Search

The quality of attractor reconstruction using the method of delays is known to be sensitive to the delay parameter, τ. Here we develop a new, computationally efficient approach to choosing τ that quantifies reconstruction expansion from the identity line of the embedding space. We show that reconstruction expansion is related to the concept of reconstruction signal strength and that increased expansion corresponds to diminished effects of measurement error. Thus, reconstruction expansion represents a simple, geometrical framework for choosing τ. Furthermore, we describe the role of dynamical error in attractor expansion and argue that algorithms for determining τ should be considered as attempts at estimating an upper bound to the optimal delay.

Detecting the presence of chaos in a dynamical system is an important problem that is solved by measuring the largest Lyapunov exponent. Lyapunov exponents quantify the exponential divergence of initially close state-space trajectories and estimate the amount of chaos in a system. We present a new method for calculating the largest Lyapunov exponent from an experimental time series. The method follows directly from the definition of the largest Lyapunov exponent and is accurate because it takes advantage of all the available data. We show that the algorithm is fast, easy to implement, and robust to changes in the following quantities: embedding dimension, size of data set, reconstruction delay, and noise level. Furthermore, one may use the algorithm to calculate simultaneously the correlation dimension. Thus, one sequence of computations will yield an estimate of both the level of chaos and the system complexity.

The average muscle fiber conduction velocity (CV) measured with multicontact surface electrodes has been reported to increase with the contraction force. To understand this behavior better, we studied the relationship between the recruitment threshold and the muscle fiber CV of single motor units (MUs). Myoelectric signals were recorded simultaneously with a linear surface electrode array and a selective needle electrode. From the signals detected by the meedle electrode, the discharges of single MUs were isolated by the decomposition technique. By using the firing of single MUs as the trigger point, we averaged the surface myoelectric signals and extracted the single MU action potentials from the interference surface signals. The CV of single MUs, calculated by a cross-correlation analysis, was higher for MUs recruited at higher contraction force. This result indicates that the larger MUs with higher muscle fiber CV contribute to increase the average CV during varying force contractions.

Twelve subjects were classified as left-handed (LH) or right-handed (RH) using Annett's hand dominance classification. Motor unit recordings were obtained from the first dorsal interosseous (FDI) muscle of each hand using a quadrifilar needle electrode. Firing occurrences of individual motor units were then identified and the firing rates of all motor units recorded during the contraction were cross-correlated. The results demonstrated significantly greater firing rate cross-correlation scores in the dominant hand than in the non-dominant hand for both LH and RH subjects. This association between hand dominance and the common drive of motor unit firing rates lends credence to the idea that one or more CNS sites may influence conjoint motor unit firing behavior.

Muscle contractions are modulated by the number of motor units recruited and their respective firing rates. The work described in this report documents an interplay between recruitment and firing rates of motor units. The recruitment of a new motor unit appears to have a disfacilitatory influence on the firing rates of previously activated motor units. It is speculated that this effect is likely to be mediated, at least partially, via the stretch reflex loop and possibly by the recurrent inhibition of the Renshaw circuit. Such a mechanism would be functionally useful in providing smooth control of muscle output via peripheral circuitry (consisting of proprioceptive reflexes and recurrent inhibition), thus lessening the amount of detailed supervision of the alpha-motoneuron pool required by the central nervous system.

Resume Les signaux myoelectriques (ME) ont ete enregistres en meme temps avec des electrodes de surface et une electrod-aiguille bipolaire au niveau des fibres medianes du deltoide. Onze sujets ont realise des contractions isometriques soutenues a des niveaux de force constants de 25%, 50% et 75% de la contraction volontaire maximale (MVC). La valeur moyenne rectifiee, la valeur de la racine carree moyenne (rms) et le rapport du voltage AC au voltage DC des signaux ME rectifies ont ete calcules. Ces expressions theoriques sont comparees aux resultats experimentaux des parametres pour determiner les effets de 5 correlats physiologiques du muscle en cours de contraction sur le signal ME. Les signaux enregistres avec electrode-aiguilles bipolaires au cours de la contraction isometrique souteneu a force constante s'averent etre principalement affectes par une diminution du taux de decharges des unites motrices. Cet effet provoque une diminution de la moyenne des valeurs rectifiees et des valeurs rms en fonction du temps de contraction. Les signaux ME enregistres avec des electrodes de surface sont egalement affectes par la diminution du taux de decharges; cependant, les valeurs moyennes rectifiees et les valeurs rms augmentent avec le temps de contraction. Cette augmentation est plus vraisemblablement due a l'effet d'une diminution de la vitesse de conduction des fibres musculaires sur les formes des potentiels d'action unitaires moteurs (MUAPs) au cours de la contraction soutenue. La diminution de la vitesse de conduction provoque une augmentation des composantes de basses frequences des MUAPs. A partir de la, un signal ME de plus grande energie passe au travers de l'effet filtre passebas des tissus corporels. Les resultats obtenus au moyen d'electrodes de surface sont egalement plus constants que ceux obtenus avec les electrodes-aiguilles bipolaires. Le recrutement avec le temps des unites motrices est egalement pris en consideration. Le rapport du voltage AC au voltage DC du signal rectifie est analyse. Ce parametre elimine les effets de modifications de formes des MUAPs. La valeur initiale des signaux ME enregistres avec electrodes de surface est a peu pres egale a la valeur d'une variable d'amplitude Gaussienne. Au fur et a mesure que le temps de contraction augmente, le rapport des voltages augmente. Ceci indique que le signal ME ne peut pas etre considere comme un processus Gaussien tout au long de la contraction. L'augmentation de ce parametre peut resulter du recrutement d'unites motrices et/ou de synchronisation. L'augmentation du rapport de voltages est tres grande pour des contractions realisees a 75% MVC alors que le recrutement lie au temps est moins susceptible de survenir, indiquant que la synchronisation peut se produire a ce niveau de force. Cette deduction trouve egalement un argument dans le comportement de la valeur rms du signal ME enregistre au moyen d'electrodes-aiguilles bipolaires. Les resultats de cette investigation concordent avec les deux hypotheses suivantes: dans le deltoide, les effets de la synchronisation des unites motrices deviennent plus prononces a des niveaux eleves de force (75% MVC); le recrutement, lie au temps, des unites motrices n'est pas important au cours de contractions isometriques a force constante realisees a moins de 50% MVC.