Abstract
Soft tissue artefacts (STAs) undermine the validity of skin-mounted approaches to measure skeletal kinematics. Magneto-inertial measurement units (MIMU) gained popularity due to their low cost and ease of use. Although the reliability of different protocols for marker-based joint kinematics estimation has been widely reported, there are still no indications on where to place MIMU to minimize STA. This study aims to find the most stable positions for MIMU placement, among four positions on the thigh, four on the shank, and three on the foot. Stability was investigated by measuring MIMU movements against an anatomical reference frame, defined according to a standard marker-based approach. To this aim, markers were attached both on the case of each MIMU (technical frame) and on bony landmarks (anatomical frame). For each MIMU, the nine angles between each versor of the technical frame with each versor of the corresponding anatomical frame were computed. The maximum standard deviation of these angles was assumed as the instability index of MIMU-body coupling. Six healthy subjects were asked to perform barefoot gait, step negotiation, and sit-to-stand. Results showed that (1) in the thigh, the frontal position was the most stable in all tasks, especially in gait; (2) in the shank, the proximal position is the least stable, (3) lateral or medial calcaneus and foot dorsum positions showed equivalent stability performances. Further studies should be done before generalizing these conclusions to different motor tasks and MIMU-body fixation methods. The above results are of interest for both MIMU-based gait analysis and rehabilitation approaches using wearable sensors-based biofeedback.
Highlights
Joint kinematics can be Magnetic and inertial measurement units (MIMU) Stability on Lower Limbs estimated by adopting either optical motion capture systems or wearable systems such as MIMU: in the former case, the position of reflective markers placed on bony landmarks is detected by stereophotogrammetric systems (Lencioni et al, 2019); in the latter MIMU measurements of acceleration, angular velocity and local magnetic field are fused together in order to obtain the estimation of the unit orientation (Sabatini, 2011; Bergamini et al, 2014; Caruso et al, 2020)
To track MIMU movements, on each considered anatomical segment, three not aligned hemispheric markers were attached on each MIMU case, defining an orthogonal technical reference frame whose axes are approximately aligned with the axes of the anatomical reference frame
The Instability Index InI ranged from 0.3° to 12.0°
Summary
Magnetic and inertial measurement units (MIMU) are widely adopted in human movement analysis due to their low cost, high miniaturization, power efficiency (Fong and Chan, 2010; Cuesta-Vargas et al, 2010; Tao et al, 2012), and their capacity to perform real-time analysis both in the laboratory and in real-world scenarios (Benson et al, 2018).In the movement analysis field, joint angular kinematics represent key determinants for the discrimination between normal and pathological gait (Saunders et al, 1953; Perry, 1992) and for the clinical decision making process in rehabilitation (Ferrarin et al, 2015). Joint kinematics can be MIMU Stability on Lower Limbs estimated by adopting either optical motion capture systems or wearable systems such as MIMU: in the former case, the position of reflective markers placed on bony landmarks is detected by stereophotogrammetric systems (Lencioni et al, 2019); in the latter MIMU measurements of acceleration, angular velocity and local magnetic field are fused together in order to obtain the estimation of the unit orientation (Sabatini, 2011; Bergamini et al, 2014; Caruso et al, 2020). Sensor drift, magnetic interference, and, as with all methods based on skin-attached devices, the soft tissue artefacts are the most relevant in human motion applications (Hughes et al, 2021) and represent the major determinants in the orientation estimation errors (Cereatti et al, 2015)
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