The inverse dynamics problem of neuromuscular control

The myoskeletal inverse dynamics problem and the myocybernetic control inverse problem were investigated with respect to their ill-posedness. The first problem consists of finding from observed experimental motion and reaction force data the resultant muscle moments that generated the observed motion, while the second aims at finding the corresponding neural controls. It is shown that both problems belong to the class of incorrectly posed (ill-posed) problems that, by definition, do not possess unique solutions. To illustrate this point, results of a forward dynamics simulation of a comprehensive neuromusculoskeletal model of the human body are presented. These results demonstrate that fairly chaotic neural control perturbations have very little influence on the resulting motion trajectory, at least in the present example. While a regularization procedure may be applied to solve successfully the myoskeletal inverse dynamics problem, the myocybernetic control inverse problem is unsolvable. The latter fact has the important implication that, based on the somatosensory inputs it receives, the pars intermedia in the cerebellum is not able to control individual motor unit stimulation rates and recruitment patterns but only whole muscles by means of a single compound signal. The latter signal is identified as the “common drive.” Presumably at the spinal level, special neural circuits are used to decompose the common drive signal into motor unit recruitment patterns and stimulation rates that are specific for a given mode of contraction and probably obey certain optimality principles. Received: 26 February 1999 / Accepted in revised form: 11 June 1999     

A fundamental problem of consent.

A consultant anaesthetist gave a diclofenac suppository for postoperative pain to a patient having four teeth extracted under general anaesthesia in the dental surgery. He did not seek the patient''s specific consent preoperatively for use of the suppository but told her afterwards what he had done. Charged before the professional conduct committee of the General Medical Council with failure to obtain informed consent and assault, the anaesthetist was found guilty of serious professional misconduct and admonished. This decision has far reaching implications and has caused great concern.     

Dynamical properties of model gene networks and implications for the inverse problem

We study the inverse problem, or the "reverse-engineering" problem, for two abstract models of gene expression dynamics, discrete-time Boolean networks and continuous-time switching networks. Formally, the inverse problem is similar for both types of networks. For each gene, its regulators and its Boolean dynamics function must be identified. However, differences in the dynamical properties of these two types of networks affect the amount of data that is necessary for solving the inverse problem. We derive estimates for the average amounts of time series data required to solve the inverse problem for randomly generated Boolean and continuous-time switching networks. We also derive a lower bound on the amount of data needed that holds for both types of networks. We find that the amount of data required is logarithmic in the number of genes for Boolean networks, matching the general lower bound and previous theory, but are superlinear in the number of genes for continuous-time switching networks. We also find that the amount of data needed scales as 2(K), where K is the number of regulators per gene, rather than 2(2K), as previous theory suggests.     

The inverse problem for DNA

The DNA distribution function for a given section of DNA of a given length depends on the nucleotide composition. It can be expressed in terms of other functions, such as specific heat, optical density, etc., if they are measured in the vicinity of the DNA melting point.     

Static optimal estimation of joint accelerations for inverse dynamics problem solution

In inverse dynamics computations, the accuracy of the solution strongly depends on the accuracy of the input data. In particular, estimated joint moments are highly sensitive to uncertainties in acceleration data. The aim of the present work was to improve classical inverse dynamics computations by providing an accurate estimation of accelerations. Accelerations are usually calculated from noise-polluted position data using numerical double differentiation, which amplifies measurement noise. The objective of the present paper is to use all available imperfect position and force measurements to extract optimum acceleration estimations. A weighted least-squares optimisation approach is used to provide optimal acceleration distributions most consistent with position and force data, and which account for the propagation of measurement uncertainties. The task chosen for comparing the solution methodology with other classical methods is a typical experimental postural movement, consisting in upper limb swings from an upright stance. The proposed method delivers a set of optimal accelerations well consistent with all available measurements. It also leads to an accurate prediction of ground reactions and it produces no residual moment at the top-most segment.     

The inverse relationship between protein dynamics and thermal stability

Protein powders that are dehydrated or mixed with a glassy compound are known to have improved thermal stability. We present elastic and quasielastic neutron scattering measurements of the global dynamics of lysozyme and ribonuclease A powders. In the absence of solvation water, both protein powders exhibit largely harmonic motions on the timescale of the measurements. Upon partial hydration, quasielastic scattering indicative of relaxational processes appears at sufficiently high temperature. When the scattering spectrum are analyzed with the Kohlrausch-Williams-Watts formalism, the exponent beta decreases with increasing temperature, suggesting that multiple relaxation modes are emerging. When lysozyme was mixed with glycerol, its beta values were higher than the hydrated sample at comparable temperatures, reflecting the viscosity and stabilizing effects of glycerol.     

Hibernation dynamics of Oryzophagus oryzae and its implications for management

The hibernation dynamics of Oryzophagus oryzae (Costa Lima) (Coleoptera, Curculionidae) was studied in a bamboo stand, in Eldorado do Sul district (30°02S, 51°23WGR), RS, Brazil. From February 1990 to February 1991, a total of 12057 insects were sampled in the litter of the study area (80 m×2 m), following a stratified random sampling programme. Four major phases were recognized in the phenology of the hibernating population: (1) a period of arrival at the hibernation site (February-March); (2) a settling period at the site; (3) a period of departure from the hibernation site (August-November); and (4) a residual period. Thus, diapause lasted about 7–8 months. The total mortality rate was 42% and a high rate of infection by Beauveria bassiana (Bals.) Vuill. was recorded among the dead insects (43%). Microclimatic profiles showed a narrower interval of variation of the air temperature and relative humidity in the bamboo litter than in the open field, hence characterizing the site as a refuge for hibernation. The concentration of high densities of hibernating adults in small areas, the long duration of diapause and the high natural occurrence of B. bassiana make the hibernation period a very appropriate and recommendable time for control of this weevil.     

Some fundamental aspects of information dynamics

Zusammenfassung Mechanisierte Sprachübersetzung, Speichern von Information und Mustererkennung stellen drei Sonderfälle der allgemeinen Operationen Lernen, Ordnen und Erkennen dar. Die Automatisierung solcher Operationen bildet nicht nur ein an und für sich interessantes Problem, sondern eine dringliche Notwendigkeit, sollen die universellen Gro Brechenanlagen nicht an der Komplexität ihrer inneren Organisation (soft-ware) ersticken. Als vielleicht einfachstes dieser drei — im Grunde nie getrennt auftretenden — Vorgänge, wird das Problem der Wiedererkennung betrachtet, d.h. dasjenige der Abbildung eines kontinuierlichen Ensembles reeller, metrischer Objekte auf eine endliche, diskrete Struktur ohne Metrik, wie sie beispielsweise das Innere einer digitalen Rechenanlage darstellt. Der notwendige Übergang vom heute einzig üblichen Speicher zum eigentlichen Gedächtnis hängt unter anderem von der Lösung dieses Problems ab. Seine Untersuchung hebt die grundsätzliche Bedeutung der Umgebung einzelner Objekte — ihres Kontextes — hervor, welche Umgebung im diskreten Raum das notwendige Äquivalent zum Kontinuum darstellt. Dies führt zu einer neuen Betrachtung des Paradoxon von Brillouin, und zur notwendigen Benützung dynamischer Methoden bei der Verarbeitung von Information, anders gesagt bei ihrer Überführung aus der bildhaft-kontinuierlichen Form in die diskrete, und zurück. Womit zumindest eine mögliche Definition nicht-trivialer Informationsverarbeitung gegeben ist.     

Snowshoe hare optimal foraging and its implications for population dynamics

The results of an optimal foraging model using linear programming with constraints for feeding time, digestive capacity, sodium requirements, and energy requirements indicate that snowshoe hare (Lepus americanus) may forage as energy maximizers. The solution provides the quantities of major food classes (leaves, herbs, fungus, twigs) included in the diet. The species composition of each diet class also is determined using a simultaneous search model based upon the probability of encounter, the probability of sufficient item size, and the probability of sufficient quality. The results also indicate that hare life history parameters (weaning size, size at first reproduction, average adult size) and potential demographic changes in hare populations may be controlled by foraging considerations.     

Physical parameter estimation from porcine ex vivo vocal fold dynamics in an inverse problem framework

This study presents a framework for a direct comparison of experimental vocal fold dynamics data to a numerical two-mass-model (2MM) by solving the corresponding inverse problem of which parameters lead to similar model behavior. The introduced 2MM features improvements such as a variable stiffness and a modified collision force. A set of physiologically sensible degrees of freedom is presented, and three optimization algorithms are compared on synthetic vocal fold trajectories. Finally, a total of 288 high-speed video recordings of six excised porcine larynges were optimized to validate the proposed framework. Particular focus lay on the subglottal pressure, as the experimental subglottal pressure is directly comparable to the model subglottal pressure. Fundamental frequency, amplitude and objective function values were also investigated. The employed 2MM is able to replicate the behavior of the porcine vocal folds very well. The model trajectories’ fundamental frequency matches the one of the experimental trajectories in \(98.6\%\) of the recordings. The relative error of the model trajectory amplitudes is on average \(9.5\%\). The experiments feature a mean subglottal pressure of 10.16 (SD \(= 2.31\)) \({\text {cmH}}_2{\text {O}}\); in the model, it was on average 7.61 (SD \(= 2.40\)) \({\text {cmH}}_2{\text {O}}\). A tendency of the model to underestimate the subglottal pressure is found, but the model is capable of inferring trends in the subglottal pressure. The average absolute error between the subglottal pressure in the model and the experiment is 2.90 (SD \(= 1.80\)) \({\text {cmH}}_2{\text {O}}\) or \(27.5\%\). A detailed analysis of the factors affecting the accuracy in matching the subglottal pressure is presented.     

The ion channel inverse problem: neuroinformatics meets biophysics

Ion channels are the building blocks of the information processing capability of neurons: any realistic computational model of a neuron must include reliable and effective ion channel components. Sophisticated statistical and computational tools have been developed to study the ion channel structure–function relationship, but this work is rarely incorporated into the models used for single neurons or small networks. The disjunction is partly a matter of convention. Structure–function studies typically use a single Markov model for the whole channel whereas until recently whole-cell modeling software has focused on serial, independent, two-state subunits that can be represented by the Hodgkin–Huxley equations. More fundamentally, there is a difference in purpose that prevents models being easily reused. Biophysical models are typically developed to study one particular aspect of channel gating in detail, whereas neural modelers require broad coverage of the entire range of channel behavior that is often best achieved with approximate representations that omit structural features that cannot be adequately constrained. To bridge the gap so that more recent channel data can be used in neural models requires new computational infrastructure for bringing together diverse sources of data to arrive at best-fit models for whole-cell modeling. We review the current state of channel modeling and explore the developments needed for its conclusions to be integrated into whole-cell modeling.     

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+1S D 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.     

Topology fingerprint approach to the inverse protein folding problem.

We describe the most general solution to date of the problem of matching globular protein sequences to the appropriate three-dimensional structures. The screening template, against which sequences are tested, is provided by a protein "structural fingerprint" library based on the contact map and the buried/exposed pattern of residues. Then, a lattice Monte Carlo algorithm validates or dismisses the stability of the proposed fold. Examples of known structural similarities between proteins having weakly or unrelated sequences such as the globins and phycocyanins, the eight-member alpha/beta fold of triose phosphate isomerase and even a close structural equivalence between azurin and immunoglobulins are found.     

Variability of 3D arterial geometry and dynamics,and its pathologic implications

Geometric parameters and features vary within the vasculature. Furthermore, at any given anatomic site, there are substantial variations in geometry among individuals. These variations can contribute to a corresponding variability in the hemodynamic environment and, to the extent that hemodynamics affects the atherosclerotic process, the progress of vascular disease. Measurements of the geometry and wall morphometry of post-mortem human coronary arteries demonstrate a relationship between these variables that supports the notion that geometric variations can contribute to a corresponding variability in the local rate of progression of arterial disease. The dynamic geometry of the coronary arteries also varies from site to site and among individuals, and this variability too may play a role in the epidemiology of coronary artery disease.     

Conclusions--scientific research and its implications.

The organizers of this meeting felt that it would be appropriate to have two short papers at the end of this volume to summarize the key points that are of relevance to scientists and the insurance and actuarial professions. This was felt to be of particular importance given the very heterogeneous backgrounds of members of the audience at the meeting, namely, scientific, medical, legal, social and financial. The objectives are to summarize the key issues in layman''s terms-trying to avoid jargon, exaggeration and a partisan approach. In some ways my task is somewhat easier than Chris Daykin''s job, since there are only four scientific and medical presentations compared with seven from the actuarial, social and legal side. I will organize this brief report, which by definition will involve repetition of some of the points made in the scientific and medical papers in this volume, into three areas: (1) introductory comments on demography and evolution relevant to human genetics; (2) the scientific opportunities; and (3) a brief comment on their implications for the health and care of individuals.