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F C van der Helm

Publications and source records attributed to F C van der Helm.

18 recordsLinked to original sources
PubMed2001

Analysis of the reflexive feedback control loop during posture maintenance.

In previous work it has been shown in posture experiments of the human arm that reflexive dynamics were substantial for narrow-band stochastic force disturbances. The estimated reflex gains varied substantially with the frequency content of the disturbances. The present study analyses a simplified linear model of the reflexive feedback control loop, to provide an explanation for the observed behaviour. The model describes co-activation and reflexive feedback. The task instruction 'minimize the displacements' is represented mathematically by a cost function that is minimized by adjusting the parameters of the model. Small-amplitude displacements allow the system to be analysed with a quasilinear approach. The optimization results clarify the limited effectiveness of reflexive feedback on the system's closed-loop behaviour, which emanates from the time delay present in the reflex loops. For low-frequency inputs less than 5 Hz, boundary-stable solutions with high reflex gains are predicted to be optimal. Input frequencies near the system's eigenfrequency (about 5 Hz), however, would be amplified and result in oscillatory behaviour. As long as the disturbance does not excite these frequencies, boundary stability will be optimal. The predicted reflex gains show a striking similarity with the estimated reflex gains from the experimental study. The present model analysis also provides a clear explanation for the negative reflex gains, estimated for near-sinusoidal inputs beyond 1.5 Hz.

Feedback↗
PubMed2001

Optimal posture control of a musculo-skeletal arm model.

In this paper maximal performance posture control of the human arm is investigated by means of model simulations. Recent experiments (F.C.T. van der Helm, submitted, 2000) have shown that the reflexive feedback during postural control varies with the bandwidth of the applied force disturbances. This paper focusses on the influence of the frequency content of force disturbances on the reflexive feedback gains by means of optimization. The arm is modelled by a nonlinear musculo-skeletal model with two degrees of freedom and six muscles. To facilitate the optimization of the model parameters, the arm model is linearized. A performance criterion is minimized for stochastic force disturbances in a two-step procedure: (1) optimization of static muscle activations using an additional energy criterion to obtain a unique and energy-efficient solution; antid (2) optimization of reflex gains using an additional control effort criterion to obtain a unique solution. The optimization reveals that for the given task and posture, the shoulder muscles have the largest contribution, whereas the bi-articular muscles have a relatively small contribution to the behaviour. The dynamics at the endpoint level are estimated so that a comparison can be made with the experiments. Compared to the experiments, the intrinsic damping of the model is relatively large (about 150%), whereas the intrinsic stiffness is relatively small (about 60%). These differences can be attributed to unmodelled mechanical effects of crossbridges in Hill-type muscle models. The optimized reflex gains show remarkable similarities with the values found in the experiments, implying that humans can adjust their reflexive feedback gains in an optimal way, weighting the performance and energy. The approach in this paper could be useful in the study of various posture tasks, for example in the prediction of the relation between the control parameters of various musculo-skeletal models and different experimental variables.

Arm↗
PubMed2001

Biomechatronics--assisting the impaired motor system.

Biomechatronics concerns the interdisciplinary field of interaction with the human neuromuscular-skeletal system with the objective to assist impaired human motor control. In this field technology is developed that integrates neuroscience, robotics, interface and sensor technology, dynamic systems and control theory. The primary issue in this field concerns the concepts of assisting impaired human motor function. The secondary, derived, issue concerns possible methods of interfacing with the human body at all hierarchical levels of the human motor system. The application of motor assist systems may serve several goals: it can take over part of the affected motor control, enable the physiological motor system to perform the desired function or aid in training the impaired physiological system. The progress in these issues are reviewed and their potential implications for assistance of the impaired human motor system are discussed.

Bone and Bones↗
PubMed2001

Three-dimensional shoulder kinematics in individuals with C5-C6 spinal cord injury.

The shoulder kinematics of five able-bodied subjects and those of five arms in three subjects with spinal cord injuries at C5 or C6 levels were measured as the subjects elevated their arms in three different planes: coronal, scapular and sagittal. The range of humeral elevation was significantly reduced in all spinal cord injury (SCI) subjects relative to able-bodied subjects. Over this restricted range of humeral motion, the scapula of SCI subjects tended to be medially rotated, relative to able-bodied subjects, and the protraction and spinal tilt angles of the scapula of the SCI subjects indicated scapular winging. These results are consistent with paralysis or at least with significant weakness of the serratus anterior muscle. If further study confirms this hypothesis, functional neuromuscular stimulation of the serratus anterior muscle via a nerve cuff electrode may be an effective intervention for improving shoulder function in C5-C6 SCI.

Biomechanical Phenomena↗
PubMed2001

Determination of functional rotation axes during elevation of the shoulder complex.

STUDY DESIGN: A cross-sectional, descriptive study of shoulder movements conducted on nonimpaired subjects. OBJECTIVE: To investigate whether a single functional rotation axis about the shoulder complex can be determined during elevation in the coronal or sagittal planes, and to identify their location. BACKGROUND: Accurate measurement of isokinetic torques about a joint requires alignment of the dynamometer axis with an assumed rotation axis of the joint. To assess shoulder function on a dynamometer, the location of a single rotation axis is not evident because the shoulder joint motion is based on several anatomical joints. Therefore, the rotation axis where humerothoracic movements occur should be judged as a functional rotation axis. METHODS AND MEASURES: During slow elevation movements in the sagittal and coronal plane, the position of the epicondyle and acromion were recorded with a motion analysis system. The motion trajectory of the elbow coordinates was fitted to a circle and considered an estimate of the functional shoulder joint rotation axis in the specified plane. RESULTS: The fitted trajectory appeared to be very accurate (root-mean-square error < 2%; N = 7). In the sagittal plane, the estimated functional rotation axis was found at the humeral head; in the coronal plane, it was located about 13 centimeters medial relative to the acromion. CONCLUSION: The shoulder complex of nonimpaired subjects can act as a hinge joint of the upper arm relative to the thorax during elevation in each measured plane.

Adult↗
PubMed2000

Length of the spine while sitting on a new concept for an office chair.

Changes in spinal length were used to evaluate a new concept for an office chair. This so-called dynamic chair imparts passive forced motion to the seated subject. The passive forced motion is a rotary movement about an axis, perpendicular to the seat with amplitude of 0.6 degrees and a frequency of 0.08 Hz. Change of stature is assumed to provide a measure for spinal load. Eight subjects were measured in two situations: static (without motion) and dynamic. In both situations the same office tasks were performed and the duration of the sitting period was 1 h. To allow for the normal shrinkage curve the starting time was the same on each of the measurement days. The results indicated a significant difference: when sitting on the dynamic chair the average spinal length increased in comparison to the spinal length in the static chair, where average spinal length decreased. It was concluded that there is spinal distress relief due to the passive motion of the chair.

Adult↗
PubMed1999

Calibration of the "Flock of Birds" electromagnetic tracking device and its application in shoulder motion studies.

In this paper the applicability in terms of measurement accuracy of the "Flock of Birds" six D.O.F. electromagnetic tracking device in shoulder research is investigated. Position measurements in a workspace of approximately 1 m3 were performed using a stylus. The andom error at the stylus tip appeared to be 1.86, 1.98 and 2.54 mm for x-, y- and z-coordinate, respectively. The error caused by distortion of the magnetic field by metal in the concrete of especially the floor was 20.8, 22.2 and 20.4 mm for the x-, y- and z-coordinate, respectively. Calibration and leaving out the measurements closest to the floor lowered this error to 2.07, 2.38 and 2.35 mm. Orientation errors of the shoulder bones evolving from the measurement inaccuracy were estimated from repeated measurements of shoulder bony landmarks of ten subjects by means of the stylus. These errors were generally below 2 degrees. This is lower than found for the same measurements using a spatial linkage digitizer. It is concluded that the "Flock of Birds" is a useful tool for shoulder kinematic studies.

Algorithms↗
PubMed1999

Effect of different arm loads on the position of the scapula in abduction postures.

OBJECTIVE: The objective of this study is to determine the relation between arm load and the three-dimensional shoulder orientations. BACKGROUND: Analysis of a musculo-skeletal system by means of an inverse dynamical simulation requires postural data of the bony elements involved. For the shoulder, the positions of the clavicle and the scapula are difficult to record due to the skin displacement. It would therefore be useful to predict the three-dimensional relation between the orientation of the arm, the clavicula and the scapula, i.e. the three-dimensional shoulder rhythm, with respect to the thorax under different load conditions. METHODS: The orientation of the clavicula and the scapula was determined with respect to the thorax by means of palpation of skeletal landmarks, for seven postures of arm elevation in the frontal plane at four load conditions: 0, 0.9, 1.9 and 2.9 kg at the wrist. The data were expressed by Cardan angles and analyzed by means of repeated measurements analysis of variance. RESULTS: While the clavicular and scapular angles were significantly related to the arm elevation, no significant relation was found with the load in the hands for the seven arm postures. CONCLUSIONS: The three-dimensional shoulder rhythm does not change under different gravitational load conditions on the arm. RELEVANCE: The fact that the magnitude of the load does not affect the shoulder postures, i.e. the moment arms of the muscles, facilitates the biomechanical, ergonomical and clinical studies on the shoulder by the reduction on the number of recordings for equal task under different load conditions, and easy imitation of real life tasks in the laboratory.

Adult↗
PubMed1998

In vivo estimation of the glenohumeral joint rotation center from scapular bony landmarks by linear regression.

In this paper, a method is described for in vivo prediction of the glenohumeral joint rotation center (GH-r), necessary for the construction of a humerus local coordinate system in shoulder kinematic studies. The three-dimensional positions of five scapula bony landmarks as well as a large number of data points on the surface of the glenoid and humeral head were collected at 36 sets of cadaver scapulae and adjacent humeri. The position of GH-r in each scapula was estimated by mathematically fitting spheres to the glenoid and humeral head. GH-r prediction from scapula geometry parameters by linear regression resulted in a RMSE between measured and predicted GH-r of 2.32 mm for the x-coordinate, 2.69 mm for the y-coordinate and 3.04 mm for the z-coordinate. Application in vivo revealed a random humerus orientation error due to measurement inaccuracies of 1.35, 0.29 and 1.26 degrees standard deviation per rotation angle. The estimated total humerus orientation error including the offset error due to the regression model inaccuracy was 2.86, 0.84 and 2.69 degrees standard deviation. As these errors were about 15 and 20% of, respectively, the intra- and inter-subject variability of the humerus orientations measured, it is concluded that the method described in this paper allows for an adequate construction of a humerus local coordinate system.

Cadaver↗
PubMed1996

Quasi-static analysis of muscle forces in the shoulder mechanism during wheelchair propulsion.

During wheelchair propulsion the largest net joint moments and net joint powers are generated around the shoulder. The analysis of the contribution of arm- and shoulder muscles to the joint moments could explain the low efficiency of wheelchair propulsion. Basically, it is assumed that a large magnitude of muscle activity will be needed to stabilize the shoulder. In addition, the muscular requirements for the minimization of negative power are assumed to be of importance. For such an analysis an inverse dynamic model is required. To utilize an inverse dynamic model of the shoulder mechanism, the trajectories of the upper extremity bones are needed. Since at this stage, dynamic non-invasive measurement techniques of scapular motion are not available, the aim of this study was to record the three-dimensional position of the scapula in static situations with the help of a palpation technique. Positions of the trunk, shoulder girdle and upper extremity, and the surface EMG of ten muscles were recorded simultaneously with forces on the rim on a stationary wheelchair ergometer. Four healthy male subjects participated in the experiment. Five hand positions on the rim and five different load levels per hand position were measured for each subject. A previously developed musculoskeletal model of the shoulder mechanism (Van der Helm, 1994a, J. Biomechanics 27(5) 551-569) was used to calculate muscle forces in an inverse static simulation. The measured EMG and the calculated muscle forces compared well except for three muscles. The moment balance between external sources and muscles around each joint axis of the shoulder mechanism is discussed. Results of the experiment indicate that large muscle contributions are needed for joint stabilization. The experimental results on the scapular motions will, in combination with experimental data collected under dynamic conditions, be used for application of the model to dynamic situations. It is concluded that the musculoskeletal model of the shoulder mechanism can be very useful in studies to determine the contribution of muscles and the mechanical load on morphological structures.

Adult↗
PubMed1996

Influence of glenohumeral prosthesis geometry and placement on shoulder muscle forces.

The authors studied the influence of a changed geometry of the glenohumeral joint on the function of the muscles with the use of a shoulder prosthesis with an anatomic design. The changed geometry is characterized by 4 parameters: orientation of the glenoid, radius of the humeral head, position of the glenohumeral joint's geometric center in relation to the scapula, and position of the glenohumeral joint's geometric center in relation to the humeral shaft. The effect of changes in these 4 parameters was investigated with an inverse dynamic 3-dimensional musculoskeletal model of the shoulder. This was done at 60 degrees and 90 degrees of abduction and flexion. Gravity was the only external force on the arm. The magnitudes of the introduced changes are assumed to be a realistic representation of a changed geometry due to the implantation of a prosthesis. In most situations, the effect of a change in the 4 parameters on the exerted muscle force was small compared with the maximum force of a muscle. However, in relation to the initial reference force in a muscle, changes with an average of 50% occurred. Changes in the geometric center's position relative to the humerus are especially important, because they are closely related to the retroversion angle and can cause changes in force of up to 300%.

Biomechanical Phenomena↗
PubMed1995

Three-dimensional recording and description of motions of the shoulder mechanism.

A measurement technique is presented for recording positions of the bones of the shoulder mechanism, i.e., thorax, clavicula, scapula and humerus, in 3-D space, based on palpating and recording positions of bony landmarks. The palpation technique implies that only static positions can be measured. Accuracy of retrieving bony landmarks is checked on-line using rigid body assumptions. The measurement error is calculated afterwards and is comparable with cinegraphic methods. Axial rotation of the clavicula is estimated by minimizing rotations in the acromioclavicular joint. A number of motion definitions is compared by means of interindividual variation and subjective interpretability. Two useful definitions are proposed for describing motions of the shoulder mechanism. Four conditions have been recorded: abduction and anteflexion of the humerus both with and without additional weight in the hand. Abduction and anteflexion result in large differences in scapular and clavicular motions. The effect of additional weight in the hand on the position of the shoulder girdle is negligible.

Adult↗
PubMed1994

Analysis of the kinematic and dynamic behavior of the shoulder mechanism.

A finite element musculoskeletal model of the shoulder mechanism consisting of the thorax, clavicula, scapula and humerus has been used for analysis of the kinematic and dynamic behavior. The model includes 16 muscles, three joints, three extracapsular ligaments and the motion constraints of the scapulothoracic gliding plane which turns the shoulder girdle into a closed-chain mechanism. Simulations are inverse dynamic. Input variables are the positions of the shoulder girdle and humerus which have been recorded in 10 subjects during unloaded and loaded humeral abduction and anteflexion. Comparisons of muscle force predictions and EMG recordings are hampered by the unknown force-length relationship and the length dependency of EMG amplitude. It is concluded that EMG amplitude cannot be used for validation of complex musculoskeletal models. Muscle function is analyzed with help of a force and moment balance of the three joints. The moment balance includes the contributions of ligaments and the reaction forces at the scapulothoracic gliding plane. The scapulothoracic gliding plane is very important for the motions and the stabilization of the shoulder girdle. The direction and magnitude of joint reaction forces are calculated as well. It is concluded that the model provides good insight into the mechanics of the shoulder mechanism and that it enables an analysis of the function of morphological structures.

Acromioclavicular Joint↗
PubMed1994

A finite element musculoskeletal model of the shoulder mechanism.

The finite element method described in this study provides an easy method to simulate the kinetics of multibody mechanisms. It is used in order to develop a musculoskeletal model of the shoulder mechanism. Each relevant morphological structure has been represented by an appropriate element. For the shoulder mechanism two special-purpose elements have been developed: a SURFACE element representing the scapulothoracic gliding plane and a CURVED-TRUSS element to represent muscles which are wrapped around bony contours. The model contains four bones, three joints, three extracapsular ligaments, the scapulothoracic gliding plane and 20 muscles and muscle parts. In the model, input variables are the positions of the shoulder girdle and humerus and the external load on the humerus. Output variables are muscles forces subject to an optimization procedure in which the mechanical stability of the glenohumeral joint is one of the constraints. Four different optimization criteria are compared. For 12 muscles, surface EMG is used to verify the model. Since the optimum muscle length and force-length relationship are unknown, and since maximal EMG amplitude is length dependent, verification is only possible in a qualitative sense. Nevertheless, it is concluded that a detailed model of the shoulder mechanism has been developed which provides good insight into the function of morphological structures.

Acceleration↗
PubMed1993

Interaction between the joints in the shoulder mechanism: the function of the costoclavicular, conoid and trapezoid ligaments.

By developing a measurement method based on the palpation of bony landmarks, the three-dimensional positions of the scapula and clavicle can be measured at several angles of humerus elevation. An analysis of these measurements shows the interaction between all joints of the shoulder mechanism. With the help of a biomechanical shoulder model the role of some of the extracapsular ligaments in the motion pattern of scapula and clavicle can be derived. In addition, the interaction between the rotations in the acromioclavicular and sternoclavicular joints is shown, and the possible implications for the treatment of joint problems in the shoulder are discussed.

Biomechanical Phenomena↗
PubMedDate not supplied

The palpator: an instrument for measuring the positions of bones in three dimensions.

In order to measure the three-dimensional (3D) position of the shoulder girdle bones, as well as the 3D positions of muscle attachments and joint surfaces in cadaver experiments, a measurement instrument, called the palpator, was developed. The palpator is composed of an open chain of four links connected by four hinges. By recording the rotation of the hinges using potentiometers, the position of the end-point of the palpator can be calculated. After the identification of 23 parameters of the palpator, a measurement accuracy of 1.43 mm is obtained.

Biomechanical Phenomena↗
PubMedDate not supplied

Model-based development of neuroprostheses for restoring proximal arm function.

Neuroprostheses with the use of functional neuromuscular stimulation (FNS) have the potential to restore elbow and shoulder function lost to paralysis because of spinal cord injury (SCI). The human shoulder is highly flexible and thus provides a large range of motion to the arm and hand, although at the expense of precarious stability of the articulations. The complexity of the shoulder has prevented widespread use of FNS at this joint. However, musculoskeletal modeling of the elbow and shoulder has the potential to significantly speed the development of neuroprostheses by allowing many mechanical issues to be resolved in simulation prior to implementation in human subjects. This paper describes our rationale for the use of musculoskeletal modeling, the model we are using, and several practical applications of the model to study the potential use of shoulder and elbow muscle FNS to restore function following cervical SCI.

Elbow Joint↗