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R Shiavi

Publications and source records attributed to R Shiavi.

At least 19 recordsLinked to original sources
PubMed1999

Detection of bursts of microneurographic activity and estimation of burst parameters.

Microneurograms are used to study the neural control of the autonomic nervous system. We developed a technique for automatically detecting the times of occurrence of bursts of microneurographic activity and their durations. It involves cross-correlating a rectified and smoothed microneurogram with a triangular shaped waveform as the signal template. Parameters of the derivative of this cross-correlation indicate the times of occurrence and the durations of the bursts of activity. The accuracy of the technique was assessed by comparing its results with those of four experienced investigators and showed significant agreement between the automated technique and the investigators.

Adrenergic Fibers↗
PubMed1998

Electromyographic signals during gait: criteria for envelope filtering and number of strides.

The use of linear envelopes to represent the electromyographic (EMG) measurements obtained during locomotion has become common practice. Guidelines for designing envelope filters and specifying the minimum number of strides needed to produce valid EMG profiles have been developed. Electromyograms from eight major muscles of the lower leg are measured from five normal young adults during self-selected slow, free and fast walking speeds. 30 strides per task are measured. The 'ideal' EMG profile is defined from the ensemble average of the rectified EMG signal. An error measure is defined and used as a criterion to assess the appropriateness of various cut-off frequencies for envelope filters and the number of strides required for establishing a good EMG profile. It is found that between six and ten strides are needed to form a representative profile, and an envelope filter with a minimum cut-off frequency of approximately 9 Hz is necessary.

Adult↗
PubMed1992

Pattern analysis of electromyographic linear envelopes exhibited by subjects with uninjured and injured knees during free and fast speed walking.

We wished to determine whether anterior cruciate ligament (ACL) injury caused changes in patterns of activity of individual or groups of muscles that control the knee joint. The electromyographic (EMG) patterns of six muscles in 26 individuals with uninjured knees and 20 individuals with ruptured ACL were studied during free and fast speed walking. A previously developed clustering technique was used for analysis. This technique involves making the Fourier transform of the average linear envelope (LE) of each muscle for each subject and using the amplitude and phase angles of the lower frequency harmonics as features to describe a pattern. These features are then grouped to subdivide the population of EMG patterns into different types. The results of analyses on single muscles showed that there exists a typical or "normal" pattern for each muscle which most uninjured and some injured subjects exhibit as well as several atypical patterns which mostly injured subjects exhibit, and that the atypical patterns are much more evident at fast walking speed. The characteristics of atypical patterns with respect to normal include time shifts in the peak of major phases of activity, the absence of a second phase of activity, and the existence of additional phases of activity. Synergy analysis showed that if one muscle has an atypical pattern, then several do; i.e., ACL injury induces major changes in the control strategy of the knee. The implications are that for rehabilitation programs one must focus on the training and strengthening of more than one muscle, and that for reconstructive procedures the changes in mechanics of the joint can possibly induce a significant change in its control strategy.

Adolescent↗
PubMed1991

Clustering analysis and pattern discrimination of EMG linear envelopes.

A technique has been developed for performing pattern analysis of EMG activities generated during locomotion. In this development it was found that the shapes of the EMG linear envelopes (LE) are mainly determined by their phase spectra; their magnitude spectra are much less important. Autoregressive (AR) parametric models and discrete Fourier transform (DFT) approaches were tested and compared. The latter was proved to be a better way to describe the EMG LE's. Feature extraction and clustering were performed by doing DFT of EMG LE's, extracting part of the phase and magnitude spectra (in less important degree) as features, and using the percent powers to weigh the corresponding harmonics. The approach was applied to the clustering analysis of EMG LE's of normal and anterior cruciate ligament (ACL) injured subjects during walking.

Anterior Cruciate Ligament Injuries↗
PubMed1990

Temporal feature extraction and clustering analysis of electromyographic linear envelopes in gait studies.

A technique for automatically clustering linear envelopes of the EMG during gait has been developed which uses a temporal feature representation and a maximum peak matching scheme. This new technique provides a viable way to define compact and meaningful EMG waveform features. The envelope matching is performed by dynamic programming, providing qualitatively the largest numbers of matched peaks and quantitatively a minimum distance measurement. The resulting averaged EMG profiles have low statistical variation and can serve as templates for EMG comparison and further classification.

Algorithms↗
PubMed1988

EMG profiles of knee joint musculature during walking: changes induced by anterior cruciate ligament deficiency.

A tear of the anterior cruciate ligament (ACL) disrupts the delicate balance of static stabilizers of the knee, leading to significant alterations in joint kinematics. Little is known about the dynamic compensatory responses of the patient to these kinematic alterations. This lack of quantitative information on the muscle synergy patterns has limited the surgeon's ability to evaluate various operative and rehabilitative techniques. Twelve subjects with documented ACL deficiency for at least 1 year and 15 normal participants were studied. Each subject was asked to walk at free and fast speeds on a 12 m walkway. The right and left foot contact patterns and the linear envelopes from the surface electromyogram (EMG) patterns of the gastrocnemius, medial and lateral hamstrings, rectus femoris, and vastus lateralis were measured. Significant differences were found in the muscle synergy patterns during walking. During the swing-to-stance transition, the ACL-deficient subjects showed significantly less activity in the quadriceps and gastrocnemius muscles and more activity in the biceps femoris than in the normal group. During early swing, the vastus lateralis is more active than normal, and during midstance and terminal stance, the hamstrings appear to be less active than normal subjects. These dynamic compensatory mechanisms suggest that use of the hamstring tendons in reconstructive procedures may alter important compensatory mechanisms about the knee joint. Application of dynamic EMG techniques to the study of reconstructive procedures should provide additional information that will assist the clinician in the rational choice of a surgical procedure.

Adult↗
PubMed1987

Normative childhood EMG gait patterns.

The population characteristics of the linear envelopes of the electromyograms measured from seven lower extremity muscles in children were studied during locomotion. The variability and changes in pattern with respect to walking speed and age were investigated using statistical properties and analysis of variance of the envelopes. All muscles studied showed changes in their patterns that were associated with different walking speeds. Some changes concerned the relative intensity of existing phases of activity whereas others concerned the existence of additional phases of activity. The most remarkable trend was that all envelopes tended to have more consistent patterns as speed increased. With regard to age, within the span of 4-11 years, only the two thigh muscles studied demonstrated appreciable differences.

Age Factors↗
PubMed1987

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.

Adult↗
PubMed1987

Helical motion analysis of the knee--I. Methodology for studying kinematics during locomotion.

A technique for investigating the three-dimensional kinematics of knee motion during dynamic functional tasks has been developed. It involves the combined usage of a six degree of freedom goniometer and helical motion analysis. A detailed procedure for coordinate system alignment and calibration must be followed. Once established this entire procedure is routinely implementable. Ensemble averages from multiple walking strides reveal that this technique is sensitive enough to differentiate between the kinematics of an uninjured and injured knee.

Humans↗
PubMed1987

Electromyographic gait assessment, Part 1: Adult EMG profiles and walking speed.

The profiles of the linear envelopes of surface electromyograms of seven major muscles in adults were studied as a function of walking speed. The ensemble statistical properties of average and standard deviation were used to quantitate the characteristics of the electromyographic patterns. Analysis of variance was used to indicate periods in the gait stride that were effected by walking speed. Two general types of pattern change were observed. One was that the fundamental phasing of muscle activity never changed but that relative amplitudes of the phases were modulated as speed increased. The other was that different phases of activity existed for different walking speeds. The timing of most phases (as expressed in percentage of the stride) decreased as speed increased. This suggests that the time base should be further normalized by stance and swing phases.

Adult↗
PubMed1987

Electromyographic gait assessment, Part 2: Preliminary assessment of hemiparetic synergy patterns.

The electromyographic (EMG) gait patterns of both lower limbs in 12 hemiparetic patients were studied during the early and late recovery periods. The linear envelopes of surface EMG were ensemble averaged to represent performance. The patients' stride parameters and EMG envelopes were compared to those of normal individuals measured at very-slow walking speeds. Both limbs could have normal or abnormal synergies. The abnormal synergy exhibited by the contralateral limb was almost always a cocontraction. Three different groups of synergies were defined depending on the change in synergy type exhibited by the contralateral limb. Patients who walked at less than a normal very-slow speed during the early recovery period never achieved a normal synergy in either limb. Seventy-five percent of the patients whose walking speed was at least 0.6 meters per second during the early recovery period improved or maintained their gait performance for at least one year.

Cerebrovascular Disorders↗
PubMed1984

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.

Biomechanical Phenomena↗
PubMed1984

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.

Animals↗