Quantifying lumbar sagittal plane kinematics using a wrist-worn inertial measurement unit

Abstract

Wearable sensors like inertial measurement units (IMUs), and those available as smartphone or smartwatch applications, are increasingly used to quantify lumbar mobility. Currently, wearable sensors have to be placed on the back to measure lumbar mobility, meaning it cannot be used in unsupervised environments. This study aims to compare lumbar sagittal plane angles quantified from a wrist-worn against that of a lumbar-worn sensor. Twenty healthy participants were recruited. An IMU was placed on the right wrist and the L3 spinal level. Participants had to position their right forearm on their abdomen, parallel to the floor. Three sets of three consecutive repetitions of flexion, and extension were formed. Linear mixed models were performed to quantify the effect of region (lumbar vs. wrist) on six outcomes [minimum, maximum, range of motion (ROM) of flexion and extension]. Only flexion ROM was significantly different between the wrist and lumbar sensors, with a mean of 4.54° (95% CI = 1.82°-7.27°). Across all outcomes, the maximal difference between a wrist-worn and lumbar-worn sensor was <8°. A wrist-worn IMU sensor could be used to measure gross lumbar sagittal plane mobility in place of a lumbar-worn IMU. This may be useful for remote monitoring during rehabilitation.

Keywords: biomechanics; mobility; range of motion; spine; wearable sensor.

Figure 1

(A) A posterior view showing the position of the lumbar and pelvis sensors, (B) an anterior view showing the position of the wrist sensor.

Figure 2

Beta coefficients with 95% confidence interval, of the linear mixed models for (A) minimum lumbar flexion angle, (B) maximum lumbar flexion angle, (C) range of motion of lumbar flexion, (D) minimum lumbar extension angle, (E) maximum lumbar extension angle, and (F) range of motion of lumbar extension.