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S Andreassen

Publications and source records attributed to S Andreassen.

17 recordsLinked to original sources
PubMed1992

Diagnostic function of the microhuman prototype of the expert system--MUNIN.

This paper describes the diagnostic function of a prototype expert system for electromyography (EMG). The prototype was restricted to a limited "Microhuman" anatomy with only 6 muscles and 8 nerves, and a corresponding limitation on the number of local nerve lesions. It attempted to give a detailed description of the most important groups of generalized nerve and muscle disorders, and the commonly used parameters from needle EMG and nerve conduction studies were included. The system can be used both for "diagnostic" and for "causal" reasoning. In diagnostic reasoning, the system's probabilistic inference engine is used to reason from test results through 14 different aspects of neuromuscular pathophysiology to disorders. In causal reasoning, the system reasons in the opposite direction from disorders through pathophysiology to expected test results. The diagnostic function of the system was illustrated by 3 cases: a normal subject, a patient with a bilateral carpal tunnel syndrome and a patient with both a diabetic polyneuropathy and a bilateral carpal tunnel syndrome.

Carpal Tunnel Syndrome↗
PubMed1991

Mechanical and electromyographic responses to stretch of the human ankle extensors.

1. During maintained ankle extension with background torques ranging from 0 to 70 N.m, the ankle extensors were stretched by a 5 degree rotation of the ankle joint. Maximal stretch velocity was 170 degrees/s. Regression analysis of simultaneous measurements of total torque and needle and surface electromyograms (EMG) from the soleus and gastrocnemius muscles showed that the soleus muscle generates about two-thirds of the maximal torque (approximately 120 Nm) with the subjects in sitting position. In addition, it was found that there is considerable cross talk between the soleus and gastrocnemius muscles when EMGs are recorded by surface electrodes. 2. The soleus EMG response to stretch began with a "phasic" response (latency 41 +/- 4 ms, mean +/- SD), consisting of two peaks, labeled M1 and M2. The phasic response ended 120-140 ms after stretch onset and was followed by a period of reduced EMG activity, ending at 170-210 ms. After this "silent period," a smaller "tonic" response was seen. The phasic responses of the soleus muscle were much larger than the corresponding responses in the anterior tibial muscle. In contrast, the tonic responses were comparable in the soleus and anterior tibial muscles. 3. The amplitudes of the phasic M1 and M2 responses were independent of the level of the background contraction. This disagrees with the "automatic gain principle," according to which the amplitudes of M1 and M2 should increase proportionally with the background EMG. In contrast to the phasic responses, the amplitude of the tonic EMG response, measured 200-400 ms after stretch onset, followed the automatic gain principle.(ABSTRACT TRUNCATED AT 250 WORDS)

Adult↗
PubMed1989

Mechanical and electromyographic responses to stretch of the human anterior tibial muscle at different levels of contraction.

The EMG response and the mechanical response to 2 degree stretch of the human anterior tibial muscle was studied during contractions ranging from 0% to 80% of maximal voluntary contraction (MVC). The EMG response showed three distinct peaks M1, M2, and M3 with peak latencies of 59 ms, 86 ms, and 120 ms respectively. At low background torques M1 dominated while M2 and M3 were small or absent. M2 and M3 dominated above 40% of MVC and M2 in particular showed "automatic gain compensation", i.e. it constituted a - more or less - constant proportion of the background EMG for all contraction levels. The ratio between M1 amplitude and background EMG steadily decreased with contraction level. Even though the summed contributions of M1, M2, and M3 to some degree showed automatic gain compensation, this was not the case for the mechanical response to stretch. Between 0% and 30% of MVC the reflex mediated mechanical response increased approximately in proportion to the contraction level, but the reflex mediated mechanical response peaked at 40% of MVC and declined to zero at 80% of MVC. This discrepancy between EMG and mechanical response was explained by a simple model. The regression line between rectified and filtered tibialis anterior EMG and torque was used to predict the mechanical response from the EMG response. At increasing contraction levels the twitch elicited by supramaximal electrical stimulation decreases, and we reduced the predicted mechanical response by the same factor as the twitch.(ABSTRACT TRUNCATED AT 250 WORDS)

Adult↗
PubMed1988

Simulation of concentric needle EMG motor unit action potentials.

Computer simulations of motor unit action potentials (MUAPs) as measured by a concentric needle (CN) electromyography (EMG) electrode in normal motor units (MUs) indicated that the MUAP amplitude is determined mainly by the proximity of the electrode to the closest muscle fiber. The area and duration of the simulated MUAPs were affected by all muscle fibers in front of the active recording surface but mainly by those that were less than 2 and 2.5 mm, respectively, from the active recording surface. The MUAP area was also affected by the proximity of the electrode to the closest muscle fiber. The number of phases of the simulated MUAPs increased when the dispersion of the arrival times of individual muscle fiber APs at the electrode was increased. Increased temporal dispersion of APs decreased the MUAP amplitude and area slightly but did not affect the MUAP duration. It is inferred that different features of the CN MUAP are determined by the distribution of muscle fibers within different portions of the MU territory and thus provide complementary information about the MU architecture.

Action Potentials↗
PubMed1988

Muscle stiffness in human ankle dorsiflexors: intrinsic and reflex components.

1. The purpose of this study was to evaluate the mechanical response to stretch in normal human ankle dorsiflexors at different levels of voluntary contraction. In an active muscle, the total mechanical response is the sum of the intrinsic response from the contractile apparatus, the response from passive tissues, and the reflex mediated response. Each of these components was investigated. 2. The total incremental stiffness was defined as the ratio between the torque increment and the amplitude of the stretch. In 14 subjects the total stiffness increased from approximately 0.6 N.m/deg to approximately 2.5 N.m/deg at 50% of MVC and remained constant (+/- 10%) from 30 to 80% of MVC. 3. The contribution to incremental stiffness from intrinsic muscle properties was measured during electrical stimulation of the deep peroneal nerve at 7-50 Hz. Intrinsic stiffness increased linearly with torque from approximately 0.5 N.m/deg to approximately 2.5 N.m/deg at 80% of MVC. 4. The reflex component (total minus intrinsic stiffness) had a maximum of 0.5-1.5 N.m/deg at 30-50% of MVC and was approximately zero at no and maximal contraction. For intermediate levels of contraction the reflex increased the stiffness with 40-100% of the intrinsic stiffness in this flexor muscle. 5. The reflex contribution to total stiffness began approximately 50 ms after onset of stretch and peaked 150-300 ms after onset of stretch. 6. Total, intrinsic, and reflex mediated stiffness were all nearly independent of the amplitude of stretch in the range from 2 to 7 degrees. The higher stiffness observed for 1 degree stretches could be due to "short range stiffness" of the cross bridges. 7. Stretching of a contracting muscle generates large force increments even for moderate amplitudes of stretch. Approximately half of this force increment is due to the stretch reflex, which makes the muscle stiffer than predicted from the intrinsic stiffness. These findings in human flexor muscles are surprisingly similar to previous findings in extensor muscles of the decerebrate cat.

Adult↗
PubMed1987

Muscle fibre conduction velocity in motor units of the human anterior tibial muscle: a new size principle parameter.

1. Twitch contractions were elicited in human anterior tibial muscle by intramuscular microstimulation of single motor axons with a bipolar needle electrode. The population of stimulated motor units studied was fairly representative for the muscle. 2. The conduction velocity of the fibres in the motor unit was calculated as the ratio between the electrode separation (15 mm) in a tripolar array of surface electrodes and the conduction delay of the motor unit potential along the electrode array. The motor unit conduction velocity ranged from 2.6 to 5.3 m/s with a mean of 3.7 m/s. 3. The contractile properties of the motor units were obtained by averaging the torque developed around the ankle joint. Twitch torques ranged from less than 10(-3) to 16 x 10(-3) N m, with a mean of 5.7 x 10(-3) N m. The twitch torque of the whole anterior tibial muscle was approximately 5 N m. Rise times were 47-80 m/s with a mean of 61 m/s, and half-relaxation times were 40-78 ms with a mean of 60 ms. 4. The mechanical properties of individual motor units were highly correlated (rise time and twitch torque: r = -0.81; rise time and half-relaxation time: r = 0.75; twitch torque and half-relaxation time: r = -0.81). 5. The motor unit conduction velocity was highly correlated to twitch torque (r = 0.87), rise time (r = -0.75) and half-relaxation time (r = -0.66). This indicates that the motor unit conduction velocity can be included in the family of interrelated 'size principle parameters'.

Axons↗
PubMed1986

Quantitative analysis of individual motor unit potentials: a proposition for standardized terminology and criteria for measurement.

The physiology of the motor unit potential (MUP) is reviewed. The aim is to identify the electrophysiological events in the motor unit that generate the individual parts of the MUP. This is based on insight gained from new experimental techniques, such as single-fiber electromyography (EMG), scanning EMG, and simulation studies of the MUP. A terminology for the different parts of the MUP is also suggested, and nine parameters used to describe different features of the MUP are delineated: duration, spike duration, amplitude, area, spike area, phases, turns, satellites, and variability. Technical aspects, such as electrode type, filtering, and sampling rate of the computers, are discussed as well. In Appendix A, different manual and computer-aided methods for quantitative MUP analysis are described. Despite minor systematic differences between the methods, MUP durations measured by different methods correlate highly with each other (Appendix B). The manual and computer-aided methods have comparable variability between repeated measurements.

Action Potentials↗
PubMed1979

Mathematical analysis of optimal multichannel filtering for nerve signals.

This paper extends recent analyses (Roberts and Hartline, 1975; Oğuztöreli and Stein, 1977) of optimal linear filters for separating neural signals from more than one electrode site. Roberts and Hartline's result, using a matched filter criterion, represents one of a class of optimal filters with different, but symmetrical, output waveforms derived by Oğuztöreli and Stein (1977). Another narrow bandwidth filter of this class will give the optimal results according to an energy criterion, but may be less useful in practical situations.

Animals↗
PubMed1979

Application of optimal multichannel filtering to simulated nerve signals.

The optimal linear filters derived in the preceding paper can be thoroughly evaluated using computer simulations, based on the properties of mammalian sensory and motor nerve fibres. Using reasonable values for action potential waveforms, conduction velocity and electrode noise, good separation of motor and sensory signals can be obtained. The performance of the filters is degraded by 1) increasing the electrode noise, 2) introducing dispersion in the conduction velocities, or 3) variation in the waveform of the action potentials from that used in designing the filters. However, the variations needed to seriously degrade performance are quite large compared to those which are likely to be present in mammalian nerves. Use of these filters to distinguish different classes of sensory (or motor) signals based on conduction velocity is discussed.

Action Potentials↗
PubMedDate not supplied

Impaired regulation of the firing pattern of single motor units.

The firing pattern of single motor units from the anterior tibial muscle was studied during constant isometric contraction. Recordings were made with bipolar, fine-wire electrodes from 4 normal subjects and from 10 patients with spasticity. A microcomputer was used to classify the motor unit potentials automatically and to measure the interspike intervals. The patients had reduced firing frequencies and reduced variability between neighbor intervals. Seven patients with spasticity due to supraspinal lesions had positive correlation between neighbor intervals.

Action Potentials↗
PubMedDate not supplied

Medical expert systems based on causal probabilistic networks.

Causal probabilistic networks (CPNs) offer new methods by which you can build medical expert systems that can handle all types of medical reasoning within a uniform conceptual framework. Based on the experience from a commercially available system and a couple of large prototype systems, it appears that CPNs are now an attractive alternative to other methods. A CPN is an intensional model of a domain, and it is therefore conceptually much closer to qualitative reasoning systems and to simulation systems than to rule-based or logic-based systems. Recent progress in Bayesian inference in networks has yielded computationally efficient methods. The inference method used follows the fundamental axioms of probability theory, and gives a sound framework for causal and diagnostic (deductive and abductive) reasoning under uncertainty. Experience with the prototypes indicates that it may be possible to use decision theory as a rational approach to test planning and therapy planning. The way in which knowledge is acquired and represented in CPNs makes it easy to express 'deep knowledge' for example in the form of physiological models, and the facilities for learning make it possible to make a smooth transition from expert opinion to statistics based on empirical data.

Artificial Intelligence↗
PubMedDate not supplied

The development of a multicenter database for reference values in clinical neurophysiology--principles and examples.

This paper describes the work undertaken to establish principles for the development of multicenter databases for reference values in clinical neurophysiology. The study was initiated because of interest of the involved laboratories in knowledge-based systems in electromyographic diagnosis, for which it was necessary to formalize the key concepts in the diagnostic process: diseases, pathophysiology and test results. The paper deals specifically with the structuring of results of motor and sensory nerve conduction studies.

Action Potentials↗