Biomedical subjects

M M Wierzbicka

Publications and source records attributed to M M Wierzbicka.

16 recordsLinked to original sources

PubMed1998

Vibration-induced postural posteffects.

It generally is known that vibration of various muscles in free-standing subjects evokes a spatially oriented postural response. Furthermore, it recently has been shown that when a vibratory stimulus is terminated, a powerful involuntary contraction of the previously vibrated muscle often occurs that, under the isotonic condition, is accompanied by movement of a limb. The aim of this study was to explore effects of a low-amplitude mechanical vibration, applied in a seated position, on the standing posture. The 30-s vibration was applied bilaterally at the ankle level to anterior or posterior tendons and at the cervical level in front or back of the neck, at one site only at a time. Center of pressure trajectories were monitored during quiet stance for </=19 min after the offset of vibration, and these measurements were compared with a previbration control trial. The results clearly indicate that vibration produced in all subjects strong, long-lasting dynamical modification of posture mainly in the anterior-posterior direction. Spatial orientation of the induced postvibratory shift in posture was dependent on the vibration side. We conclude that sustained Ia sensory inflow, evoked by vibration, has a powerful after-effect on the motor system at the postural level.

Adult↗

PubMed1998

Effect of agonist load on fastest muscle contractions in human elbow flexors.

The purpose of this study was to investigate whether the central nervous system (CNS) motor program subserving most-rapid voluntary force pulse generation is modified according to the level of agonist muscle load maintained prior to the pulse. Five normal subjects produced most-rapid force pulses to a target of 20% maximum voluntary force (MVF) above different levels of steady tonic contraction. Time to reach peak force, the first agonist burst duration and area were relatively constant for loads up to approximately 40% of MVF and then increased substantially with larger loads. These results indicate that the CNS adjusts agonist burst amplitude and duration in order to accomplish the same rapid motor task depending on the load. It is argued that these adjustments are necessary to compensate for physiological limitations of motor unit behavior and non-linearity between EMG activity and phasic force output.

Adult↗

PubMed1996

The "gap paradigm" leads to express-like saccadic reaction times in Parkinson's disease.

Application of the "gap paradigm" reduces mean saccadic reaction time (SRT). It enhances oculomotor response, sometimes showing express-like reactions within about 100-140 ms from target onset, which are called "express saccades." Despite some controversy, there is evidence that express and regular saccades may be controlled by different pathways, but it is still not clear which pathways are involved in the generation of express saccades. It was suggested that the substantia nigra (SN) could take part in the control of these saccades. We compared saccadic reaction times of 14 patients with mild to moderate Parkinson's disease (PD), which commonly affects the SN, with those of nine elderly normal subjects. The results show a clear gap effect and no significant difference between mean saccadic reaction times and reaction time distributions of the PD and the control group. In the gap condition, express saccades are present in several subjects of both groups. Therefore, it is concluded that the deficits in the SN pars compacta due to PD, which affect directly and indirectly the activity of the SN pars reticulata that projects to SC, do not dominantly influence the process of preparing and executing express and regular saccades in patients with mild to moderate PD.

Aged↗

PubMed1996

Accuracy of motor responses in subjects with and without control of antagonist muscle.

1. The aim of this study was to investigate the role of the antagonist muscle in determining the accuracy of fast, single-joint motor responses to a target. We recently found that C5/C6 tetraplegic subjects, who lacked voluntary control of their triceps muscle, were less accurate than control subjects in producing fast flexion movements to a target. 2. Two hypotheses are proposed to account for these larger errors: 1) the ability of tetraplegic subjects to compensate for errors arising early in the motor response is impaired because of the lack of antagonist muscle activation; or 2) tetraplegic subjects lack antagonist (braking) force, so they must use much smaller accelerative forces when they move, in order to avoid overshooting their target. Because studies have shown that low levels of force are produced with less relative accuracy than larger forces, this relative inaccuracy of force generation by the motor control system at low force levels is responsible for the inaccuracy of tetraplegics' movements. To test these two hypotheses, we compared the variability of "fast and accurate as possible" force pulses in four control subjects and four C5/C6 tetraplegic subjects to targets at 15, 30, and 45% of maximum voluntary contraction. Multiple regression analyses were performed to look for patterns of agonist or antagonist muscle activation consistent with compensatory adjustments for early trajectory errors in both groups of subjects. 3. Force rise time was significantly prolonged in tetraplegic subjects, although there was some overlap between groups. At similar levels of effort, there were no significant differences in constant and variable errors of control and tetraplegic subjects. We also found no consistent statistical evidence for the presence of compensatory electromyographic activity in either group of subjects. Subjects who lacked the ability to make corrections involving the triceps muscle performed as well as subjects with normal triceps strength. This suggests that a corrective mechanism involving the triceps must have a weak role, if any, in these experiments. 4. Together with our observation that lower force targets are indeed associated with larger relative variable errors, in both control and tetraplegic subjects, the above results lead us to conclude that the second hypothesis listed above is more likely correct. The antagonist muscle clearly enables the production of briefer force pulses. In addition, the antagonist indirectly contributes to the accuracy of isotonic movements because antagonist braking allows larger agonist forces to be used. These larger agonist forces are less variable, and produce more accurate movements, than the smaller forces used by tetraplegic subjects.

Adult↗

PubMed1993

Discharge properties of single motor units in patients with spinal cord injuries.

To study neurophysiological correlates of spastic paresis, we analyzed the discharge pattern of single motor units (SMUs) during sustained voluntary contraction from muscles weakened by spinal cord injury (SCI) and from muscles of near normal strength just at or above the level of injury. The average firing rate of SMUs was reduced in patients' biceps brachii and tibialis anterior muscles compared with controls, but not in the triceps brachii. Floating serial correlation coefficients obtained from successive interdischarge intervals were significantly more positive in patients than in controls in all three muscles. One statistical measure of regularity of discharge, akin to a coefficient of variation, was best able to differentiate patient and control SMUs. Increased discharge variability in muscles just above the level of injury suggested that subtle effects of traumatic SCIs were more extended than was clinically apparent. Although consistent statistical differences could be measured, these changes were not specific to SCI, nor were all SMUs equally affected.

Humans↗

PubMed1993

Relationship between tremor and the onset of rapid voluntary contraction in Parkinson's disease.

It has been suggested that initiation of voluntary movement in Parkinson's disease (PD) is synchronous with tremor beats. This hypothesis was tested by examining quantitatively the onset of voluntary contraction in relation to the tremor cycle. Five PD patients with tremor at rest produced fast isometric abduction of the index finger in reaction time and self-paced trials. For each trial, the time interval from last tremor peak to contraction onset was evaluated and its frequency distribution was compared with proposed statistical models. It was found that patients most often initiated contractions during the descending phase of the tremor oscillation and that the phasic EMG burst most commonly occurred after the mid-point of the EMG tremor cycle in both self-paced and reaction time conditions. These results indicate that voluntary contraction is not initiated arbitrarily with respect to the tremor oscillation. Rather, there is a systematic phase relationship between the onset of the voluntary response and tremor. Thus attraction of voluntary movement to the tremor oscillator in PD results from abnormally synchronised motor units and failure to activate the motor neuron pool voluntarily soon after the tremor discharge.

Aged↗

PubMed1992

Effects of weak antagonist on fast elbow flexion movements in man.

By using a mathematical model and experiments involving electrical simulation of antagonistic muscles, we have formed the hypothesis (Wierzbicka et al. 1986) that in one-joint movements the antagonist muscle not only provides braking torque but also controls movement time. To get additional experimental support for this hypothesis, we studied elbow flexion movements performed by patients with spinal cord injury at the C5-6 level who had relatively normal strength in their biceps muscle and little or no voluntary control of the triceps. Seven quadriplegic patients and six control subjects performed elbow flexion movements of 10 degrees, 20 degrees, and 30 degrees "as fast and accurately as possible". Despite the lack of antagonist, patients used the same "pulse height" strategy as control subjects to scale their responses with movement amplitude. However, patients' movement time was on average twice that of control subjects, and durations of both accelerative and decelerative phases of movement were increased. Movement speed and acceleration were reduced to 20-50% of the corresponding values of control subjects. Patients tended to overshoot the target to a larger extent than control subjects, particularly 10 degrees targets, with nearly twice the error. We performed the same experiments using an external torque motor to assist the weak triceps. When a constant extensor torque of 2.5 or 5 Nm was provided by the motor, patients were able to move faster, and movement accuracy improved to within the normal range. These results provide direct evidence that the lack of an antagonist has an important effect on completion time and accuracy of fast goal-directed movements.

Adult↗

PubMed1992

Kinematic models and human elbow flexion movements: quantitative analysis.

The smoothness with which movements are customarily performed has led Hogan (1984) to formulate a model for trajectory planning by the central nervous system in which the goal is to maximize smoothness, one measure of which is the integrated mean squared magnitude of jerk (jerk cost). We tested the applicability of this minimum-jerk model to one-joint goal directed movements performed by human subjects at different speeds and amplitudes, by comparing kinematic parameters and the jerk cost predicted by the mathematical model with values calculated from experimental data. We also tested a higher order, minimum-snap kinematic model. Normal subjects performed elbow flexions of 5 to 50 degrees "as rapidly and accurately as possible" and also at slower speeds. The boundary conditions of both models were adjusted to account for the failure of subjects to produce movements which reached equilibrium precisely at the target (so that acceleration and velocity reached zero together). Typically, fast movements (less than 300 ms duration) were fairly symmetric in that the durations and amplitudes of acceleration and deceleration were approximately equal; slower movements (greater than 300 ms) were asymmetric with strong, brief acceleration peaks and broad, slow deceleration peaks. In fast movements, the calculated jerk cost was consistently higher than predicted by the minimum-jerk model; a good fit to all kinematic parameters was provided by the minimum-snap model (a seventh-order polynomial). Neither model consistently predicted the trajectories of slower movements. We conclude that muscle/limb dynamics can account for the success of the minimum-snap model with fast movements, and that there is no evidence of planning for maximal smoothness in slower movements.

Brain↗

PubMed1992

[The dependence of rapid voluntary contractions on the tremor phase in parkinsonism].

Parkinsonian patients sometimes have problems to produce really fast motor responses. Often, these are significantly delayed in comparison to those of normal controls. In patients with tremor-at-rest, this delay might be induced by some attraction of the movement execution to the tremor oscillator, according to a hypothesis of Hallet et al. (1977). This study now examined the phase relationship between the on-going tremor and the onset of fast voluntary contractions in Parkinson's disease (PD), quantitatively. Five patients with prominent tremor-at-rest performed rapid isometric index finger abductions under self paced (SP) and reaction time (RT) conditions, and force and surface EMG signals from the FDI muscle were analyzed. In both conditions the SP and the RT, data showed that the onset of the contraction mostly occurs during the descending phase of the tremor oscillation and, accordingly, the phasic EMG burst synchronously with the tremor bursts confirming the above mentioned hypothesis.

Aged↗

PubMed1991

Abnormal single motor unit behavior in the upper motor neuron syndrome.

We studied the discharge pattern of single motor units (SMUs) in the left and right biceps muscles from a patient with nonspastic weakness of the left arm. Detailed statistical analysis of the behavior of discharge patterns of 4 of 4 single motor units on the affected side showed abnormalities with characteristic features of an upper motor neuron lesion. Five out of 5 single motor units recorded from the right biceps were normal. An upper motor neuron lesion affecting the left arm, predicted by our results, was confirmed by magnetic resonance imaging (MRI), which showed a lesion in the right precentral gyrus. It appears that changes in single motor unit firing characteristics, caused by an upper motor neuron lesion, can be detected at a time when there is no evidence of increased "tone" and/or hyperreflexia (spasticity) in the affected extremity.

Arm↗

PubMed1991

Abnormal most-rapid isometric contractions in patients with Parkinson's disease.

Fast isometric elbow flexor muscle contractions of specified amplitude in six normal subjects were compared with those of 11 patients with Parkinson's disease. Despite treatment, all patients exhibited deficits in this motor task. Three patients were able to produce rapid force pulses with normal contraction times, but the variability of their force responses was increased in comparison with the highly stereotyped responses produced by normal subjects. The other eight patients had prolonged contraction times and segmentation of the force profiles. The integrated area of the first agonist EMG burst and the rate of development of force (dF/dt) were less at any target level than what was needed to produce a fast response. The area of the EMG burst, however, did increase with target amplitude, and the relative increase of dF/dt, with target amplitude, was normal. It is concluded that the motor program subserving fast muscle contraction is preserved in Parkinson's disease, but its execution is characterised by improper scaling of motor output.

Adult↗

PubMed1988

Motor unit synchronization in physiologic, enhanced physiologic, and voluntary tremor in man.

Synchronization between pairs of single motor units simultaneously recorded from wrist extensor muscles was quantitated in 3 normal subjects during physiologic tremor (PT), beta-adrenergically enhanced physiologic tremor (EPT), and fast voluntary wrist flexion-extension movements mimicking tremor (VT). Cross-correlation histograms generated from the two spike trains of each motor unit pair demonstrated central or paracentral peaks in 13/19 recordings during PT, 22/36 during EPT, and 6/7 during VT. Relative peak area was used as a quantitative index of synchronization between the two motor units of each pair. It was lowest in PT, progressively increased in EPT as tremor amplitude increased, and highest in VT. In PT and lower amplitude EPT, the synchronization indexes were higher between motor units that discharged at the same or nearly the same frequency. In contrast, in higher amplitude EPT and VT, motor units with different firing frequencies were sometimes strongly synchronized as a consequence of double discharges in faster-firing motor units that had burst repetition rates in the range of slower-firing motor units discharging as singlets. Greater motor unit synchronization with increasing tremor amplitude in EPT may be secondary to a simultaneous increase in muscle spindle afferent activity from the tremulous muscle. Greatest synchronization in VT presumably reflects near maximal supraspinal and segmental common synaptic input onto motoneurons that generate VT. These results support a longstanding hypothesis that synchronization of motor units is the physiological basis for higher amplitude tremor.

Electromyography↗

PubMed1987

A method for assessing significance of peaks in cross-correlation histograms.

Cross-correlation histograms have been widely used to analyze the interdependence of two simultaneously observed trains of neuronal spikes or muscle motor unit discharges. Here, a formula is presented for calculating a synchronization index from such a histogram to reliably detect subtle correlations such as short-term synchronization, even in the case of relatively sparse data, as well as allowing comparison of the degree of synchronization of grouped or correlated motor unit discharges. The index takes into account the number of counts in the histogram, number of bins, and width of the histogram peak. A table of critical values of the index, at several levels of statistical significance, is included.

Animals↗

PubMed1987

Electromyographic studies of motor control in humans.

Electromyography and electroneurography have proved to be useful in investigation and understanding of a variety of neurologic disorders. In most laboratories, however, these electrodiagnostic techniques have been used to help in the diagnosis of diseases that affect the peripheral nerves, neuromuscular junctions, or skeletal muscle fibers. Although major advances in electronic and computer technology have made it possible to study, quantitate, and document reflex activity in intact human subjects, most neurologists still rely on gross clinical observations and most electromyographers continue to use conventional techniques of EMG and nerve conduction studies to differentiate "myopathy" from "neuropathy." This article is a review of some of the electromyographic techniques that have been used in the authors' laboratory for the study of normal and abnormal motor control in man and the treatment of patients with disorders of motor control.

Biofeedback, Psychology↗

PubMed1986

Role of agonist and antagonist muscles in fast arm movements in man.

Fast goal-directed voluntary movements of the human upper extremity are known to be associated with three distinct bursts of EMG activity in antagonistic muscles. The role of each burst (AG1, ANT, AG2) in controlling motion is not fully understood, largely because overall limb response is a complex function of the entire sequence of bursts recorded during experimental trials. In order to investigate the role of each burst of muscle activity in controlling motion, we studied fast voluntary arm movements and also developed two simulation techniques, one employing a mathematical model of the limb and the other using electrical stimulation of human arm muscles. These techniques show that two important movement parameters (peak displacement, time to reach peak displacement) are non-linear functions of the magnitude of the antagonist input (torque and stimulation voltage, respectively, in our two simulations). In the fastest movements, the agonist muscle is primarily responsible for the distance moved, while the antagonist muscle provides an effective means of reducing movement time. The third component of the triphasic pattern moderates the antagonist braking forces and redirects the movement back to the target.

Arm↗

PubMedDate not supplied

Inhibition of EMG activity in isometrically loaded agonist muscle preceding a rapid contraction.

It was recently suggested that a premotion silent period in isometrically loaded agonist muscle reflects transition process in switching motor program from an isometric to an isotonic condition. To test this hypothesis we investigated changes in EMG activity of a biceps muscle during motor preparatory phase in an entirely isometric paradigm. Five healthy volunteers produced rapid isometric elbow flexions superimposed on a slightly sustained contraction during self-paced and reaction time conditions. In addition to surface EMG, single motor units were recorded in agonist muscle prior to a rapid contraction. Silent period was found in responses of three out of five subjects, whereas inhibition of some but not all tonically active motor units was observed in all studied subjects prior to the phasic EMG burst. Inhibition of motor unit activity occurred more often in self-paced than reaction time contractions. Our results indicate that inhibition is not always powerful enough to produce a complete electrical silence, and weaker inhibitory effects were observed as a declining number of firing motor units. We conclude that the silent period or depression of the EMG activity is associated with changes in motor program from tonic to phasic muscle activation and is not merely a consequence of changes from an isometric to an isotonic condition.

Action Potentials↗