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Biomedical subjects

O Levin

Publications and source records attributed to O Levin.

18 recordsLinked to original sources
PubMed2005

Principal component analysis of complex multijoint coordinative movements.

Principal components analysis (PCA) has not been very much in vogue within the field of movement coordination even though it is useful to reduce data dimensionality and to reveal underlying data structures. Traditionally, studies of coordination between two joints have predominantly made use of relative phase analyses. This has resulted in the identification of principal constraints that govern the Central Nervous System's organization and the control of coordination patterns. However, relative phase analyses on pairwise joints have some drawbacks because they are not optimal for revealing convergent patterns among multijoint coordination modes and for unraveling generic control strategies. In this paper, we present a method to analyze multijoint coordination based on the properties of PC, more specifically the eigenvalues and eigenvectors of the covariance matrix. The comparison between relative phase analysis and PCA shows that both provide similar and consistent results, underscoring the latter technique's sensitivity to the study of coordination performance. In addition, it provides a method for automatic pattern detection as well as an index of performance for each joint within the context of the global coordination pattern. Finally, the merit of the PCA technique within the context of central pattern generators (CPG) will be discussed.

Adult↗
PubMed2004

Dynamical changes in corticospinal excitability during imagery of unimanual and bimanual wrist movements in humans: a transcranial magnetic stimulation study.

This study explored the dynamical changes in corticospinal excitability during the imagination of cyclical unimanual and bimanual wrist flexion-extension movements. Transcranial magnetic stimulation was applied over the left motor cortex to evoke motor evoked potentials in the right wrist flexor and extensor muscles. Findings provided evidence for increased reciprocal excitability changes during imagery of symmetrical in-phase movements as compared to asymmetrical (anti-phase) or unimanual movements. This suggests that in-phase movements may reinforce whereas anti-phase movements may reduce the temporal representation of the task in the corticospinal motor networks of the brain.

Adult↗
PubMed2003

Frequency-dependent effects of muscle tendon vibration on corticospinal excitability: a TMS study.

The aim of the present study was to investigate the effects of muscle tendon vibration at different frequencies on corticospinal excitability by means of transcranial magnetic stimulation (TMS). A second objective was to describe whether the observed modulations in motor evoked potentials (MEPs), as a function of vibration frequency, reflect the behavior of Ia afferents during and after vibration. In ten subjects, muscle tendon vibration (duration 30 s) was applied to the flexor carpi radialis (FCR) muscle at three different frequencies (20, 75 and 120 Hz). MEPs following single-pulse TMS were recorded from the targeted muscle during a previbration, vibration, and postvibration period. Muscle tendon vibration at 75 Hz increased the MEP amplitude significantly during vibration, whereas a smaller but still significant effect was observed during 120 Hz vibration. No significant MEP changes could be observed during 20 Hz vibration and during the postvibration period for each frequency. Our findings indicate that muscle tendon vibration exerts a frequency-dependent effect on corticospinal excitability. Furthermore, evidence is provided for the notion that the excitatory effect of muscle tendon vibration on the primary motor cortex is mediated by Ia afferent input.

Adult↗
PubMed2003

Head movements destabilize cyclical in-phase but not anti-phase homologous limb coordination in humans.

The present study addressed the role of head movements in the coordination of the homologous upper or lower limbs in supine normal subjects. Consistent with previous research, in-phase mirror symmetrical movements were performed more accurately and consistently than anti-phase movements. However, inclusion of head movements destabilized in-phase but not anti-phase homologous limb coordination, in contrast to previous work demonstrating a higher vulnerability of anti-phase than in-phase coordination to various experimental perturbations. It was observed that the head moved in the same direction as the limbs during anti- but not during in-phase coordination. Furthermore, the interlimb patterns also affected the head rotations that were lower in spatiotemporal consistency and less consistently coupled with the limbs during in-phase than during anti-phase coordination. These findings provide new insights into the coalition of egocentric and allocentric constraints during interlimb coordination.

Adult↗
PubMed2001

Directional tuning effects during cyclical two-joint arm movements in the horizontal plane.

The present study explored the effect of different movement orientations on the arm end-effector kinematic features, levels of muscle activity and intermuscular coordination between shoulder and elbow muscles during cyclical movement. Subjects were instructed to trace cyclical lines with their dominant arm along vertical, horizontal, and right (low inertia) or left diagonal (high inertia) orientations. EMG activity from the biceps, triceps and anterior and posterior deltoids were monitored along with the displacements of the end-effector of the arm. The results suggested a differential role for the shoulder versus elbow muscles in the manipulation of the hand end-effector trajectory. The activity in the shoulder flexors was predominantly in anti-phase with that of the shoulder extensors and was therefore presumed to manipulate the global features of the trajectory. Biceps and triceps tended to show less orchestrated activity and were therefore assumed to be responsible for making the fine adjustments and to compensate for intersegmental interactions. The most pronounced differences in kinematics and EMG features among the four principal movement orientations were observed between the two diagonal orientations, which differed profoundly in arm inertial resistance. The findings converged upon the principle of 'inertial anisotropy,' as previously identified for discrete movement, suggesting that the central nervous system did not fully preplan the actual kinematic requirements of cyclical task performance. Moreover, inertial anisotropy was evident in spite of the fact that movement was performed under temporal constraints (metronome pacing) and with availability of a visual template of the task, suggesting that enhancement of the feedback loop did not fully eliminate these effects.

Action Potentials↗
PubMed2001

Constraints during bimanual coordination: the role of direction in relation to amplitude and force requirements.

The present study addressed the status of spatial encoding during a bimanual task paradigm. This was based on the premise that patterns of contralateral interference during bimanual coordination provide a window into those movement parameters that are primarily encoded within the central nervous system. Results showed that both direction and amplitude were subject to (bilateral) interference when different specifications were to be generated simultaneously for each limb. Directional interference was found to be partially independent of the amount and pattern of underlying muscle activation, suggesting that direction is encoded at a rather abstract level in the central nervous system. The findings are consistent with single-cell recording studies that have pointed to the role of directional tuning in various brain areas. Moreover, the findings suggest that spatial parameters of movement constrain the coordination of limb movements in addition to temporal parameters.

Adult↗
PubMed2001

Proprioceptive control of cyclical bimanual forearm movements across different movement frequencies as revealed by means of tendon vibration.

The effect of unilateral tendon vibration on the performance of cyclical bimanual forearm movements was investigated across different cycling frequencies (from 0.67 to 2.53 Hz). The spatiotemporal features of the individual limb motions as well as their coordination were studied. Tendon vibration was found to result in a substantial reduction in the amplitude of the vibrated arm, leaving the nonvibrated arm unaffected. The vibration-induced amplitude reduction decreased from 26% to 11% as cycling frequency increased even though significant reductions were still observed at the highest cycling frequencies. Tendon vibration was also found to result in an increase of the phase lead of the dominant arm with respect to the nondominant arm, but this effect was not modulated by cycling frequency. The data argue in favor of a closed-loop mode of movement control during cyclical high-speed movements. It is suggested that kinesthetic afferent information is processed and used to guide action up to near-maximal movement speeds, reinforcing recent claims with respect to visual information processing.

Adolescent↗
PubMed2000

Transcutaneous FES of the paralyzed quadriceps: is knee torque affected by unintended activation of the hamstrings?

This study addresses the question whether unintended response of the knee flexors (hamstrings) accompanies transcutaneous functional electrical stimulation (FES) of the quadriceps and whether the knee torque is hereby affected. Transcutaneous FES of the right quadriceps of two paraplegic subjects was applied and measurements were made of the net torque and of the myoelectric activities of the quadriceps and hamstrings muscles of the right leg. A low correlation was obtained between the peak-to-peak amplitudes of the M-waves of the two muscles. This correlation decreased further with the development of fatigue, which indicated that the electromyography (EMG) signals from the hamstrings were not the result of cross-talk between adjacent recording sites. The force profile of each muscle was determined from a developed model incorporating EMG-based activation, muscle anthropometry as obtained from in vivo magnetic resonance imaging of the thigh, and metabolic fatigue function, based on data acquired by 31P nuclear magnetic resonance spectroscopy. A sensitivity analysis revealed that the muscle specific tension and the muscle moment arms have a major influence on the resulting muscle forces and should therefore be accurately provided. The results show that during the unfatigued phase of contraction the estimated maximal force in the hamstrings was lower than 20% of that in the quadriceps and could be considered to be practically negligible. As fatigue progressed the hamstrings-to-quadriceps force ratio increased, reaching up to 45%, and the effect of co-activation on the torque partition between the two muscles was no longer negligible.

Adult↗
PubMed1999

EMG and metabolite-based prediction of force in paralyzed quadriceps muscle under interrupted stimulation.

A major issue associated with functional electrical stimulation (FES) of a paralyzed limb is the decay with time of the muscle force as a result of fatigue. A possible means to reduce fatigue during FES is by using interrupted stimulation, in which fatigue and recovery occur in sequence. In this study, we present a model which enables us to evaluate the temporal force generation capacity within the electrically activated muscle during first stimulation fatigue, i.e., when the muscle is activated from unfatigued initial conditions, and during postrest stimulation, i.e., after different given rest durations. The force history of the muscle is determined by the activation as derived from actually measured electromyogram (EMG) data, and by the metabolic fatigue function expressing the temporal changes of muscle metabolites, from existing data acquired by in vivo 31P MR spectroscopy in terms of the inorganic phosphorus variables, Pi or H2PO4-, and by the intracellular pH. The model was solved for supra-maximal stimulation in isometric contractions separated by rest periods, and compared to experimentally obtained measurements. EMG data were fundamental for prediction of the ascending force during its posttetanic response. On the other hand, prediction of the decaying phase of the force was possible only by means of the metabolite-based fatigue function. The prediction capability of the model was assessed by means of the error between predicted and measured force profiles. The predicted force obtained from the model in first stimulation fatigue fits well with the experimental one. In postrest stimulation fatigue, the different metabolites provided different prediction capabilities of the force, depending on the duration of the rest period. Following rest duration of 1 min, Pi provided the best prediction of force; H2PO4- extended the prediction capacity of the model to up to 6 min and pH provided a reliable prediction for rest durations longer than 12 min. The results presented shed light on the roles of EMG and of metabolites in prediction of the force history of a paralyzed muscle under conditions where fatigue and recovery occur in sequence.

Algorithms↗
PubMed1998

Standing sway: iterative estimation of the kinematics and dynamics of the lower extremities from force-plate measurements.

In this study, a model for the estimation of the dynamics of the lower extremities in standing sway from force plate data only is presented. A three-dimensional, five-segment, four-joint model of the human body was used to describe postural standing sway dynamics. Force-plate data of the reactive forces and centers of pressure were measured bilaterally. By applying the equations of motion to these data, the transversal trajectory of the center of gravity (CG) of the body was resolved in the sagittal and coronal planes. An inverse kinematics algorithm was used to evaluate the kinematics of the body segments. The dynamics of the segments was then resolved by using the Newton-Euler equations, and the model's estimated dynamic quantities of the distal segments were compared with those actually measured. Differences between model and measured dynamics were calculated and minimized, using an iterative algorithm to re-estimate joint positioning and anthropometric properties. The above method was tested with a group of 11 able-bodied subjects, and the results indicated that the relative errors obtained in the final iteration were of the same order of magnitude as those reported for closed loop problems involved in direct kinematic measurements of human gait.

Algorithms↗
PubMed1987

Mutagenicity of pan residues and gravy from fried meat.

Lean pork meat was fried with or without the addition of frying-fat at 200 or 250 degrees C. The pan residues were collected by washing the hot pan with boiling water. When producing thickened gravy the water was substituted by a mixture of water and flour, milk and flour or cream and flour. The basic extracts were tested for mutagenicity in Ames' Salmonella test on strain TA98 with the addition of S9 mix. High amounts of mutagenicity were found in all samples. The amounts of mutagenicity in the pan residues were at a comparable level of the amounts found in the meat crusts. Thickening of the gravy caused only small changes in the mutagenicity.

Animals↗
PubMed1984

High mutagenic activity formed in pan-broiled pork.

Lean pork was pan-broiled at various temperatures between 100 and 290 degrees C. Cooking was performed in an open frying pan common for domestic use in Sweden. No fat was added. Cooking procedures are clearly defined in order to facilitate inter-laboratory comparisons. The crust was extracted with organic solvents of varying polarity. The mutagenic activity was assayed with Ames' Salmonella mutagenicity test. Large amounts of mutagenic activity were detected in samples pan-broiled at 200-290 degrees C. The mutagenic activity recovered was about 10 times higher than that reported by previous investigators to be found during cooking of meat under similar conditions. This discrepancy could be due to differences in the composition of Swedish pork as compared to the meat samples used by other investigators or to different methodology in cooking and extraction procedures.

Animals↗
PubMedDate not supplied

Influence of frying fat on mutagenic activity in lean pork meat.

Mutagenic activity in lean pork meat fried at two different pan temperatures, 200 degrees C and 250 degrees C, with or without the addition of fat, was measured in Ames' Salmonella test on strain TA98. 9 different fats with varying chemical composition were tested. All fried meat samples were shown to be mutagenic. At the frying temperature of 200 degrees C differences between meat samples fried in different fats or without fat, respectively, were small. All meat samples fried at 250 degrees C were considerably more mutagenic than the samples fried at 200 degrees C. At 250 degrees C, the addition of fat caused a significant rise in mutagenic activity. We believe this is mainly an effect of more efficient heat transfer from the bottom of the frying-pan to the meat samples, although other factors may also contribute.

Animals↗