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. 2014 Sep 6:11:133.
doi: 10.1186/1743-0003-11-133.

Infant trunk posture and arm movement assessment using pressure mattress, inertial and magnetic measurement units (IMUs)

Andraž Rihar  1 , Matjaž Mihelj, Jure Pašič, Janko Kolar, Marko Munih
Affiliations

Infant trunk posture and arm movement assessment using pressure mattress, inertial and magnetic measurement units (IMUs)

Andraž Rihar et al. J Neuroeng Rehabil. .

Abstract

Background: Existing motor pattern assessment methods, such as digital cameras and optoelectronic systems, suffer from object obstruction and require complex setups. To overcome these drawbacks, this paper presents a novel approach for biomechanical evaluation of newborn motor skills development. Multi-sensor measurement system comprising pressure mattress and IMUs fixed on trunk and arms is proposed and used as alternative to existing methods. Observed advantages seem appealing for the focused field and in general. Combined use of pressure distribution data and kinematic information is important also for posture assessment, ulcer prevention, and non-invasive sleep pattern analysis of adults.

Methods: Arm kinematic parameters, such as root-mean-square acceleration, spectral arc length of hand velocity profile, including arm workspace surface area, and travelled hand path are obtained with the multi-sensor measurement system and compared to normative motion capture data for evaluation of adequacy. Two IMUs per arm, only one IMU on upper arm, and only one IMU on forearm sensor placement options are studied to assess influence of system configuration on method precision. Combination of pressure mattress and IMU fixed on the trunk is used to measure trunk position (obtained from mat), rotation (from IMUs) and associated movements on surface (from both). Measurement system is first validated on spontaneous arm and trunk movements of a dedicated baby doll having realistic anthropometric characteristics of newborns. Next, parameters of movements in a healthy infant are obtained with pressure mattress, along with trunk and forearm IMU sensors to verify appropriateness of method and parameters.

Results: Evaluation results confirm that full sensor set, comprising pressure mattress and two IMUs per arm is a reliable substitution to optoelectronic systems. Motor pattern parameter errors are under 10% and kinematic estimation error is in range of 2 cm. Although, use of only forearm IMU is not providing best possible kinematic precision, the simplicity of use and still acceptable accuracy are convincing for frequent practical use. Measurements demonstrated system high mobility and usability.

Conclusions: Study results confirm adequacy of the proposed multi-sensor measurement system, indicating its enviable potential for accurate infant trunk posture and arm movement assessment.

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Figures

Figure 1
Figure 1
Experimental setup. Baby doll is positioned on top of two pressure mattresses, and equipped with five IMU bracelets (red ellipses), positioned one on baby doll’s chest, one on each forearm, and one on each upper arm. Ten Optotrak markers (one on baby doll’s forehead, one on each cheek, one on the frontal side of lower abdomen, one on the frontal side of each shoulder joint, one on the lateral side of each elbow joint, one on the dorsal side of each hand) serve as reference (white rectangles). Referential Optotrak and IMU coordinate system orientation is indicated in the lower right corner (white arrows).
Figure 2
Figure 2
Arm kinematics. 2 IMUs per arm (blue lines), only 1 IMU on the upper arm (red lines), and only 1 IMU on the forearm (green lines) sensor placement options are presented. SH, EL, and H represent shoulder, elbow, and hand positions, respectively. φ stands for the elbow flexion angle, lUA and lFA represent upper and forearm segment lengths, while εUA and εFA stand for Euclidean distances of upper and forearm sensor placement simplifications. RUA and RFA indicate upper and forearm coordinate systems.
Figure 3
Figure 3
Pressure data processing. Bias values matrix (a), loaded matrix before (b) and after (c) noise removal, and matrix after interpolation (d). (e) depicts final data processing results with labelled trunk and head imprints, arm orientation (green lines), trunk orientation (red line), C O Pmat (white circle), and shoulder positions (purple circles).
Figure 4
Figure 4
R M S Earm values for various IMU sensor placements, compared to referential optoelectronic motion capture (Optotrak) values. EL2IMUs, H2IMUs, EL1UA, H1UA, EL1FA, and H1FA represent the R M S Earm values of elbow and hand coordinates for the 2 IMUs per arm, 1 IMU on the upper arm, and 1 IMU on the forearm sensor placements, respectively.
Figure 5
Figure 5
Spectral arc length ( SAL ) dependency. x and y axes represent SAL values, calculated from referential motion capture system (Optotrak) and IMU based hand velocity, respectively. S A L G y r o 2 (red circles) and S A L G y r o 1 (green triangles) denote SAL values of hand velocity, determined from angular velocity for 2 IMUs per arm and 1 IMU per forearm sensor placements, respectively. S A L A c c (blue squares) presents SAL results of hand velocity, calculated by integration of forearm IMU acceleration vector. Best fitting ellipses indicate level of linearity. Pearson correlation coefficients R for the three possibilities in relation to referential motion capture system (Optotrak) are presented in top left corner.
Figure 6
Figure 6
Absolute differences of SAL results for referential motion capture system (Optotrak) based hand velocity and various IMU based approaches. First box presents results for hand velocity determination as integral of IMU dynamic acceleration vector (S A L A c c), while second and third box present angular velocity based hand velocity calculation for 1 IMU per forearm (S A L G y r o 1) and 2 IMUs per arm (S A L G y r o 2) sensor placement.
Figure 7
Figure 7
Left arm workspace surface envelope results. Results are presented for referential motion capture system (Opto - black line), 2 IMUs per arm (2IMUs - green patch), 1 IMU per forearm (1FA - red patch), and 1 IMU per upper arm (1UA - blue patch) sensor placements. Patches with alternative, mixed colours represent areas, where results overlap. Right half of the figure presents from top to bottom views on baby doll’s coronal, sagittal, and transverse planes.
Figure 8
Figure 8
Normalized workspace volume (left) and normalized surface area (right) values. Results are presented for referential motion capture system (Opto), 2 IMUs per arm (2IMUs), 1 IMU per upper arm (1UA), and 1 IMU per forearm (1FA) sensor placement.
Figure 9
Figure 9
RMSE values for centre-of-pressure, shoulder and head coordinates. C O Pmat-opto, SH, and HEAD represent RMSE values for centre-of-pressure, shoulder and head coordinates, determined with pressure data processing and with Optotrak.
Figure 10
Figure 10
Right arm workspace surface envelope results for the healthy infant. Left part of the figure presents the diagonal view, while the right half presents views on the infant’s coronal, sagittal, and transverse planes.
Figure 11
Figure 11
Referential video recordings.
See this image and copyright information in PMC

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