Soft tissue displacement over pelvic anatomical landmarks during 3-D hip movements.

The position, in a pelvis-embedded anatomical coordinate system, of skin points located over the following anatomical landmarks (AL) was determined while the hip assumed different spatial postures: right and left anterior superior and posterior superior iliac spines, and the sacrum. Postures were selected as occurring during walking and during a flexion-extension and circumduction movement, as used to determine the hip joint centre position (star-arc movement). Five volunteers, characterised by a wide range of body mass indices (22-37), were investigated. Subject-specific MRI pelvis digital bone models were obtained. For each posture, the pose of the pelvis-embedded anatomical coordinate system was determined by registering this bone model with points digitised over bony prominences of the pelvis, using a wand carrying a marker-cluster and stereophotogrammetry. The knowledge of how the position of the skin points varies as a function of the hip posture provided information regarding the soft tissue artefact (STA) that would affect skin markers located over those points during stereophotogrammetric movement analysis. The STA was described in terms of amplitude (relative to the position of the AL during an orthostatic posture), diameter (distance between the positions of the AL which were farthest away from each other), and pelvis orientation. The STA amplitude, exhibited, over all postures, a median [inter-quartile] value of 9[6] and 16[11]mm, for normal and overweight volunteers, respectively. STA diameters were larger for the star-arc than for the walking postures, and the direction was predominantly upwards. Consequent errors in pelvic orientation were in the range 1-9 and 4-11 degrees, for the two groups respectively.

A constrained extended Kalman filter for the optimal estimate of kinematics and kinetics of a sagittal symmetric exercise.

This paper presents a method for real-time estimation of the kinematics and kinetics of a human body performing a sagittal symmetric motor task, which would minimize the impact of the stereophotogrammetric soft tissue artefacts (STA). The method is based on a bi-dimensional mechanical model of the locomotor apparatus the state variables of which (joint angles, velocities and accelerations, and the segments lengths and inertial parameters) are estimated by a constrained extended Kalman filter (CEKF) that fuses input information made of both stereophotogrammetric and dynamometric measurement data. Filter gains are made to saturate in order to obtain plausible state variables and the measurement covariance matrix of the filter accounts for the expected STA maximal amplitudes. We hypothesised that the ensemble of constraints and input redundant information would allow the method to attenuate the STA propagation to the end results. The method was evaluated in ten human subjects performing a squat exercise. The CEKF estimated and measured skin marker trajectories exhibited a RMS difference lower than 4mm, thus in the range of STAs. The RMS differences between the measured ground reaction force and moment and those estimated using the proposed method (9N and 10Nm) were much lower than obtained using a classical inverse dynamics approach (22N and 30Nm). From the latter results it may be inferred that the presented method allows for a significant improvement of the accuracy with which kinematic variables and relevant time derivatives, model parameters and, therefore, intersegmental moments are estimated.

Rigid and non-rigid geometrical transformations of a marker-cluster and their impact on bone-pose estimation.

When stereophotogrammetry and skin-markers are used, bone-pose estimation is jeopardised by the soft tissue artefact (STA). At marker-cluster level, this can be represented using a modal series of rigid (RT; translation and rotation) and non-rigid (NRT; homothety and scaling) geometrical transformations. The NRT has been found to be smaller than the RT and claimed to have a limited impact on bone-pose estimation. This study aims to investigate this matter and comparatively assessing the propagation of both STA components to bone-pose estimate, using different numbers of markers. Twelve skin-markers distributed over the anterior aspect of a thigh were considered and STA time functions were generated for each of them, as plausibly occurs during walking, using an ad hoc model and represented through the geometrical transformations. Using marker-clusters made of four to 12 markers affected by these STAs, and a Procrustes superimposition approach, bone-pose and the relevant accuracy were estimated. This was done also for a selected four marker-cluster affected by STAs randomly simulated by modifying the original STA NRT component, so that its energy fell in the range 30-90% of total STA energy. The pose error, which slightly decreased while increasing the number of markers in the marker-cluster, was independent from the NRT amplitude, and was always null when the RT component was removed. It was thus demonstrated that only the RT component impacts pose estimation accuracy and should thus be accounted for when designing algorithms aimed at compensating for STA.

A model of the soft tissue artefact rigid component.

When using stereophotogrammetry and skin-markers, the reconstruction of skeletal movement is affected by soft-tissue artefact (STA). This may be described by considering a marker-cluster as a deformable shape undergoing a geometric transformation formed by a non-rigid (change in size and shape) and a rigid component (translation and rotation displacements). A modal decomposition of the STA, relative to an appropriately identified basis, allows the separation of these components. This study proposes a mathematical model of the STA that embeds only its rigid component and estimates the relevant six mode amplitudes as linear functions of selected proximal and distal joint rotations during the analysed task. This model was successfully calibrated for thigh and shank using simultaneously recorded pin- and skin-marker data of running volunteers. The root mean square difference between measured and model-estimated STA rigid component was 1.1(0.8)mm (median (inter-quartile range) over 3 subjects × 5 trials × 33 markers coordinates), and it was mostly due to the wobbling not included in the model. Knee joint kinematics was estimated using reference pin-marker data and skin-marker data, both raw and compensated with the model-estimated STA. STA compensation decreased inaccuracy on average from 6% to 1% for flexion/extension, from 43% to 18% for the other two rotations, and from 69% to 25% for the linear displacements. Thus, the proposed mathematical model provides an STA estimate which can be effectively used within optimal bone pose and joint kinematics estimators for artefact compensation, and for simulations aimed at their comparative assessments.

Assessment of waveform similarity in clinical gait data: the linear fit method.

The assessment of waveform similarity is a crucial issue in gait analysis for the comparison of kinematic or kinetic patterns with reference data. A typical scenario is in fact the comparison of a patient's gait pattern with a relevant physiological pattern. This study aims to propose and validate a simple method for the assessment of waveform similarity in terms of shape, amplitude, and offset. The method relies on the interpretation of these three parameters, obtained through a linear fit applied to the two data sets under comparison plotted one against the other after time normalization. The validity of this linear fit method was tested in terms of appropriateness (comparing real gait data of 34 patients with cerebrovascular accident with those of 15 healthy subjects), reliability, sensitivity, and specificity (applying a cluster analysis on the real data). Results showed for this method good appropriateness, 94.1% of sensitivity, 93.3% of specificity, and good reliability. The LFM resulted in a simple method suitable for analysing the waveform similarity in clinical gait analysis.

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.

Generalized mathematical representation of the soft tissue artefact.

While reconstructing skeletal movement using stereophotogrammetry, the relative movement between a skin marker and the underlying bone is regarded as an artefact (soft tissue artefact: STA). Similarly, the consequent pose, size and shape variations that affect a cluster of markers associated with a bony segment, or any arbitrary change of configuration in the marker local positions as representative of the skin envelope shape variation, may also be looked upon as an STA. Bone pose estimators able to compensate for these artefacts must embed relevant a priori knowledge in the form of an STA mathematical model. Prior to tackling this modeling exercise, an appropriate definition and mathematical representation of the STA time histories must be accomplished. Relevant appropriateness is based on the degree of approximation of the STA reconstruction and on the number of parameters involved. The objective of this study was to propose a generalized mathematical representation of the STA which would be applicable for most plausible definitions of it. To this purpose, a modal approach was used that, most importantly, allows for the splitting of a given STA into additive components (modes). For each STA definition, these modes may be ranked according to the contribution that each of them gives to the reconstruction of the STA. In this way, the STA definition leading to the minimum number of modes, and, therefore, of parameters, that provides an adequate approximation for further purposes can be selected, allowing a trade-off between complexity and effectiveness of the STA model. Using information available in the literature and data provided by an ex-vivo experiment, it is shown that the modes corresponding to the different STA definitions (individual marker displacements, marker-cluster geometrical transformations, and skin envelope shape variations) can be ranked and selected leading, respectively, to a large, moderate or low number of parameters embedded in the STA mathematical representation.

Tibio-femoral joint constraints for bone pose estimation during movement using multi-body optimization.

When using skin markers and stereophotogrammetry for movement analysis, bone pose estimation may be performed using multi-body optimization with the intent of reducing the effect of soft tissue artefacts. When the joint of interest is the knee, improvement of this approach requires defining subject-specific relevant kinematic constraints. The aim of this work was to provide these constraints in the form of plausible values for the distances between origin and insertion of the main ligaments (ligament lengths), during loaded healthy knee flexion, taking into account the indeterminacies associated with landmark identification during anatomical calibration. Ligament attachment sites were identified through virtual palpation on digital bone templates. Attachments sites were estimated for six knee specimens by matching the femur and tibia templates to low-dose stereoradiography images. Movement data were obtained using stereophotogrammetry and pin markers. Relevant ligament lengths for the anterior and posterior cruciate, lateral collateral, and deep and superficial bundles of the medial collateral ligaments (ACL, PCL, LCL, MCLdeep, MCLsup) were calculated. The effect of landmark identification variability was evaluated performing a Monte Carlo simulation on the coordinates of the origin-insertion centroids. The ACL and LCL lengths were found to decrease, and the MCLdeep length to increase significantly during flexion, while variations in PCL and MCLsup length was concealed by the experimental indeterminacy. An analytical model is given that provides subject-specific plausible ligament length variations as functions of the knee flexion angle and that can be incorporated in a multi-body optimization procedure.

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 cadaver's 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 ).

Control of the upper body movements during level walking in patients with facioscapulohumeral dystrophy.

Facioscapulohumeral dystrophy (FSHD) is a muscular disease usually spreading from upper to lower body and characterised by asymmetric muscle weakness. Walking ability is compromised in these patients, with a consequent high risk of falls. A quantitative analysis of the upper body oscillations may unveil useful information about the capacity of these patients to stabilise the head, maintain balance, and compensate for lower limb muscle weakness during walking. This study involved 13 patients with FSHD and 13 healthy volunteers. The trajectories of three points located on the cranio-caudal axis, at head, shoulder, and pelvis levels, during level walking, were analysed. The range of motion of these three points and the attenuation of the relevant accelerations going from pelvis to head level were used to describe the upper body movements during walking. The patients had wider and less symmetrical oscillations than the healthy controls both in antero-posterior and medio-lateral directions. Furthermore, the capacity of the patients to attenuate the accelerations going from pelvis to head level was reduced. These features may be related not only to upper body muscle weakness, but also to a strategy functional to the compensation of proximal leg muscle weakness. In conclusion, this study highlighted that the control of upper body oscillations and of head stability is reduced in patients with FSHD, suggesting that the assessment of the upper body movements should be included in the treatment decision process.

Should the instructions issued to the subject in traditional static posturography be standardised?

The postural ability of a subject is usually evaluated through the observation of the centre of pressure parameters obtained through posturography. These parameters are known to be sensitive to various factors and standards have consequently been proposed for data acquisition and analysis. A factor usually not taken into due consideration but likely to influence the postural exam is the kind of standing posture (e.g. natural or immobile) a subject is instructed to maintain. This study aimed at investigating whether instructions issued in a traditional static posturographic test influence its outcome and hence should be considered in the standardisation of the posturography protocols. Two groups of young healthy subjects were each issued one of two common instructions, "stand quietly" or "stand as still as possible", by means of projected instructions. Differences between the two groups were investigated for commonly calculated centre of pressure parameters. All these parameters, but the mean frequency, were significantly different, with variations in the range between 8% (mean velocity) and 71% (confidence circle area). These results suggest that instructions given to the subjects strongly influence the outcome of posturography and should, hence, be standardised.

A neurofuzzy inference system based on biomechanical features for the evaluation of the effects of physical training.

The current study aimed to evaluate physical training effects. For this purpose, a classifier was implemented by taking into account biomechanical features selected from force-plate measurements and a neurofuzzy algorithm for data management and relevant decision-making. Measurements included two sets of sit-to-stand (STS) trials involving two homogeneous groups, experimental and control, of elders. They were carried out before and after a 12-week heavy resistance strength-training program undergone by the experimental group. Pre- and post-training differences were analysed, and percentages of membership to "trained" and "untrained" fuzzy sets calculated. The method was shown to be appropriate for detecting significant training-related changes. Detection accuracy was higher than 87%. Slightly weaker results were obtained using a neural approach, suggesting the need for a larger sample size. In conclusion, the use of a set of biomechanical features and of a neurofuzzy algorithm allowed to propose a global score for evaluating the effectiveness of a specific training program.

Mobility assessment of patients with facioscapulohumeral dystrophy.

BACKGROUND: Facioscapulohumeral muscular dystrophy is the third most common form of inherited myopathies with a prevalence of 1:20,000. Since both muscle involvement and disease progression are heterogeneous and unpredictable, quantitative assessment tools are needed to evaluate the effects of pharmacological and physical training treatments. METHODS: The instrumented movement analysis of 12 patients with facioscapulohumeral dystrophy and 12 control subjects was conducted using a 9-camera stereophotogrammetric system and 2 force platforms. Subjects performed four tasks of different difficulties: arm movement, level walking, step ascending, and squatting. Manual muscle test, clinical severity scale and magnetic resonance imaging were used to clinically assess the patients. FINDINGS: Walking speed and centre of mass vertical displacement during squatting were reduced in patients and can be used to assess their motor capacity. Features common in the patient sample were: the reduction of shoulder range of motion, the excessive ankle plantar-flexion during walking and step ascending, and the reduction of knee flexion-extension moment during squatting. These parameters were correlated with magnetic resonance imaging results at relevant structure level and can be used to assess the corresponding body functioning. Furthermore, instrumented movement analysis was able to distinguish from normal controls also a group of patients in which clinical assessments did not show any obvious abnormalities and had been evaluated as normal. INTERPRETATION: The quantitative assessment tool devised in this study provides suitable information in terms of both motor capacity and impairment severity of patients with facioscapulohumeral dystrophy, and, thus, encouraging its use for the evaluation of therapeutic trial outcomes for this disease.

Femoral anatomical frame: assessment of various definitions.

The reliability of the estimate of joint kinematic variables and the relevant functional interpretation are affected by the uncertainty with which bony anatomical landmarks and underlying bony segment anatomical frames are determined. When a stereo-photogrammetric system is used for in vivo studies, minimising and compensating for this uncertainty is crucial. This paper deals with the propagation of the errors associated with the location of both internal and palpable femoral anatomical landmarks to the estimation of the orientation of the femoral anatomical frame and to the knee joint angles during movement. Given eight anatomical landmarks, and the precision with which they can be identified experimentally, 12 different rules were defined for the construction of the anatomical frame and submitted to comparative assessment. Results showed that using more than three landmarks allows for more repeatable anatomical frame orientation and knee joint kinematics estimation. Novel rules are proposed that use optimization algorithms. On the average, the femoral frame orientation dispersion had a standard deviation of 2, 2.5 and 1.5 degrees for the frontal, transverse, and sagittal plane, respectively. However, a proper choice of the relevant construction rule allowed for a reduction of these inaccuracies in selected planes to 1 degrees rms. The dispersion of the knee adduction-abduction and internal-external rotation angles could also be limited to 1 degrees rms irrespective of the flexion angle value.

The sensitivity of posturographic parameters to acquisition settings.

The objective of this study was to evaluate the sensitivity of posturographic parameters (PP) to changes in acquisition settings. A group of eight young adults underwent a set of typical orthostatic posture trials, and selected PP were then calculated from a set of centre of pressure (CoP) displacement time series obtained by applying different cut-off frequencies to the same set of raw data. Four PP out of 11 showed significant changes with respect to cut-off frequency. Statistical mechanics parameters exhibited smaller sensitivity than summary measures. On the basis of the results obtained, a proposal for a standard cut-off frequency and a sampling rate value is embodied in the paper together with some suggestions on measurement settings, with a view to standardized use of instrumentation for quantitative analysis in orthostatic posturography.

Hemodynamics as a possible internal mechanical disturbance to balance.

The postural control system is assessed by observing body sway while the subject involved aims at maintaining a specified up-right posture. Internal masses generate internal reaction forces that constitute an internal mechanical stimulus that may contribute to cause segmental displacements, i.e. body sway. Thus, gaining knowledge about the amplitude and direction of these reaction forces would contribute to gain insights into the mechanisms that influence the maintenance of balance and into its control. The 3-D force vector that acts on the body centre of mass (COM) and is associated with the transient blood movement at each cardiac cycle was assessed in a population sample of 20 young adults during the maintenance of a quiet up-right posture. Typical patterns of the three components of this force vector were identified. Relevant parameters were selected and submitted to sample statistics. For a number of them, linear correlation with subject-specific parameters was found. The antero-posterior force component was characterised by a triphasic major wave, the peaks of which had values up to 0.40 N. The vertical component showed a repeatable triphasic wave with peak-to-peak values in the range 1.3-3.0 N. The medio-lateral component showed relatively low peak-to-peak values (in the range 0.05-0.10 N). The resultant vector had an amplitude that underwent several oscillations during the cardiac cycle and reached its maximal value in the range 0.6-1.7 N.