No description available.

No description available.

Frog sartorius muscles were illuminated with laser light (λ = 457.9 - 632.8nm), stimulated electrically and stretched at the plateau of an isometric tetanus. An electro-optic system was used to rapidly (~7kHz) switch the electric vector of the incident beam through π/2 radians (between φ = 0° and 90°, relative to the muscle long axis) and 'simultaneous' recordings of transmitted light intensity (Ia) were made at orthogonal beam orientations during single contractions, using a sample-hold device. Stretch causes Ia to fall: this is represented either as a decrease of transparency or an increase of turbidity (τ). The amplitudes of the optical transients vary in direct proportion to the tension increment generated by stretch (which is related to the extent of actin-myosin filament overlap) and are highly anisotropic with respect to both λ and φ. At any given λ the amplitude for φ = 0° > φ - 90°. Both 0° and 90° signals vary inversely with λ: the wavelength exponent for the former is -2.39 and for the latter, -3.87. An attempt is made to analyse the changes in conservative dichroism (= Δτ0° - Δτ90°) using a model in which the scattering elements are forced to undergo a change of angular orientation (Δγ). The size of the scattering particles is estimated to be ~17nm (long axis) and to occupy ~0.038 of the fibre volume. It is postulated that the dichroic signal is due to a change in cross-bridge head (= S-1 HMM) orientation. The linear dimension of the actin-myosin interactive surface is estimated (from Δγ) to be >4.8nm. The results are interpreted in terms of a multi-step force generating cycle, based on the Huxley-Simmons model in which each head progresses through as many as 5 to 7 discrete positions during its working stroke, each separated from its neighbour by a potential difference energy of ~0.46x10-20J.

No description available.

No description available.

The underlying principles of how the nervous system selects specific muscle activation pattern, among many that produce the same movement, remain unknown. Experimental studies suggest that the nervous system may use fixed groups of muscles, referred to as muscle synergies, to produce functional motor outputs relevant to the task. In contrast, predictions from biomechanical models suggest that minimizing muscular effort may be the criteria how a muscle coordination pattern is organized for muscle synergies. However, both experimental and modeling evidence shows that stability, as well as energetic efficiency, also needs to be considered. Based on the hypothesis that the nervous system uses functionally stable muscle activation pattern for a muscle synergy, we investigated the stability of muscle patterns using a neuromechanical model of the cat hindlimb. Five unique muscle patterns that generate each of the five experimentally-identified muscle synergy force vectors at the endpoint were found using a minimum-effort criterion. We subjected the model to various perturbed conditions and evaluated functional stability of each of the five minimum-effort muscle synergies using a set of empirical criteria derived from experimental observations. Results show that minimum-effort muscle synergies can be functionally stable or unstable, suggesting that minimum-effort criterion is not always sufficient to predict physiologically relevant postural muscle synergies. Also, linearized system characteristics can robustly predict the behavior exhibited by fully dynamic and nonlinear biomechanical simulations. We conclude that functional stability, which assesses stability of a biomechanical system in a physiological context, must be considered when choosing a muscle activation pattern for a given motor task.

Thesis (Ph. D.)--Oregon State University, 2006. / Blank pages 143 and 159 not microfilmed. Includes bibliographical references. Also available online (PDF file) by a subscription to the set or by purchasing the individual file.

Thesis (Ph. D.)--Oregon State University, 2006. / Blank pages 143 and 159 not microfilmed. Includes bibliographical references.

The response patterns of human stretch receptors in the finger extensor muscles of the forearm were studied using the microneurography technique. Single-unit recordings were obtained from one-hundred and twenty-four afferents. A procedure was developed to classify the units in muscle spindle primary afferents, secondary afferents, and Golgi tendong organ afferents. The procedure allows an objective and reproducible classification on the basis of the afferents’ responses to a series of tests which individually are non-conclusive. It was demonstrated that maximal twitch contractions can be elicited in the finger extensor muscles of the forearm, without causing undue discomfort to the subjects, or hazarding the single-unit recording. The response of the units to this test allowed, in most cases but not always, a separation in muscle spindle and tendon organ afferents. Thus the test was not adequate for an unequivocal classification. Three discrete response parameters were extracted from ramp-and-hold stretches, viz. the presence or absence of an initial burst and a deceleration response, and prompt silencing at slow muscle shortening. The distributions of the parameters were significantly different among the three unit types. These parameters which were pair-wise independent constituted a set of considerable discriminative power. It was shown that human muscle spindles have about the same static position sensitivity to fractional muscle stretch as previously found in animals. Stretch sensitization was demonstrated by rapid, repeated stretches of the muscle which enhanced the réponse to subsequent slow stretches of muscle spindles. Sensitization was different with primary and secondary muscle spindle afferents whereas Golgi tendon organ afferents never displayed stretch sensitization. One-to-one driving with small-amplitude sinusoidal stretches superimposed on ramp-and- hold stretches was almost exclusively seen with primary muscle spindle afferents, whereas secondaries seldom and tendon organ afferents never displayed driving. The afferent responses during slowly increasing isometric contractions and rapid relaxations were analysed. An increased discharge rate on relaxation was common among spindle afferents whereas it was never seen in tendon organs afferents. Two separate groups of spindles afferents were found with regard to fusimotor recruitment. The largest group was recruited at rather low and variable contractile forces whereas the smaller group was not recruited at all. The proportions of the three unit types, spindle primary, spindle secondary, and Golgi tendon organ afferents were estimated from a preliminary classification and the distribution of the eight response features were analyzed for each class of afferents. On the basis of these estimates and the response pattern of the individual unit Bayes’ theorem was used to calculate the probabilities that the unit was a spindle primary, a spindle secondary, or a tendon organ afferent. Estimates indicate that about 19 out of 20 muscle afferents are correctly classified when all eight features are analyzed. / <p>Diss. (sammanfattning) Umeå : Umeå universitet, 1988, härtill 6 uppsatser.</p> / digitalisering@umu