coefficients (mean r=0.93) between spectral peaks and force in human trials.

Wearable system successfully estimates tibial load in real-world conditions.

Abstract

Forces transmitted by bones are routinely studied in human biomechanics, but it is challenging to measure them non-invasively, especially outside of laboratory settings. We introduce a technique for non-invasive, in vivo measurement of tibial compressive force using flexural waves propagating in the tibia. Modelling the tibia as an axially compressed Euler-Bernoulli beam, we show that tibial flexural waves have load-dependent frequency spectra. Specifically, under physiological conditions, peak locations in the wave acceleration spectra vary linearly with the compressive force on the tibia and may be used as proxies for the compressive force. We test the validity of this technique using a proof-of-concept wearable system that generates flexural waves via a skin-mounted mechanical transducer and measures the spectra of these waves using a skin-mounted accelerometer. In agreement with beam theory, data from 9 participants demonstrate linear relationships between tibial compressive force and spectral peak location, with Pearson correlation coefficients $r=0.82 - 0.99$ (mean $r=0.93$) for medial-lateral swaying and $r=0.81 - 0.98$ (mean $r=0.93$) for walking trials. This flexural wave-based technique could give rise to a new class of wearable sensors for non-invasive physiological bone load monitoring and measurement, impacting research in human locomotion and sports medicine.