Estimating infant upper extremities motion with an RGB-D camera and markerless deep neural network tracking: A validation study.

Quantitative biomarkers of infant motion may be predictive of the development of movement disorders. This study presents and validates a low cost, markerless motion tracking method for the estimation of upper body kinematics of infants from which proper biomarkers may be extracted. The method requires a single RGB-D camera, a 2D motion tracking software publicly available (DeepLabCut) and an algorithm generating 3D point coordinates from the 2D tracked points, dealing with missing data, originating from various sources, for estimating joint kinematics. The proposed method is validated using known point kinematics obtained from a doll, with size and shape of an infant, lying on a turntable rotating at 33⅓ rpm. Two camera image plane orientations are tested: parallel to the turntable motion plane and forming a 45° angle with respect to the motion plane. The latter enhances the occurrence of body parts occlusions during motion as expected in live infant motion recordings. The length of upper body segments, elbow and shoulder joint angles and the linear point velocity determined with the proposed method are evaluated against reference values obtained from the known motion of the turntable. The relevant Mean Absolute Errors (MAE) found indicate the margin of error to expect when processing live infant motion. The proposed method may be improved if enhanced hardware and tracking software are employed, therefore reducing the above-mentioned margin of error. Clinical Relevance - The validation of the proposed method carried out in this study allows clinicians to select proper quantitative biomarkers obtained from infants upper body motion that may be useful for predicting movement disorders.

A wearable multi-sensor system for real world gait analysis.

Gait analysis is commonly performed in standardized environments, but there is a growing interest in assessing gait also in ecological conditions. In this regard, an important limitation is the lack of an accurate mobile gold standard for validating any wearable system, such as continuous monitoring devices mounted on the trunk or wrist. This study therefore deals with the development and validation of a new wearable multi-sensor-based system for digital gait assessment in free-living conditions. In particular, results obtained from five healthy subjects during lab-based and real-world experiments were presented and discussed. The in-lab validation, which assessed the accuracy and reliability of the proposed system, shows median percentage errors smaller than 2% in the estimation of spatio-temporal parameters. The system also proved to be easy to use, comfortable to wear and robust during the out-of-lab acquisitions, showing its feasibility for free-living applications.

Accuracy of the Orientation Estimate Obtained Using Four Sensor Fusion Filters Applied to Recordings of Magneto-Inertial Sensors Moving at Three Rotation Rates.

Magneto-Inertial technology is a well-established alternative to optical motion capture for human motion analysis applications since it allows prolonged monitoring in free-living conditions. Magneto and Inertial Measurement Units (MIMUs) integrate a triaxial accelerometer, a triaxial gyroscope and a triaxial magnetometer in a single and lightweight device. The orientation of the body to which a MIMU is attached can be obtained by combining its sensor readings within a sensor fusion framework. Despite several sensor fusion implementations have been proposed, no well-established conclusion about the accuracy level achievable with MIMUs has been reached yet. The aim of this preliminary study was to perform a direct comparison among four popular sensor fusion algorithms applied to the recordings of MIMUs rotating at three different rotation rates, with the orientation provided by a stereophotogrammetric system used as a reference. A procedure for suboptimal determination of the parameter filter values was also proposed. The findings highlighted that all filters exhibited reasonable accuracy (rms errors <; 6.4°). Moreover, in accordance with previous studies, every algorithm's accuracy worsened as the rotation rate increased. At the highest rotation rate, the algorithm from Sabatini (2011) showed the best performance with errors smaller than 4.1° rms.

Inter-leg Distance Measurement as a Tool for Accurate Step Counting in Patients with Multiple Sclerosis.

Step detection is commonly performed using wearable inertial devices. However, methods based on the extraction of signals features may deteriorate their accuracy when applied to very slow walkers with abnormal gait patterns. The aim of this study is to test and validate an innovative step counter method (DiSC) based on the direct measurement of inter-leg distance. Data were recorded using an innovative wearable system which integrates a magneto-inertial unit and multiple distance sensors (DSs) attached to the shank. The method allowed for the detection of both left and right steps using a single device and was validated on thirteen people affected by multiple sclerosis (0 <; EDSS <; 6.5) while performing a six-minute walking test. Two different measurement ranges for the distance sensor were tested (DS200: 0-200 mm; DS400: 0-400 mm). Accuracy was evaluated by comparing the estimates of the DiSC method against video recordings used as gold standard. Preliminary results showed a good accuracy in detecting steps with half the errors in detecting the step of the instrumented side compared to the non-instrumented (mean absolute percentage error 2.4% vs 4.8% for DS200; mean absolute percentage error 2% vs 5.4% for DS400). When averaging errors across patients, over and under estimation errors were compensated, and very high accuracy was achieved (E%<; 1.2% for DS200; E%<; 0.7% for DS400). DS400 is the suggested configuration for patients walking with a large base of support.

Position and orientation in space of bones during movement: experimental artefacts.

This paper deals with the experimental problems related to the reconstruction of the position and orientation of the lower limb bones in space during the execution of locomotion and physical exercises. The inaccuracies associated with the relative movement between markers and underlying bone are analysed. Quantitative information regarding this movement was collected by making experiments on subjects who had suffered fractures and were wearing either femoral or tibial external fixators. These latter devices provided frames that were reliably rigid with the bone involved, and hence the possibility of assessing the relative movement between markers mounted on the skin and this bone. Anatomical frames associated with thigh and shank were reconstructed using technical frames based on different clusters of skin markers and their rotation with respect to the relevant bone evaluated. Marker movement was also assessed in subjects with intact musculoskeletal structures using digital videofluoroscopy.

Position and orientation in space of bones during movement: anatomical frame definition and determination.

This paper deals with methodological problems related to the reconstruction of the position and orientation of the human pelvis and the lower limb bones in space during the execution of locomotion and physical exercises using a stereophotogrammetric system. The intention is to produce a means of quantitative description of joint kinematics and dynamics for both research and application. Anatomical landmarks and bone-embedded anatomical reference systems are defined. A contribution is given to definition of variables and relevant terminology. The concept of anatomical landmark calibration is introduced and relevant experimental approaches presented. The problem of data sharing is also addressed. This material is submitted to the scientific community for consideration as a basis for standardization. RELEVANCE: In order to make movement analysis effective in the solution of clinical problems, a structured conceptual background is needed in addition to standardized definitions and methods. Technical solutions which make data sharing and relevant data banks possible are also of primary importance. This paper makes suggestions in this context.