Use of induced acceleration to quantify the (de)stabilization effect of external and internal forces on postural responses

Due to the mechanical coupling between the body segments, it is impossible to see with the naked eye the causes of body movements and understand the interaction between movements of different body parts. The goal of this paper is to investigate the use of induced acceleration analysis to reveal the causes of body movements. We derive the analytical equations to calculate induced accelerations and evaluate its potential to study human postural responses to support-surface translations. We measured the kinematic and kinetic responses of a subject to sudden forward and backward translations of a moving platform. The kinematic and kinetics served as input to the induced acceleration analyses. The induced accelerations showed explicitly that the platform acceleration and deceleration contributed to the destabilization and restabilization of standing balance, respectively. Furthermore, the joint torques, coriolis and centrifugal forces caused by swinging of the arms, contributed positively to stabilization of the center of mass. It is concluded that induced acceleration analyses is a valuable tool in understanding balance responses to different kinds of perturbations and may help to identify the causes of movement in different pathologies

Modeling movement disorders - CRPS-related dystonia explained by abnormal proprioceptive reflexes

AbstractHumans control their movements using adaptive proprioceptive feedback from muscle afferents. The interaction between proprioceptive reflexes and biomechanical properties of the limb is essential in understanding the etiology of movement disorders. A non-linear neuromuscular model of the wrist incorporating muscle dynamics and neural control was developed to test hypotheses on fixed dystonia. Dystonia entails sustained muscle contractions resulting in abnormal postures. Lack of inhibition is often hypothesized to result in hyperreflexia (exaggerated reflexes), which may cause fixed dystonia. In this study the model-simulated behavior in case of several abnormal reflex settings was compared to the clinical features of dystonia: abnormal posture, sustained muscle contraction, increased stiffness, diminished voluntary control and activity-aggravation.The simulation results were rated to criteria based on characteristic features of dystonia. Three abnormal reflex scenarios were tested: (1) increased reflex sensitivity—increased sensitivity of both the agonistic and antagonistic reflex pathways; (2) imbalanced reflex offset—a static offset to the reflex pathways on the agonistic side only; and (3) imbalanced reflex sensitivity—increased sensitivity of only the agonistic reflex pathways.Increased reflex sensitivity did not fully account for the features of dystonia, despite distinct motor dysfunction, since no abnormal postures occurred. Although imbalanced reflex offset did result in an abnormal posture, it could not satisfy other criteria. Nevertheless, imbalanced reflex sensitivity with unstable force feedback in one of the antagonists closely resembled all features of dystonia. The developed neuromuscular model is an effective tool to test hypotheses on the underlying pathophysiology of movement disorders

Comparison of different methods to identify and quantify balance control

The goal of this paper is to clarify the methodological aspects of studies of human balance during quiet standing and perturbed standing. Centre of mass (CoM), centre of pressure (CoP) and electromyogram (EMG) or similar measures are commonly recorded to quantify human balance control. In this paper we show that to identify the rigid body dynamics and the physiological mechanism that controls the body separately, one has to externally perturb the body with known perturbations and to use the indirect (IA) or joint input–output approach (JA) for identification. However, in many balance control studies the direct approach (DA) have been used, which is well suited to study open-loop systems but will give erroneous results when applied to a closed-loop system, as in human balance control. The cross-correlation function and linear regression are examples of the erroneous application of the DA approach in human balance control studies. The consequences of this erroneous DA are given. In addition a new application of the JA is presented that identifies physiological mechanisms that control balance, including passive and active feedback pathways. This new method is compared with existing identification schemes that use the IA and an existing JA that estimates the active pathway. Also it is shown how descriptive measures such as the power spectral densities (PSD) or the stabilogram diffusion plot (SDP) of the CoP and/or CoM depends on the PSD of internal perturbations and sensor noise, which are not measured. Although descriptive measures can be used to describe the state of the balance control system for a particular situation, it does not separate the dynamics of unknown processes that perturb balance from the dynamics of the active and passive feedback mechanisms that controls balance. Only the IA and the preferred JA can give estimates of the passive and active passive feedback mechanisms that control balance

Quantification of Error Sources with Inertial Measurement Units in Sports

Background: Inertial measurement units (IMUs) offer the possibility to capture the lower body motions of players of outdoor team sports. However, various sources of error are present when using IMUs: the definition of the body frames, the soft tissue artefact (STA) and the orientation filter. Methods to minimize these errors are currently being used without knowing their exact influence on the various sources of errors. The goal of this study was to present a method to quantify each of the sources of error of an IMU separately. Methods: An optoelectronic system was used as a gold standard. Rigid marker clusters (RMCs) were designed to construct a rigid connection between the IMU and four markers. This allowed for the separate quantification of each of the sources of error. Ten subjects performed nine different football-specific movements, varying both in the type of movement, and in movement intensity. Results: The error of the definition of the body frames (11.3–18.7 deg RMSD), the STA (3.8–9.1 deg RMSD) and the error of the orientation filter (3.0–12.7 deg RMSD) were all quantified separately for each body segment. Conclusions: The error sources of IMU-based motion analysis were quantified separately. This allows future studies to quantify and optimize the effects of error reduction techniques

The size of the supraspinatus outlet during elevation of the arm in the frontal and sagittal plane: A 3-D model study

Objective. To quantify the size of the supraspinatus outlet as it is dictated by both the three-dimensional geometry of the shoulder and the relative orientation of the humerus with respect to the scapula during motions of the arm. Design. Previously obtained data of shoulder kinematics were brought into a geometrical model of the shoulder, derived from a cadaver study. Background. Knowledge of the parameters dictating the size of the supraspinatus outlet is essential for a better understanding of the impingement syndrome of the shoulder. Methods. A geometrical model, based on fitting spheres to various anatomical items of the shoulder was derived from three-dimensional position data of the gleno-humeral joint and coraco-acromial arch of 32 cadaver shoulders. Kinematical data were collected from 10 healthy volunteers. The geometrical and kinematical data were combined to study the supraspinatus outlet during elevation of the humerus in the frontal and sagittal plane. Results. No single geometry parameter correlated significantly with the initial size of the outlet. During arm elevation, the greater tuberosity was moved away from the coraco-acromial arch quite effectively resulting in narrowing of the outlet during elevation in the frontal plane from 60° to 120° only. Deviations from the average were quite substantial. This was caused by kinematical and especially geometrical variability. Conclusion. The size of the outlet is dictated by both the geometry and kinematics of the gleno-humeral joint. Assessment of the individual susceptibility to impingement requires three-dimensional viewing techniques including three-dimensional movements of both the scapula and humerus. Relevance. Little is known about etiology and pathogenesis of various shoulder disorders such as the impingement syndrome. The supraspinatus outlet plays probably a key role. More knowledge on the architecture of the outlet is required for a better understanding

Frequency Domain Characterization of the Somatosensory Steady State Response in Electroencephalography

A continuous somatosensory stimulation evokes a steady state response in the cortex, which can be measured using electroencephalography. We applied mechanical multisine stimulation of the wrist to investigate the properties of the steady state response in the frequency domain. Our results show a response in the contralateral sensorimotor cortex at the stimulated frequencies yet with more power at their even harmonics, indicating substantial nonlinear behavior. The observed cortical response to a mechanical somatosensory stimulation is time invariant and nonlinear, however shows no subharmonics, allowing for the application of a broad range of (non)linear system identification tools

Influence of biomechanical models on joint kinematics and kinetics in baseball pitching

In baseball pitching, biomechanical parameters have been linked to ball velocity and potential injury risk. However, although the features of a biomechanical model have a significant influence on the kinematics and kinetics of a motion, this influence have not been assessed for pitching. The aim of this study was to evaluate the choice of the trunk and shoulder features, by comparing two models using the same input. The models differed in thoraco-humeral joint definition (moving or fixed with the thorax), joint centre estimation, values of the inertial parameters and computational framework. One professional pitcher participated in the study. We found that the different features of the biomechanical models have a substantial influence on the kinematics and kinetics of the pitchers. With a fixed thoraco-humeral joint the peak average thorax angular velocity was delayed and underestimated by 17% and the shoulder internal rotation velocity was overestimated by 7%. The use of a thoraco-humeral joint fixed to the thorax will lead to an overestimation of the rotational power at the shoulder and will neglect the power produced by the forward and upward translation of the shoulder girdle. These findings have direct implications for the interpretation of shoulder muscle contributions to the pitch

Unveiling neural coupling within the sensorimotor system: directionality and nonlinearity

Neural coupling between the central nervous system and the periphery is essential for the neural control of movement. Corticomuscular coherence is a popular linear technique to assess synchronised oscillatory activity in the sensorimotor system. This oscillatory coupling originates from ascending somatosensory feedback and descending motor commands. However, corticomuscular coherence cannot separate this bidirectionality. Furthermore, the sensorimotor system is nonlinear, resulting in cross-frequency coupling. Cross-frequency oscillations cannot be assessed nor exploited by linear measures. Here, we emphasise the need of novel coupling measures, which provide directionality and acknowledge nonlinearity, to unveil neural coupling in the sensorimotor system. We highlight recent advances in the field and argue that assessing directionality and nonlinearity of neural coupling will break new ground in the study of the control of movement in healthy and neurologically impaired individuals

Fixating the pelvis in the horizontal plane affects gait characteristics

In assistive devices for neuro-rehabilitation, natural human motions are partly restricted by the device. This may affect the normality of walking during training. This research determines effects on gait of fixating the pelvis translations in the horizontal plane during treadmill walking. Direct effects on the motion of the pelvis and external forces acting on the pelvis were measured. Several gait descriptors (step parameters, trunk angles and a ground reaction force parameter) were defined and measured to indicate changes. We observed the effect of the pelvis fixation on these parameters while varying gait velocity (0.35, 0.60 and 0.90 m/s). It was shown that the fixation caused a reduction of step width by 33%, and an increase of step length of 19%. Sagittal and coronal trunk rotations changed with +68% and −54% respectively. The fixation also significantly changed the effect of speed on most descriptors. It can therefore be concluded that a fixation of the pelvis severely affects gait dynamics and that it should be avoided if natural walking should be possible during training