Andrea Cereatti

Joint kinematics estimate using wearable inertial and magnetic sensing modules

BACKGROUND AND AIMS In many applications, it is essential that the evaluation of a given motor task is not affected by the restrictions of the laboratory environment. To accomplish this requirement, miniature triaxial inertial and magnetic sensors can be used. This paper describes an anatomical calibration technique for wearable inertial and magnetic sensing modules based on the direct measure of the direction of anatomical axes using palpable anatomical landmarks. An anatomical frame definition for the estimate of joint angular kinematics of the lower limb is also proposed. METHODS The performance of the methodology was evaluated in an upright posture and a walking trial of a single able-bodied subject. The repeatability was assessed with six examiners performing the anatomical calibration, while its consistency was evaluated by comparing the results with those obtained using stereophotogrammetry. RESULTS Results relative to the up-right posture trial revealed an intra- and inter-examiner variability which is minimal in correspondence to the flex-extension angles (0.2-2.9 degrees ) and maximal to the internal-external rotation (1.6-7.3 degrees ). For the level walking, the root mean squared error between the kinematics estimated with the two measurement techniques varied from 2.5% to 4.8% of the range of motion for the flex-extension, whereas it ranged from 13.1% to 41.8% in correspondence of the internal-external rotation. CONCLUSION The proposed methodology allowed for the estimate of lower limb joint angular kinematics in a repeatable and consistent manner, enabling inertial and magnetic sensing based systems to be used especially for outdoor human movement analysis applications.

Estimation of stride length in level walking using an inertial measurement unit attached to the foot: A validation of the zero velocity assumption during stance

In a variety of applications, inertial sensors are used to estimate spatial parameters by double integrating over time their coordinate acceleration components. In human movement applications, the drift inherent to the accelerometer signals is often reduced by exploiting the cyclical nature of gait and under the hypothesis that the velocity of the sensor is zero at some point in stance. In this study, the validity of the latter hypothesis was investigated by determining the minimum velocity of progression of selected points of the foot and shank during the stance phase of the gait cycle while walking at three different speeds on level ground. The errors affecting the accuracy of the stride length estimation resulting from assuming a zero velocity at the beginning of the integration interval were evaluated on twenty healthy subjects. Results showed that the minimum velocity of the selected points on the foot and shank increased as gait speed increased. Whereas the average minimum velocity of the foot locations was lower than 0.011 m/s, the velocity of the shank locations were up to 0.049 m/s corresponding to a percent error of the stride length equal to 3.3%. The preferable foot locations for an inertial sensor resulted to be the calcaneus and the lateral aspect of the rearfoot. In estimating the stride length, the hypothesis that the velocity of the sensor can be set to zero sometimes during stance is acceptable only if the sensor is attached to the foot.

Bilateral step length estimation using a single inertial measurement unit attached to the pelvis

BackgroundThe estimation of the spatio-temporal gait parameters is of primary importance in both physical activity monitoring and clinical contexts. A method for estimating step length bilaterally, during level walking, using a single inertial measurement unit (IMU) attached to the pelvis is proposed. In contrast to previous studies, based either on a simplified representation of the human gait mechanics or on a general linear regressive model, the proposed method estimates the step length directly from the integration of the acceleration along the direction of progression.MethodsThe IMU was placed at pelvis level fixed to the subjects belt on the right side. The method was validated using measurements from a stereo-photogrammetric system as a gold standard on nine subjects walking ten laps along a closed loop track of about 25 m, varying their speed. For each loop, only the IMU data recorded in a 4 m long portion of the track included in the calibrated volume of the SP system, were used for the analysis. The method takes advantage of the cyclic nature of gait and it requires an accurate determination of the foot contact instances. A combination of a Kalman filter and of an optimally filtered direct and reverse integration applied to the IMU signals formed a single novel method (Kalman and Optimally filtered Step length Estimation - KOSE method). A correction of the IMU displacement due to the pelvic rotation occurring in gait was implemented to estimate the step length and the traversed distance.ResultsThe step length was estimated for all subjects with less than 3% error. Traversed distance was assessed with less than 2% error.ConclusionsThe proposed method provided estimates of step length and traversed distance more accurate than any other method applied to measurements obtained from a single IMU that can be found in the literature. In healthy subjects, it is reasonable to expect that, errors in traversed distance estimation during daily monitoring activity would be of the same order of magnitude of those presented.

Hip joint centre location: an ex vivo study.

The human hip joint is normally represented as a spherical hinge and its centre of rotation is used to construct femoral anatomical axes and to calculate hip joint moments. The estimate of the hip joint centre (HJC) position using a functional approach is affected by stereophotogrammetric errors and soft tissue artefacts. The aims of this study were (1) to assess the accuracy with which the HJC position can be located using stereophotogrammetry and (2) to investigate the effects of hip motion amplitude on this accuracy. Experiments were conducted on four adult cadavers. Cortical pins, each equipped with a marker cluster, were implanted in the pelvis and femur, and eight skin markers were attached to the thigh. Recordings were made while an operator rotated the hip joint exploiting the widest possible range of motion. For HJC determination, a proximal and a distal thigh skin marker cluster and two recent analytical methods, the quartic sphere fit (QFS) method and the symmetrical centre of rotation estimation (SCoRE) method, were used. Results showed that, when only stereophotogrammetric errors were taken into account, the analytical methods performed equally well. In presence of soft tissue artefacts, HJC errors highly varied among subjects, methods, and skin marker clusters (between 1.4 and 38.5 mm). As expected, larger errors were found in the subject with larger soft tissue artefacts. The QFS method and the distal cluster performed generally better and showed a mean HJC location accuracy better than 10mm over all subjects. The analysis on the effect of hip movement amplitude revealed that a reduction of the amplitude does not improve the HJC location accuracy despite a decrease of the artefact amplitude.

A spot check for assessing static orientation consistency of inertial and magnetic sensing units

Despite the widespread use of Magnetic and Inertial Measurement Units (MIMUs) for movement reconstruction, only a few studies have tackled issues related to their accuracy. It has been proved that their performance decreases over a period of use since calibration parameters become no longer effective. Good practice rules recommend to assess, prior to any experimental session, the instrumental errors associated to the relevant measures. Aim of this study was to provide a practical and reproducible spot check for assessing the performance of MIMUs in terms of consistency in determining their orientation with respect to a common (inter-MIMUs consistency, IC) and invariant (self-MIMU consistency, SC) global frame. IC was assessed by verifying the hypothesis that the orientation of 9 MIMUs aligned to each other on a rigid Plexiglas plank coincided at any orientation of the plank. SC was assessed separately by verifying differences between measured and imposed known rotations imparted to each MIMU. The orientation of MIMUs relative to the global frame was expressed in terms of quaternion. IC test showed that MIMUs defined their orientation differently. This difference was not constant but varied according to the planks orientation. The least consistent MIMU showed discrepancy up to 5.7°. SC test confirmed the same MIMU as that affected by the highest inaccuracy (8.4°), whereas it revealed errors within limits (1°) in correspondence to other MIMUs. A tool has been proposed that allows the users to be aware of the errors that may be expected when using MIMUs for the estimate of absolute and relative segments kinematics.

Accuracy, sensitivity and robustness of five different methods for the estimation of gait temporal parameters using a single inertial sensor mounted on the lower trunk

In the last decade, various methods for the estimation of gait events and temporal parameters from the acceleration signals of a single inertial measurement unit (IMU) mounted at waist level have been proposed. Despite the growing interest for such methodologies, a thorough comparative analysis of methods with regards to number of extra and missed events, accuracy and robustness to IMU location is still missing in the literature. The aim of this work was to fill this gap. Five methods have been tested on single IMU data acquired from fourteen healthy subjects walking while being recorded by a stereo-photogrammetric system and two force platforms. The sensitivity in detecting initial and final contacts varied between 81% and 100% across methods, whereas the positive predictive values ranged between 94% and 100%. For all tested methods, stride and step time estimates were obtained; three of the selected methods also allowed estimation of stance, swing and double support time. Results showed that the accuracy in estimating step and stride durations was acceptable for all methods. Conversely, a statistical difference was found in the error in estimating stance, swing and double support time, due to the larger errors in the final contact determination. Except for one method, the IMU positioning on the lower trunk did not represent a critical factor for the estimation of gait temporal parameters. Results obtained in this study may not be applicable to pathologic gait.

A Machine Learning Framework for Gait Classification Using Inertial Sensors: Application to Elderly, Post-Stroke and Huntington’s Disease Patients

Machine learning methods have been widely used for gait assessment through the estimation of spatio-temporal parameters. As a further step, the objective of this work is to propose and validate a general probabilistic modeling approach for the classification of different pathological gaits. Specifically, the presented methodology was tested on gait data recorded on two pathological populations (Huntington’s disease and post-stroke subjects) and healthy elderly controls using data from inertial measurement units placed at shank and waist. By extracting features from group-specific Hidden Markov Models (HMMs) and signal information in time and frequency domain, a Support Vector Machines classifier (SVM) was designed and validated. The 90.5% of subjects was assigned to the right group after leave-one-subject–out cross validation and majority voting. The long-term goal we point to is the gait assessment in everyday life to early detect gait alterations.

Metrics for Describing Soft-Tissue Artefact and Its Effect on Pose, Size, and Shape of Marker Clusters

In human movement analysis based on stereophotogrammetry, bone pose is reconstructed by observing a cluster of skin markers. Each marker undergoes a displacement relative to the underlying bone that is regarded as an artefact (soft-tissue artefact, STA) since it affects accuracy in bone pose estimation. This paper proposes a set of metrics for the statistical description of the STA and its effects on cluster pose, size, and shape, with the intent of contributing to a clearer knowledge of its characteristics, and consequently of setting the bases for the development of more accurate bone pose estimators than presently available. Skin marker clusters behave as deformable bodies in motion relative to the underlying bone. Their motion can be described, based on Procrustes analysis, as the composition of four independent transformations: translation and rotation (rigid motion, RM), and change in size and shape (nonrigid motion, NRM). Statistical parameters describing the time histories of both the individual marker STA and the cluster transformations listed earlier were defined. For demonstration purposes, data collected ex vivo were used. The lower limbs of three cadavers were made to undergo movements with prevailing flexion-extension components. Femur pose was accurately measured using pin markers and the movement of twelve thigh skin markers observed relative to it. The STAs of all possible clusters of four skin markers were analysed. RM and NRM exhibited similar magnitudes and therefore impact on bone pose estimation. Thus bone pose estimators should not account for NRM only, as is normally the case, but also for RM.

Is the human acetabulofemoral joint spherical

The human acetabulofemoral joint is commonly modelled as a pure ball-and-socket joint, but there has been no quantitative assessment of this assumption in the literature. Our aim was to test the limits and validity of this hypothesis. We performed experiments on four adult cadavers. Cortical pins, each equipped with a marker cluster, were implanted in the pelvis and the femur. Movements were recorded using stereophotogrammetry while an operator rotated the cadavers acetabulofemoral joint, exploiting the widest possible range of movement. The functional consistency of the acetabulofemoral joint as a pure spherical joint was assessed by comparing the magnitude of the translations of the hip joint centre as obtained on cadavers, with the centre of rotation of two metal segments linked through a perfectly spherical hinge. The results showed that the radii of the spheres containing 95% of the positions of the estimated centres of rotation were separated by less than 1 mm for both the acetabulofemoral joint and the mechanical spherical hinge. Therefore, the acetabulofemoral joint can be modelled as a spherical joint within the considered range of movement (flexion/extension 20 degrees to 70 degrees ; abduction/adduction 0 degrees to 45 degrees ; internal/external rotation 0 degrees to 30 degrees ).

Comparative assessment of different methods for the estimation of gait temporal parameters using a single inertial sensor: application to elderly, post-stroke, Parkinson's disease and Huntington's disease subjects

The estimation of gait temporal parameters with inertial measurement units (IMU) is a research topic of interest in clinical gait analysis. Several methods, based on the use of a single IMU mounted at waist level, have been proposed for the estimate of these parameters showing satisfactory performance when applied to the gait of healthy subjects. However, the above mentioned methods were developed and validated on healthy subjects and their applicability in pathological gait conditions was not systematically explored. We tested the three best performing methods found in a previous comparative study on data acquired from 10 older adults, 10 hemiparetic, 10 Parkinsons disease and 10 Huntingtons disease subjects. An instrumented gait mat was used as gold standard. When pathological populations were analyzed, missed or extra events were found for all methods and a global decrease of their performance was observed to different extents depending on the specific group analyzed. The results revealed that none of the tested methods outperformed the others in terms of accuracy of the gait parameters determination for all the populations except the Parkinsons disease subjects group for which one of the methods performed better than others. The hemiparetic subjects group was the most critical group to analyze (stride duration errors between 4-5 % and step duration errors between 8-13 % of the actual values across methods). Only one method provides estimates of the stance and swing durations which however should be interpreted with caution in pathological populations (stance duration errors between 6-14 %, swing duration errors between 10-32 % of the actual values across populations).