reliability of assessing postural control during seated balancing using a physical human robot interaction abstract this study evaluated the within and between visit reliability of a seated balance test for quantifying trunk motor control using input output data thirty healthy subjects performed a seated balance test under three conditions eyes open eo eyes closed ec and eyes closed with vibration to the lumbar muscles vib each subject performed three trials of each condition on three different visits the seated balance test utilized a torque controlled robotic seat which together with a sitting subject resulted in a physical human robot interaction phri two degrees of freedom with upper and lower body rotations subjects balanced the phri by controlling trunk rotation in response to pseudorandom torque perturbations applied to the seat in the coronal plane performance error was expressed as the root mean square rmse of deviations from the upright position in the time domain and as the mean bandpass signal energy emb in the frequency domain intra class correlation coefficients icc quantified the between visit reliability of both rmse and emb the empirical transfer function estimates etfe from the perturbation input to each of the two rotational outputs were calculated coefficients of multiple correlation cmc quantified the within and between visit reliability of the averaged etfe iccs of rmse and emb for all conditions were 0 84 the mean within and between visit cmcs were all 0 96 for the lower body rotation and 0 89 for the upper body rotation therefore our seated balance test consisting of phri to assess coronal plane trunk motor control is reliable

the equivalence of multi axis spine systems recommended stiffness limits using a standardized testing protocol abstract the complexity of multi axis spine testing often makes it challenging to compare results from different studies the aim of this work was to develop and implement a standardized testing protocol across three six axis spine systems compare them and provide stiffness and phase angle limits against which other test systems can be compared standardized synthetic lumbar specimens n 5 comprising three springs embedded in polymer at each end were tested on each system using pure moments in flexion extension lateral bending and axial rotation tests were performed using sine and triangle waves with an amplitude of 8 nm a frequency of 0 1 hz and with axial preloads of 0 and 500 n the stiffness phase angle and r2 value of the moment against rotation in the principal axis were calculated at the center of each specimen the tracking error was adopted as a measure of each test system to minimize non principal loads defined as the root mean squared difference between actual and target loads all three test systems demonstrated similar stiffnesses with small

on some geometric properties of the generalized ces production functions abstract in this paper we prove that the generalized ces production function has constant return to scale if and only if the corresponding hypersurface is developable moreover we establish that this production function has decreasing increasing return to scale if and only if the corresponding hypersurface has positive negative gaussian curvature these results are a generalization of some recent results concerning the generalized cobb douglas production functions g e vilcu a geometric perspective on the generalized cobb douglas production functions appl math lett 24 2011 777 783

anderson transition driven by running fractal dimensions in a fractal shaped structure abstract anderson transition in a fractal shaped structure is driven by running fractal dimension from two to three we analyzed the spectral statistics of a quantum particle confined in a fractal shaped structure generated using the dielectric breakdown model in three dimensions when the fractal dimension is almost three the spectral statistics of low energy states are similar to those of a gaussian orthogonal ensemble as the fractal dimension decreases to two the spectral statistics become poissonian this indicates the onset of quantum localization in the fractal shaped structure so the anderson transition takes place as the fractal dimension changes the effects of introducing random potentials are also analyzed when a random magnetic field affects quantum states extended over our fractal shaped structure the spectral statistics are similar to those of a gaussian unitary ensemble on the other hand a random on site potential encourages quantum localization in our fractal shaped structure

kinetic analysis of the lower limbs during walking what information can be gained from a three dimensional model kinetic analyses joint moments powers and work of the lower limbs were performed during normal walking to determine what further information can be gained from a three dimensional model over planar models it was to be determined whether characteristic moment and power profiles exist in the frontal and transverse planes across subjects and how much work was performed in these planes kinetic profiles from nine subjects were derived using a three dimensional inverse dynamics model of the lower limbs and power profiles were then calculated by a dot product of the angular velocities and joint moments resolved in a global reference system characteristic joint moment profiles across subjects were found for the hip knee and ankle joints in all planes except for the ankle frontal moment as expected the major portion of work was performed in the plane of progression since the goal of locomotion is to support the body against gravity while generating movements which propel the body forward however the results also showed that substantial work was done in the frontal plane by the hip during walking 23 of the total work at that joint the characteristic joint profiles suggest defined motor patterns and functional roles in the frontal and transverse planes kinetic analysis in three dimensions is necessary particularly if the hip joint is being examined as a substantial amount of work was done in the frontal plane of the hip to control the pelvis and trunk against gravitational forces

on the corporate votes and their relation with daisy models abstract the distribution of votes of one of the corporate parties in mexico during elections of 2000 2003 and 2006 is analyzed after proper normalization and unfolding the agreement of the vote distributions with those of daisy models of several ranks is good these models are generated by retaining each r 1 level in a sequence which follows a poisson distribution beyond the fact that rank 2 daisy model resembles the distribution of the quasi optimal distances for the traveling salesman problem no clear explanation exists for this behavior but the agreement is not fortuitous and the possibility of a universal phenomena for corporate vote is discussed

least squares identification of the dynamic relation between the electromyogram and joint moment the dynamic relation between surface emg of an agonist and isometric joint moment is important in models for the prediction of dynamic joint moment from emg an efficient approach for determining the best fitting linear transfer function relating emg and isometric moment using a least squares approach is presented and illustrated for a second order model

computation of trunk equilibrium and stability in free flexion extension movements at different velocities velocity of movement has been suggested as a risk factor for low back disorders the effect of changes in velocity during unconstrained flexion extension movements on muscle activations spinal loads base reaction forces and system stability was computed in vivo measurements of kinematics and ground reaction forces were initially carried out on young asymptomatic subjects the collected kinematics of three subjects representing maximum mean and minimum lumbar rotations were subsequently used in the kinematics driven model to compute results during the entire movements at three different velocities estimated spinal loads and muscle forces were significantly larger in fastest pace as compared to slower ones indicating the effect of inertial forces spinal stability was improved in larger trunk flexion angles and fastest movement partial or full flexion relaxation of global extensor muscles occurred only in slower movements some local lumbar muscles especially in subjects with larger lumbar flexion and at slower paces also demonstrated flexion relaxation results confirmed the crucial role of movement velocity on spinal biomechanics predictions also demonstrated the important role on response of the magnitude of peak lumbar rotation and its temporal variation

response of trunk muscle coactivation to changes in spinal stability the goal of this effort was to assess the neuromuscular response to changes in spinal stability biomechanical models suggest that antagonistic co contraction may be related to stability constraints during lifting exertions a two dimensional biomechanical model of spinal equilibrium and stability was developed to predict trunk muscle co contraction as a function of lifting height and external load the model predicted antagonistic co contraction must increase with potential energy of the system even when the external moment was maintained at a constant value predicted trends were compared with measured electromyographic emg data recorded during static trunk extension exertions wherein subjects held weighted barbells at specific horizontal and vertical locations relative to the lumbo sacral spine junction the task was designed to assure the applied moment was identical during each height condition thereby changing potential energy without influencing moment measured emg activity in the trunk flexors increased with height of the external load as predicted by the model gender difference in spinal stability were also noted results empirically demonstrate that the neuromuscular system responds to changes in spinal stability and provide insight into the recruitment of trunk muscle activity

scaling and universality in proportional elections a most debated topic of the last years is whether simple statistical physics models can explain collective features of social dynamics a necessary step in this line of endeavor is to find regularities in data referring to large scale social phenomena such as scaling and universality we show that in proportional elections the distribution of the number of votes received by candidates is a universal scaling function identical in different countries and years this finding reveals the existence in the voting process of a general microscopic dynamics that does not depend on the historical political and or economical context where voters operate a simple dynamical model for the behavior of voters similar to a branching process reproduces the universal distribution

critical helix of hierarchical structures abstract electronic structures in a hierarchical helix with inter round couplings i e a multiply twisted helix are investigated using the spectral statistics of a quantum particle the hierarchical structure is constructed using a procedure in which a helical chain of components is twisted producing a doubly twisted helix which if itself is twisted produces a triply twisted helix and so on we compare three types of inter round couplings which enable electrons to return to their original positions when the strength of inter round couplings is sufficiently smaller than that along a basal helix the type of inter round couplings can be distinguished by the behavior of spectral statistics the multiply twisted helix of inter round couplings through a single site is unique in the sense that its electronic structure is robust against any change in the basal helix so is called a critical helix

measurement of a spinal motion segment stiffness matrix the six degrees of freedom elastic behavior of spinal motion segments can be approximated by a stiffness matrix a method is described to measure this stiffness matrix directly with the motion segment held under physiological conditions of axial preload and in an isotonic fluid bath by measuring the forces and moments associated with each of the six orthogonal translations and rotations the stiffness matrix was obtained from the load displacement measurements by linear least squares assuming a symmetric matrix results from a pig lumbar spinal motion segment in an isotonic bath with and without a 500 n axial preload showed a large stiffening effect with axial preload

models of intermediate spectral statistics based on numerical results it is conjectured that the spectral statistics of certain pseudointegrable billiards have a special form similar to that of the anderson model at the transition point a simple theoretical model where such statistics can be obtained analytically is briefly discussed a few models with similar behavior are considered in particular we analytically found the eigenvalue statistics of a poisson distributed matrix perturbed by a rank one matrix which is a good model for spectral statistics of a singular billiard

brazilian elections voting for a scaling democracy abstract the proportional elections held in brazil in 1998 and 2002 display identical statistical signatures in particular the distribution of votes among candidates includes a power law regime we suggest that the rationale behind this robust scaling invariance is a multiplicative process in which the voter s choice for a candidate is governed by a product of probabilities

intermediate statistics in quantum maps we present a one parameter family of quantum maps whose spectral statistics are of the same intermediate type as observed in polygonal quantum billiards our central result is the evaluation of the spectral two point correlation form factor at small argument which in turn yields the asymptotic level compressibility for macroscopic correlation lengths

expressing the joint moments of drop jumps and sidestep cutting in different reference frames does it matter abstract joint moments help us understand joint loading and muscle function during movement however the interpretation depends on the choice of reference frame but the different reference frames have not been compared in dynamic high impact sporting movements we have compared the magnitude and the resulting ranking of hip and knee joint moments expressed in the laboratory coordinate system the local system of the distal segment and projected or decomposed to the joint coordinate system jcs axes hip and knee joint moments of drop jumps and sidestep cutting in 70 elite female handball players were calculated based on recordings from an eight camera 240 hz system and two force platforms and expressed with the four methods the greatest variations in magnitude between conditions were seen for drop jump hip internal rotation range 0 31 0 71 nm kg and sidestep cutting knee flexion 2 87 3 39 nm kg and hip internal rotation 0 87 2 36 nm kg and knee internal rotation 0 10 0 40 nm kg moments the rank correlations were highest between conditions for flexion moments 0 88 1 00 and sidestep cutting abduction moments 0 71 0 98 the rank correlations ranged from 0 64 to 0 73 for drop jump knee abduction moments and between 0 17 and 0 67 for hip and knee internal rotation moments expression of joint moments in different reference systems affects the magnitude and ranking of athletes this lack of consistency may complicate the comparison and combination of results projection to the jcs is the only method where joint moments correspond to muscle and ligament loading more widespread adoption of this convention could facilitate comparison of studies and ease the interpretation of results

sensory contributions to stabilization of trunk posture in the sagittal plane trunk stabilization is required to control posture and movement during daily activities various sensory modalities such as muscle spindles golgi tendon organs and the vestibular system might contribute to trunk stabilization and our aim was to assess the contribution of these modalities to trunk stabilization in 35 healthy subjects upper body sway was evoked by continuous unpredictable force controlled perturbations to the trunk in the anterior direction subjects were instructed to either maximally resist the perturbation or to relax but remain upright with eyes closed frequency response functions frfs of admittance the amount of movement per unit of force applied and reflexes the modulation of trunk extensor activity per unit of trunk displacement were obtained to these frfs we fitted physiological models to estimate intrinsic trunk stiffness and damping as well as feedback gains and delays the different model versions were compared to assess which feedback loops contribute to trunk stabilization intrinsic stiffness and damping and muscle spindle short delay feedback alone were sufficient to accurately describe trunk stabilization but only with unrealistically low reflex delays addition of muscle spindle acceleration feedback or inhibitory golgi tendon organ feedback yielded realistic delays and improved the model fit with a significantly better model fit with acceleration feedback addition of vestibular feedback did not improve the model fit in conclusion muscle spindle feedback and intrinsic mechanical properties are sufficient to describe trunk stabilization in the sagittal plane under small mechanical perturbations provided that muscle spindles encode acceleration in addition to velocity and position information

interpretations of some parameter dependent generalizations of classical matrix ensembles two types of parameter dependent generalizations of classical matrix ensembles are defined by their probability density functions pdfs as the parameter is varied one interpolates between the eigenvalue pdf for the superposition of two classical ensembles with orthogonal symmetry and the eigenvalue pdf for a single classical ensemble with unitary symmetry while the other interpolates between a classical ensemble with orthogonal symmetry and a classical ensemble with symplectic symmetry we give interpretations of these pdfs in terms of probabilities associated to the continuous robinson schensted knuth correspondence between matrices with entries chosen from certain exponential distributions and non intersecting lattice paths and in the course of this probability measures on partitions and pairs of partitions are identified the latter are generalized by using macdonald polynomial theory and a particular continuum limit the jacobi limit of the resulting measures is shown to give pdfs related to those appearing in the work of anderson on the selberg integral and also in some classical work of dixon by interpreting anderson s and dixon s work as giving the pdf for the zeros of a certain rational function it is then possible to identify random matrices whose eigenvalue pdfs realize the original parameter dependent pdfs this line of theory allows sampling of the original parameter dependent pdfs their dixon anderson type generalizations and associated marginal distributions from the zeros of certain polynomials defined in terms of random three term recurrences

on some geometric properties of quasi product production models abstract a production function f is called quasi sum if there are continuous strict monotone functions f h 1 h n with f 0 such that f x f h 1 x 1 h n x n cf aczel and maksa 1996 1 a quasi sum production function is called quasi linear if at most one of f h 1 h n is a nonlinear function for a production function f the graph of f is called the production hypersurface of f in this paper we obtain a very simple necessary and sufficient condition for a quasi sum production function f to be quasi linear in terms of graph of f moreover we completely classify quasi sum production functions whose production hypersurfaces have vanishing gauss kronecker curvature

effects of external trunk loads on lumbar spine stability abstract stability of the lumbar spine is an important factor in determining spinal response to sudden loading using two different methods this study evaluated how various trunk load magnitudes and directions affect lumbar spine stability the first method was a quick release procedure in which effective trunk stiffness and stability were calculated from trunk kinematic response to a resisted force release the second method combined trunk muscle emg data with a biomechanical model to calculate lumbar spine stability twelve subjects were tested in trunk flexion extension and lateral bending under nine permutations of vertical and horizontal trunk loading the vertical load values were set at 0 20 and 40 of the subject s body weight bw the horizontal loads were 0 10 and 20 of bw effective spine stability as obtained from quick release experimentation increased significantly p

trunk stiffness and dynamics during active extension exertions abstract spinal stability is related to the recruitment and control of active muscle stiffness stochastic system identification techniques were used to calculate the effective stiffness and dynamics of the trunk during active trunk extension exertions twenty one healthy adult subjects 10 males 11 females wore a harness with a cable attached to a servomotor such that isotonic flexion preloads of 100 135 and 170 n were applied at the t10 level of the trunk a pseudorandom stochastic force sequence bandwidth 0 10 hz amplitude 30 n was superimposed on the preload causing small amplitude trunk movements nonparametric impulse response functions of trunk dynamics were computed and revealed that the system exhibited underdamped second order behavior second order trunk dynamics were determined by calculating the best least squares fit to the irf the quality of the model was quantified by comparing estimated and observed displacement variance accounted for vaf and quality of the second order fits was calculated as a percentage and referred to as fit accuracy mean vaf and fit accuracy were 87 8 4 0 and 96 0 4 3 respectively indicating that the model accurately represented active trunk kinematic response the accuracy of the kinematic representation was not influenced by preload or gender mean effective stiffness was 2 78 0 96 n mm and increased significantly with preload p 0 001 but did not vary with gender p 0 425 mean effective damping was 314 72 n s m and effective trunk mass was 37 0 9 3 kg we conclude that stochastic system identification techniques should be used to calculate effective trunk stiffness and dynamics

identifying intrinsic and reflexive contributions to low back stabilization motor control deficits have been suggested as potential cause and or effect of a specific chronic low back pain and its recurrent behavior therefore the goal of this study is to identify motor control in low back stabilization by simultaneously quantifying the intrinsic and reflexive contributions upper body sway was evoked using continuous force perturbations at the trunk while subjects performed a resist or relax task frequency response functions frfs and coherences of the admittance kinematics and reflexes semg were obtained in comparison with the relax task the resist task resulted in a 61 decrease in admittance and a 73 increase in reflex gain below 1 1 hz intrinsic and reflexive contributions were captured by a physiologically based neuromuscular model including proprioceptive feedback from muscle spindles position and velocity and golgi tendon organs force this model described on average 90 of the variance in kinematics and 39 of the variance in semg while resulting parameter values were consistent over subjects

effects of low pass filter combinations on lower extremity joint moments in distance running abstract inverse dynamics is a standard tool in biomechanics which requires low pass filtering of external force and kinematic signals unmatched filtering procedures are reported to affect joint moment amplitudes in high impact movements like landing or cutting but are also common in the analysis of distance running we analyzed the effects of cut off frequencies in 94 rearfoot runners at a speed of 3 5 m s additionally we investigated whether the evaluation of footwear interventions is affected by the choice of cut off frequencies we performed 3d inverse dynamics for the hip knee and ankle joints using different low pass filter cut off frequency combinations for a recursive fourth order butterworth filter we observed fluctuations of joint moment curves in the first half of stance which were most pronounced for the most unmatched cut off frequency combination kinematics 10 hz ground reaction forces grfs 100 hz and for more proximal joints peak sagittal plane hip joint moments were altered by 94 on average we observed a change in the ranking of subjects based on joint moment amplitude we found significant p

semi poisson statistics and beyond semi poisson statistics are shown to be obtained by removing every other number from a random sequence retaining every r 1 mathrm th level we obtain a family of sequences which we call daisy models their statistical properties coincide with those of bogomolny s nearest neighbor interaction coulomb gas if the inverse temperature coincides with the integer r in particular the case r 2 reproduces closely the statistics of quasioptimal solutions of the traveling salesman problem

constraining spine stability levels in an optimization model leads to the prediction of trunk muscle cocontraction and improved spine compression force estimates a major limitation of optimization models of the spine has been the inability to accurately predict trunk muscle co activity antagonist muscle activity is thought to be necessary to maintain adequate levels of spine stability but in turn creates increased loading on the spine it is thus hypothesized that the cns attempts to optimize the relationship between spine loading and spine stability in determining muscular activation patterns this study presents an optimization model of the spine in which stability was constrained to target levels predicted from regression equations of independent loading variables objective functions were set to either minimize the sum of the cubed muscle forces or minimize the sum of the squared intervertebral forces at the l4 l5 disc level results demonstrate that the inclusion of stability constraints in optimization simulations produced realistic predictions of antagonist muscle activity and predictions of spine compression levels that agree more closely with emg based estimates compared to simulations in which stability was unconstrained it was concluded that spinal stability is a vital consideration for the cns when dictating trunk muscle recruitment patterns

numerical accuracy of bogomolny s semiclassical quantization scheme in quantum billiards we use the semiclassical quantization scheme of bogomolny to calculate eigenvalues of the limacon quantum billiard corresponding to a conformal map of the circle billiard we use the entire billiard boundary as the chosen surface of section and use a finite approximation for the transfer operator in coordinate space computation of the eigenvalues of this matrix combined with a quantization condition determines a set of semiclassical eigenvalues which are compared with those obtained by solving the schrodinger equation the classical dynamics of this billiard system undergoes a smooth transition from integrable circle to completely chaotic motion thus providing a test of bogomolny s semiclassical method in coordinate space in terms of the morphology of the wavefunction we analyse the results for billiards which exhibit both soft and hard chaos

on the implications of interpreting the stability index a spine example quantifying the stability of the spinal column offers a perspective on the effectiveness of the motor control strategy to ensure a stable spine and minimize the risk of injury from experiencing an unstable event there are essentially three energy based methods of calculating a stability index for the lumbar spine all three methods involve mathematical manipulation of an 18 x 18 hessian matrix the purpose of this paper was to consider the mathematical implications for the three methods of determining a single stability index and examine the effects of biological factors such as muscle activation in each of these methods the first approach computes the hessian s determinant and is thought to represent a more global or average perspective on stability a second approach computes the smallest eigenvalue of the hessian matrix to determine the weakest link of the spine the final method determines an average critical stiffness difference for the spine and is intended to effectively determines how far a human spine is from instability and allows comparison between tasks this study shows that the same interpretation of stability is achieved via all three computational approaches they agree as to whether the spine is stable or not however they appear to differ in their sensitivity to the effect of muscle activation patterns

alterations in trunk bending stiffness following changes in stability and equilibrium demands of a load holding task abstract the contribution of the trunk neuromuscular system tns to spine stability has been shown in earlier studies by characterizing changes in antagonistic activity of trunk muscles following alterations in stability demands of a task whether and or how much such changes in the response of tns to alteration in stability demand of the task alter spinal stiffness remains unclear to address this research gap a repeated measure study was conducted on twenty gender balanced asymptomatic individuals to evaluate changes in trunk bending stiffness throughout the lumbar spine s range of flexion following alterations in both stability and equilibrium demands of a load holding task trunk bending stiffness was determined using trunk stiffness tests in upright posture on a rigid metal frame under different equilibrium and stability demands on the lower back increasing the stability demand by increasing the height of lifted load 30 cm only increased trunk bending stiffness 39 over the lower range of lumbar flexion and under the low equilibrium demand condition similarly increasing the equilibrium demand of the task by increasing the weight of lifted load by 3 5 kg only increased trunk bending stiffness 55 over the low range of lumbar flexion and under the low stability demand condition our results suggest a non linear relationship between changes in stability and equilibrium demands of a task and the contribution of tns to trunk bending stiffness specifically alterations in tns response to changes in stability and equilibrium demand of a given task will increase stiffness of the trunk only if the background stiffness is low

a database of lumbar spinal mechanical behavior for validation of spinal analytical models data from two experimental studies with eight specimens each of spinal motion segments and or intervertebral discs are presented in a form that can be used for comparison with finite element model predictions the data include the effect of compressive preload 0 250 and 500n with quasistatic cyclic loading 0 0115hz and the effect of loading frequency 1 0 1 0 01 and 0 001hz with a physiological compressive preload mean 642n specimens were tested with displacements in each of six degrees of freedom three translations and three rotations about defined anatomical axes the three forces and three moments in the corresponding axis system were recorded during each test linearized stiffness matrices were calculated that could be used in multi segmental biomechanical models of the spine and these matrices were analyzed to determine whether off diagonal terms and symmetry assumptions should be included these databases of lumbar spinal mechanical behavior under physiological conditions quantify behaviors that should be present in finite element model simulations the addition of more specimens to identify sources of variability associated with physical dimensions degeneration and other variables would be beneficial supplementary data provide the recorded data and matlab codes for reading files linearized stiffness matrices derived from the tests at different preloads revealed few significant unexpected off diagonal terms and little evidence of significant matrix asymmetry

zero vs one dimensional parametric vs non parametric and confidence interval vs hypothesis testing procedures in one dimensional biomechanical trajectory analysis biomechanical processes are often manifested as one dimensional 1d trajectories it has been shown that 1d confidence intervals cis are biased when based on 0d statistical procedures and the non parametric 1d bootstrap ci has emerged in the biomechanics literature as a viable solution the primary purpose of this paper was to clarify that for 1d biomechanics datasets the distinction between 0d and 1d methods is much more important than the distinction between parametric and non parametric procedures a secondary purpose was to demonstrate that a parametric equivalent to the 1d bootstrap exists in the form of a random field theory rft correction for multiple comparisons to emphasize these points we analyzed six datasets consisting of force and kinematic trajectories in one sample paired two sample and regression designs results showed first that the 1d bootstrap and other 1d non parametric cis were qualitatively identical to rft cis and all were very different from 0d cis second 1d parametric and 1d non parametric hypothesis testing results were qualitatively identical for all six datasets last we highlight the limitations of 1d cis by demonstrating that they are complex design dependent and thus non generalizable these results suggest that i analyses of 1d data based on 0d models of randomness are generally biased unless one explicitly identifies 0d variables before the experiment and ii parametric and non parametric 1d hypothesis testing provide an unambiguous framework for analysis when one s hypothesis explicitly or implicitly pertains to whole 1d trajectories

effects of emg processing on biomechanical models of muscle joint systems sensitivity of trunk muscle moments spinal forces and stability biomechanical models are in use to estimate parameters such as contact forces and stability at various joints in one class of these models surface electromyography emg is used to address the problem of mechanical indeterminacy such that individual muscle activation patterns are accounted for unfortunately because of the stochastical properties of emg signals emg based estimates of muscle force suffer from substantial estimation errors recent studies have shown that improvements in muscle force estimation can be achieved through adequate emg processing specifically whitening and high pass hp filtering of the signals the aim of this paper is to determine the effect of such processing on outcomes of a biomechanical model of the lumbosacral joint and surrounding musculature goodness of fit of estimated muscle moments to net moments and also estimated joint stability significantly increased with increasing cut off frequencies in hp filtering whereas no effect on joint contact forces was found whitening resulted in moment estimations comparable to those obtained from optimal hp filtering with cut off frequencies over 250 hz moreover compared to hp filtering whitening led to a further increase in estimated joint stability based on theoretical models and on our experimental results we hypothesize that the processing leads to an increase in pick up area this then would explain the improvements from a better balance between deep and superficial motor unit contributions to the signal

critical statistics in quantum chaos and calogero sutherland model at finite temperature we investigate the spectral properties of a generalized gaussian orthogonal ensemble capable of describing critical statistics the joint distribution of eigenvalues of this model is expressed as the diagonal element of the density matrix of a gas of particles governed by the calogero sutherland cs hamiltonian taking advantage of the correspondence between cs particles and eigenvalues and utilizing a recently conjectured expression by kravtsov and tsvelik for the finite temperature density density correlations of the cs model we show that the number variance of our random matrix model is asymptotically linear with a slope depending on the parameters of the model such linear behavior is a signature of critical statistics this random matrix model may be relevant for the description of spectral correlations of complex quantum systems with a self similar or fractal poincare section of its classical counterpart this is shown in detail for two examples the anisotropic kepler problem and a kicked particle in a well potential in both cases the number variance and the delta 3 statistic are accurately described by our analytical results

solutions to homogeneous monge ampere equations of homothetic functions and their applications to production models in economics in economics homothetic functions are production functions whose marginal technical rate of substitution is homogeneous of degree zero mathematically a homothetic function is a function of the form f x f h x1 xn where f is a monotonically increasing function and h is a homogeneous function of any degree d 0 in this paper we classify homothetic functions satisfying the homogeneous monge ampere equation several applications to production models in economics will also be given

strategies used to stabilize the elbow joint challenged by inverted pendulum loading abstract the stiffness of activated muscles may stabilize a loaded joint by preventing perturbations from causing large displacements and injuring the joint here the elbow muscle recruitment patterns were compared with the forearm loaded vertically a potentially unstable inverted pendulum configuration and with horizontal loading eighteen healthy subjects were studied with the forearm vertical and supinated and the elbow flexed approximately 90 in the first experiment emg electrodes recorded activity of biceps triceps and brachioradialis muscles for joint torques produced a by voluntarily exerting a horizontal force isometrically b by voluntarily flexing and extending the elbow while the forearm was loaded vertically with 135 n the relationship between the emg and the torque generated was quantified by the linear regression slope and zero torque intercept in a second experiment a vertical load increasing linearly with time up to 300 n was applied in experiment 1 the emg torque relationships for biceps and triceps had an intercept about 10 of maximum voluntary effort greater with the vertical compared to the horizontal force the inverse was found for brachioradialis but the emg torque slopes for both agonist and antagonistic muscles were not different in experiment 2 there were 29 trials with minimal elbow displacement and all the three muscles activated on the order of 11 of maximum activation to stabilize the elbow 19 trials had small elbow extension and 14 trials small flexion requiring altered muscle forces for equilibrium 7 trials ended in large unstable displacement or early termination of the test an analysis indicate that the observed levels of muscle activation would only provide stability if the muscles short range stiffness was at the high end of the published range hence the elbow was marginally stable the stability analysis also indicated that the small elbow extension increased stability and flexion decreased stability

a direct comparison of spine rotational stiffness and dynamic spine stability during repetitive lifting tasks stability of the spinal column is critical to bear loads allow movement and at the same time avoid injury and pain however there has been a debate in recent years as to how best to define and quantify spine stability with the outcome being that different methods are used without a clear understanding of how they relate to one another therefore the goal of the present study was to directly compare lumbar spine rotational stiffness calculated with an emg driven biomechanical model to local dynamic spine stability calculated using lyapunov analyses of kinematic data during a series of continuous dynamic lifting challenges twelve healthy male subjects performed 30 repetitive lifts under three varying load and three varying rate conditions with an increase in the load lifted constant rate there was a significant increase in mean maximum and minimum spine rotational stiffness po 0 001 and a significant increase in local dynamic stability po 0 05 both stability measures were moderately to strongly related to one another r 0 55 to 0 71 with an increase in lifting rate constant load there was also a significant increase in mean and maximum spine rotational stiffness po 0 01 however there was a non significant decrease in the minimum rotational stiffness and a non significant decrease in local dynamic stability p40 05 weak linear relationships were found for the varying rate conditions r 0 02 to 0 27 the results suggest that spine rotational stiffness and local dynamic stability are closely related to one another as they provided similar information when movement rate was controlled however based on the results from the changing lifting rate conditions it is evident that both models provide unique information and that future research is required to completely understand the relationship between the two models using both techniques concurrently may provide the best information regarding the true effects of in stability under different loading and movement scenarios and in comparing healthy and clinical populations

geometric classifications of homogeneous production functions in this paper we completely classify homogeneous production functions with an arbitrary number of inputs whose production hypersurfaces are flat as an immediate consequence we obtain a complete classification of homogeneous production functions with two inputs whose production surfaces are developable

effect of low pass filtering on joint moments from inverse dynamics implications for injury prevention analyses of joint moments are important in the study of human motion and are crucial for our understanding of e g how and why acl injuries occur such analyses may be affected by artifacts due to inconsistencies in the equations of motion when force and movement data are filtered with different cut off frequencies the purpose of this study was to quantify the effect of these artifacts and compare joint moments calculated with the same or different cut off frequency for the filtering of force and movement data 123 elite handball players performed sidestep cutting while the movement was recorded by eight 240 hz cameras and the ground reaction forces were recorded by a 960 hz force plate knee and hip joint moments were calculated through inverse dynamics with four different combinations of cut off frequencies for signal filtering movement 10 hz force 10 hz 10 10 movement 15 hz force 15 hz movement 10 hz force 50 hz 10 50 movement 15 hz force 50 hz the results revealed significant differences especially between conditions with different filtering of force and movement mean sd peak knee abduction moment for the 10 10 and 10 50 condition were 1 27 0 53 and 1 64 0 68 nm kg respectively ranking of players based on knee abduction moments were affected by filtering condition out of 20 players with peak knee abduction moment higher than mean 1 sd with the 10 50 condition only 11 were still above mean 1 sd when the 10 10 condition was applied hip moments were very sensitive to filtering cut off mean sd peak hip flexion moment was 3 64 0 75 and 5 92 1 80 under the 10 10 and 10 50 conditions respectively based on these findings force and movement data should be processed with the same filter conclusions from previous inverse dynamics studies where this was not the case should be treated with caution

competitive cluster growth in complex networks in this work we propose an idealized model for competitive cluster growth in complex networks each cluster can be thought of as a fraction of a community that shares some common opinion our results show that the cluster size distribution depends on the particular choice for the topology of the network of contacts among the agents as an application we show that the cluster size distributions obtained when the growth process is performed on hierarchical networks e g the apollonian network have a scaling form similar to what has been observed for the distribution of a number of votes in an electoral process we suggest that this similarity may be due to the fact that social networks involved in the electoral process may also possess an underlining hierarchical structure

periodic orbits contribution to the 2 point correlation form factor for pseudo integrable systems using heuristic arguments based on the trace formulas we relate the 2 point correlation form factor k2 at small values of with sums over classical periodic orbits for typical examples of pseudo integrable systems the later sums have been explicitly calculated for the following models i plane billiards in the form of right triangles with one angle n and ii rectangular billiards with the aharonov bohm flux line in the first model using the properties of the veech structure it is shown that k2 0 n e n 3 n 2 where e n 0 for odd n e n 2 for even n not divisible by 3 and e n 6 for even n divisible by 3 for completeness we also recall informally the main features of the veech construction in the second model the answer depends on arithmetical properties of ratios of flux line coordinates to the corresponding sides of the rectangle when these ratios are non commensurable irrational numbers k2 0 1 3 where is the fractional part of the flux through the rectangle when and it is symmetric with respect to the line when the comparison of these results with numerical calculations of the form factor is discussed in detail the above values of k2 0 differ from all known examples of spectral statistics thus confirming analytically the peculiarities of statistical properties of the energy levels in pseudo integrable systems

segment interaction analysis of the stance limb in sprint running a high angular velocity of the thigh of the stance limb generated by hip extensor musculature is commonly thought to be a performance determining factor in sprint running however the thigh segment is a component of a linked system i e the lower limb therefore it is unlikely that the kinematics of the thigh will be due exclusively to the resultant joint moment rjm at the hip the purpose of this study was to quantify by means of segment interaction analysis the determinants of sagittal plane kinematics of the lower limb segments during the stance phase of sprint running video and ground reaction force data were collected from four male athletes performing maximal effort sprints the analysis revealed that during the first third of the stance phase a hip extension moment was the major determinant of the increasing angular velocity of the thigh however during the mid third of stance hip and knee extension moments and segment interaction effects all contributed to the thigh attaining its peak angular velocity extension moments at the ankle and to a lesser extent the knee were attributed with preventing the collapse of the shank under the effects of the interactive moment due to ground reaction force the angular acceleration of the foot was determined almost completely by the rjm at the ankle and the interactive moment due to ground reaction force further research is required to determine if similar results exit for a wide range of athletes and for other stages of a sprint race e g early acceleration maximal velocity and deceleration phases

comparison of muscle forces and joint load from an optimization and emg assisted lumbar spine model towards development of a hybrid approach abstract the purpose of this study was to determine whether the same estimates of individual muscle and l4 l5 lumbar joint compressive forces result from an optimization opt compared to an electromyography emg assisted approach for solving the inderminate moment equilibrium equations in the same anatomical model four male subjects performed near maximum isometric ramp efforts in trunck flexion extension and lateral bending in a testing apparatus the emg approach was sensitive to subject and trial differences in the magnitudes of individual muscle forces needed to produce the same reaction moment in contrast the opt method converged on a similar estimate of muscle forces for all subjects and trials producing the same moment the opt method predicted lower l4 l5 joint compression values on average by 32 43 and 23 in trunk extension flexion and lateral bending respectively because unlike the emg method it could not predict co contraction of anatomically antagonistic muscles we incorporated the opt method s advantage of forcing an equilibrium in the reaction moments into the emg method in a new approach we have called emg assisted optimization emgao muscle force estimates from the emg and emgao methods differed from those from the opt method on average by 123 rms for flexion and extension and by 218 for lateral bends data from the two approaches result in different conclusions about spine mechanics we have more confidence in the emg assisted methods because they respond to variation in muscle synergy and co contraction patterns commonly observed in different trials and subjects for the same reaction moments

on pseudo homogeneous triangular norms triangular conorms and proper uninorms in this paper quasi homogeneous t norms are generalized and then pseudo homogeneous t norms are introduced and characterized it is shown that only some particular strict t norms and the minimum t norm t m are pseudo homogeneous the concepts of pseudo homogeneous t conorms and proper uninorms are given as well different from t norms only the maximum t conorm s m is pseudo homogeneous and any proper uninorm is not pseudo homogeneous

the simulation of aerial movement ii a mathematical inertia model of the human body a mathematical inertia model which permits the determination of personalized segmental inertia parameter values from anthropometric measurements is described the human body is modelled using 40 geometric solids which are specified by 95 anthropometric measurements a stadium solid is introduced for modelling the torso segments using perimeter and width measurements this procedure is more accurate than the use of elliptical discs of given width and depth and permits a smaller number of such solids to be used inertia parameter values may be obtained for body models of up to 20 segments errors in total body mass estimates from this and other models are discussed with reference to the unknown lung volumes

statistical energy analysis modelling of complex structures as coupled sets of oscillators ensemble mean and variance of energy expressions are derived for the ensemble means and variances of the subsystem energies of built up systems comprising two subsystems the approach is based on the statistical energy analysis of two spring coupled oscillators and sets of oscillators or coupled continuous subsystems described by mace and ji the statistical energy analysis of coupled sets of oscillators proceedings of the royal society a 1824 2007 the paper focuses on spring coupling although similar results hold for more general forms of conservative coupling randomness is introduced into the system by assuming that the natural frequency spacings in each subsystem conform to certain statistical distributions a coupling coefficient parameter is introduced which together with the coupling strength parameter defined by mace and ji 2007 accounts for the statistics of the coupling stiffness various approximations and assumptions are made it is seen that the variance of the excited subsystem depends primarily on the variance of the input power which in turn depends on the variance of the number of modes of the excited subsystem in the frequency band of excitation and their mode shapes the variance of the undriven subsystem on the other hand depends primarily on the variance of the intermodal coupling coefficients which in turn depend on the variances of the number of in band modes of both subsystems and their mode shapes the cases of poisson and gaussian orthogonal ensemble natural frequency spacing statistics are considered numerical examples of two plates coupled by one or a number of springs are presented

the generic modeling fallacy average biomechanical models often produce non average results computational biomechanics models constructed using nominal or average input parameters are often assumed to produce average results that are representative of a target population of interest to investigate this assumption a stochastic monte carlo analysis of two common biomechanical models was conducted consistent discrepancies were found between the behavior of average models and the average behavior of the population from which the average models input parameters were derived more interestingly broadly distributed sets of non average input parameters were found to produce average or near average model behaviors in other words average models did not produce average results and models that did produce average results possessed non average input parameters these findings have implications on the prevalent practice of employing average input parameters in computational models to facilitate further discussion on the topic the authors have termed this phenomenon the generic modeling fallacy the mathematical explanation of the generic modeling fallacy is presented and suggestions for avoiding it are provided analytical and empirical examples of the generic modeling fallacy are also given

intermediate statistics in the regime of mixed classical dynamics we explain the arguments in support of the berry robnik 1984 17 2413 picture of the energy level statistics in the asymptotic strict far semiclassical limit of sufficiently small where the entire energy spectrum can be represented as a statistically independent superposition of regular and irregular level sequences the regular ones obeying the poissonian statistics and the irregular ones the rmt statistics goe or gue we generalize the results to describe not only the level spacing distribution the number variance and the delta statistics but also arbitrary statistics the probability to find k levels inside an interval of length l after unfolding very useful and effective approximations for are described we demonstrate very clearly that this regime is excellently described by this picture for k as high as the outer energy scale even when taking into account the regular component and only one the dominant chaotic component this we show numerically for the compactified standard map and for the quartic 2d billiard

human ankle joint stiffness over the full range of muscle activation levels abstract system identification techniques have been used to track changes in dynamic stiffness of the human ankle joint over a wide range of muscle contraction levels subjects lay supine on an experimental table with their left foot encased in a rigid low inertia cast which was fixed to an electro hydraulic actuator operating as a position servo subjects generated tonic plantarflexor or dorsiflexor torques of different magnitudes ranging from rest to maximum voluntary contractions mvc during repeated presentations of a stochastic ankle angular position perturbation compliance impulse response functions irf were determined from every 2 5 s perturbation sequence the gain g natural freqency n and damping parameters of the second order model providing the best fit to each irf were determined and used to compute the corresponding inertial i viscous b and elastic k stiffness parameters the behaviour of these parameters with mean torque was found to follow two simple rules first the elastic parameter k increased in proportion to mean ankle torque as it was varied from rest to mvc these changes were considerable involving increases of more than an order of magnitude second the damping parameter remained almost invariant over the entire range of contractions despite the dramatic changes in k

does the personal lift assist device affect the local dynamic stability of the spine during lifting abstract the personal lift assist device plad is an on body ergonomic aid that reduces low back physical demands through the restorative moment of an external spring element which possesses a mechanical advantage over the erector spinae although the plad has proven effective at reducing low back muscular demand spinal moments and localized muscular fatigue during laboratory and industrial tasks the effects of the device on the neuromuscular control of spinal stability during lifting have yet to be assessed thirty healthy subjects 15m 15f performed repetitive lifting for three minutes at a rate of 10 lifts per minute with and without the plad maximum finite time lyapunov exponents representing short term max s and long term max l divergence were calculated from the measured trunk kinematics to estimate the local dynamic stability of the lumbar spine using a mixed design repeated measures anova it was determined that wearing the plad did not significantly change max s np 0 335 p 0 321 p 0 225 but did significantly reduce max l np 0 0024 p 0 0011 p 0 014 2 0 197 there were no between subject effects of sex or significant interactions p 0 720 the present results indicated that max s was not statistically different between the device conditions but that the plad significantly reduced max l to a negative stable value this shows that subjects neuromuscular systems were able to respond to local perturbations more effectively when wearing the device reflecting a more stable control of spinal movements these findings are important when recommending the plad for long term industrial or clinical use

computation of trunk stability in forward perturbations effects of preload perturbation load initial flexion and abdominal preactivation spine stability demand influences active passive coordination of the trunk response especially during sudden perturbations the objective of this study was to look at the role of passive stationary active and reflexive subsystems on spinal stability spine stability was evaluated here during pre and post perturbation phases by computing the minimum i e critical muscle stiffness coefficient required to maintain stability the effects of pre perturbation conditions preloading initial posture and abdominal antagonistic coactivation as well as perturbation magnitude were studied results revealed that higher preload initially flexed trunk posture and abdominal pre activation enhanced pre perturbation stiffness and stability in contrast to the preload however larger sudden load initial flexion and abdominal preactivation significantly increased post perturbation stability margin as a result much lower critical muscle stiffness coefficient was required post perturbation compared to the pre perturbation phase the trunk stiffness and stability substantially increased post perturbation demanding thus a much lower critical muscle stiffness coefficient overall these findings highlight the crucial role of the ligamentous spine and muscles in both passive and active states in augmenting the trunk stiffness and hence stability during pre and post perturbation phases a role much evident in the presence of initial trunk flexion