Richard Shiavi

Muscular synergism. II: A minimum-fatigue criterion for load sharing between synergistic muscles

P6new physiological criterion for muscular load sharing is developed. The criterion is based on the assumption that the endurance time of muscular contractions is maximized, hence muscular fatigue is minimized. The optimization problem is cast in the form of a linearly constrained, non-linear MINIMAX optimization. The new method predicts that: (1) there is synergistic muscle action, (2) muscle force increases non-linearly with external force (load), (3) relatively more force is allocated to muscles that have a large maximum force (large muscles), (4) relatively more force is allocated to muscles with a high percentage of slow-twitch fibers (muscles that are fatigue-resistant), (5) the load sharing does not depend on the moment arm of the muscles (although the absolute force levels do depend on this variable). The predicted load sharing between two cat muscles during standing and walking is in good agreement with direct force measurement data from the literature.

Acoustical properties of speech as indicators of depression and suicidal risk

Acoustic properties of speech have previously been identified as possible cues to depression, and there is evidence that certain vocal parameters may be used further to objectively discriminate between depressed and suicidal speech. Studies were performed to analyze and compare the speech acoustics of separate male and female samples comprised of normal individuals and individuals carrying diagnoses of depression and high-risk, near-term suicidality. The female sample consisted of ten control subjects, 17 dysthymic patients, and 21 major depressed patients. The male sample contained 24 control subjects, 21 major depressed patients, and 22 high-risk suicidal patients. Acoustic analyses of voice fundamental frequency (F/sub 0/), amplitude modulation (AM), formants, and power distribution were performed on speech samples extracted from audio recordings collected from the sample members. Multivariate feature and discriminant analyses were performed on feature vectors representing the members of the control and disordered classes. Features derived from the formant and power spectral density measurements were found to be the best discriminators of class membership in both the male and female studies. AM features emerged as strong class discriminators of the male classes. Features describing F/sub 0/ were generally ineffective discriminators in both studies. The results support theories that identify psychomotor disturbances as central elements in depression and suicidality.

Muscular synergism—I. On criteria for load sharing between synergistic muscles

The use of optimization techniques to predict individual muscle forces in redundant biomechanical systems implies the formulation of a criterion for load sharing between the muscles. In part I of this paper, the characteristics and performance of several linear and non-linear criteria reported in the literature have been compared for static-isometric knee flexion. The results show that linear criteria inherently predict discrete muscle action (orderly recruitment of muscles) whereas non-linear criteria can predict synergistic action. All criteria predict that relatively more force is allocated to muscles with large moment arms. When muscle stresses (or ratios of muscle force to maximum muscle force) are used as the decision variables in the objective function, then relatively more force is allocated to muscles with large maximum possible force as well. Future formulations of the optimization should consider the differences in fiber type composition among the muscles. Such an approach is presented in part II of the paper.

Analysis of raw microneurographic recordings based on wavelet de-noising technique and classification algorithm: wavelet analysis in microneurography

We propose a new technique for analyzing the raw neurogram which enables the study of the discharge behavior of individual and group neurons. It utilizes an ideal bandpass filter, a modified wavelet de-noising procedure, an action potential detector, and a waveform classifier. We validated our approach with both simulated data generated from muscle sympathetic neurograms sampled at high rates in five healthy subjects and data recorded from seven healthy subjects during lower body negative pressure suction. The modified wavelet method was superior to the classical discriminator method and the regular wavelet de-noising procedure when applied to simulated neuronal signals. The detected spike rate and spike amplitude rate of the action potentials correlated strongly with number of bursts detected in the integrated neurogram (r = 0.79 and 0.89, respectively, p < 0.001). Eight major action potential waveform classes were found to describe more than 81% of all detected action potentials in all subjects. One class had characteristics similar in shape and in average discharge frequency (27.4/spl plusmn/5.1 spikes/min during resting supine position) to those of reported single vasoconstrictor units. The newly proposed technique allows a precise estimate of sympathetic nerve activity and characterization of individual action potentials in multiunit records.

Human muscle sympathetic nerve activity and plasma noradrenaline kinetics in space

Astronauts returning from space have reduced red blood cell masses, hypovolaemia and orthostatic intolerance, marked by greater cardio–acceleration during standing than before spaceflight, and in some, orthostatic hypotension and presyncope. Adaptation of the sympathetic nervous system occurring during spaceflight may be responsible for these postflight alterations. We tested the hypotheses that exposure to microgravity reduces sympathetic neural outflow and impairs sympathetic neural responses to orthostatic stress. We measured heart rate, photoplethysmographic finger arterial pressure, peroneal nerve muscle sympathetic activity and plasma noradrenaline spillover and clearance, in male astronauts before, during (flight day 12 or 13) and after the 16 day Neurolab space shuttle mission. Measurements were made during supine rest and orthostatic stress, as simulated on Earth and in space by 7 min periods of 15 and 30 mmHg lower body suction. Mean (±s.e.m.) heart rates before lower body suction were similar pre–flight and in flight. Heart rate responses to −30 mmHg were greater in flight (from 56 ± 4 to 72 ± 4 beats min−1) than pre–flight (from 56 ± 4 at rest to 62 ± 4 beats min−1, P < 0.05). Noradrenaline spillover and clearance were increased from pre–flight levels during baseline periods and during lower body suction, both in flight (n= 3) and on post–flight days 1 or 2 (n= 5, P < 0.05). In–flight baseline sympathetic nerve activity was increased above pre–flight levels (by 10–33 %) in the same three subjects in whom noradrenaline spillover and clearance were increased. The sympathetic response to 30 mmHg lower body suction was at pre–flight levels or higher in each subject (35 pre–flight vs. 40 bursts min−1 in flight). No astronaut experienced presyncope during lower body suction in space (or during upright tilt following the Neurolab mission). We conclude that in space, baseline sympathetic neural outflow is increased moderately and sympathetic responses to lower body suction are exaggerated. Therefore, notwithstanding hypovolaemia, astronauts respond normally to simulated orthostatic stress and are able to maintain their arterial pressures at normal levels.

Investigation of vocal jitter and glottal flow spectrum as possible cues for depression and near-term suicidal risk

Among the many clinical decisions that psychiatrists must make, assessment of a patients risk of committing suicide is definitely among the most important, complex, and demanding. When reviewing his clinical experience, one of the authors observed that successful predictions of suicidality were often based on the patients voice independent of content. The voices of suicidal patients judged to be high-risk near-term exhibited unique qualities, which distinguished them from nonsuicidal patients. We investigated the discriminating power of two excitation-based speech parameters, vocal jitter and glottal flow spectrum, for distinguishing among high-risk near-term suicidal, major depressed, and nonsuicidal patients. Our sample consisted of ten high-risk near-term suicidal patients, ten major depressed patients, and ten nondepressed control subjects. As a result of two sample statistical analyses, mean vocal jitter was found to be a significant discriminator only between suicidal and nondepressed control groups (p<0.05). The slope of the glottal flow spectrum, on the other hand, was a significant discriminator between all three groups (p<0.05). A maximum likelihood classifier, developed by combining the a posteriori probabilities of these two features, yielded correct classification scores of 85% between near-term suicidal patients and nondepressed controls, 90% between depressed patients and nondepressed controls, and 75% between near-term suicidal patients and depressed patients. These preliminary classification results support the hypothesized link between phonation and near-term suicidal risk. However, validation of the proposed measures on a larger sample size is necessary.

Clinical Determinants of Biomechanics Platform Measures of Balance in Aged Women

The cross‐sectional associations between clinical variables and biomechanics platform measures of balance (sway) were determined in a random sample of 50 aged single women living in high rise apartment buildings. A history of falling in the previous year was associated with increased areas of sway. Increased body mass was associated with decreased velocity of sway. Poor near, but not far, visual acuity was associated with increased areas of sway. A postural drop of 10 mmHg or more in diastolic pressure was associated with increased velocity of sway. The associations between these variables and the balance measures persisted after adjustment for age and each other to adjust for potential confounding.

Effect of loss of balance on biomechanics platform measures of sway: Influence of stance and a method for adjustment

This paper describes a method for adjusting biomechanics platform measures of sway for loss of balance. Area and velocity measures of sway were determined in forty-seven elderly women, in double and single leg stance, first with their eyes open, then closed. Subjects were rarely able to complete 10 s trials during single leg stances. Therefore, a method was developed for eliminating data associated with loss of balance. Monitoring changes in vertical force and velocity by computer, those points exceeding trial specific thresholds associated with loss of balance were truncated. In double leg stances, loss of balance increased area measures by 0.3%, but did not effect velocity measures. In contrast, the loss of balance increased area measures by 0-3%, but did not effect velocity measures. In contrast, the loss of balance experienced by most subjects in single leg stance exaggerated area measures by 16-38%, and velocity measures by up to 10%. In double leg stances the correlations between unadjusted area measures and area measures adjusted for loss of balance ranged from 0.98 to 1.00. In single leg stances, the correlations for the area measures ranged from 0.69 to 0.89. The correlations between adjusted and unadjusted velocity measures were 1.00 and 0.93 for the double and single leg stances respectively. Although the question of which sway measure is best remains unanswered, this study provides useful data for future research. First, it demonstrates a method for modifying area representations of the center of pressure excursions for the effects of loss of balance.(ABSTRACT TRUNCATED AT 250 WORDS)

Simultaneous measurement of body center of pressure and center of gravity during upright stance. Part I: Methods

Abstract Clinical and experimental assessments of balance use measures of center of pressure (COP) excursion to quantify postural stability during standing. This assumes that the greater the COP excursions, the greater the imbalance. However, it is the position of the body center of gravity (COG), specifically in relation to the base of support, that determines static stability during upright standing. The reason the COG is frequently ignored is that it cannot be measured directly, whereas the COP is readily obtained from a force platform. We report here on a method for simultaneous measurement of the COP and COG displacements. To compute the COG displacements, an optoelectric imaging system operating in synchrony with a force platform was used to measure the three-dimensional positions of the body joints and body segment endpoints. The COG displacements were then computed from the segment kinematics using subject-specific anthropometric measurements. In Part II of this paper, we compare summary measures of the COP and COG excursions obtained from six subjects during three different upright stances.

Helical motion analysis of the knee--II. Kinematics of uninjured and injured knees during walking and pivoting

The knee kinematics of eight individuals with uninjured knees and of seven individuals with ruptured anterior cruciate ligaments have been investigated during walking and pivoting. The kinematics were measured using a six degree of freedom goniometer and quantitated using helical motion analysis. The helical motion variables reveal clearly that the knee is definitely neither a hinge nor a planar joint and its dynamic behavior changes over the stride. Ligamentous loss results in more adduction and external rotation during certain periods of the stride. Also, the range of translation of the tibia in the medial/lateral direction is reduced, and its average translation is more medial.