M. Margaret Wierzbicka

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 Parkinsons 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 Parkinsons disease, but its execution is characterised by improper scaling of motor output.

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

SummaryFast 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.

Cervical root stimulation in the diagnosis of radiculopathy

Cervical root stimulation (CRS) was compared with conventional EMG, nerve conduction, and late response studies in 34 patients with possible cervical radiculopathy. Cervical roots were stimulated by monopolar needles inserted into paraspinal muscles, recording compound muscle action potentials in biceps, triceps, and abductor digiti minimi muscles. In 18 patients with clinical evidence of radiculopathy, EMG was abnormal in 11 (61%), but CRS was abnormal in all 18. Of 16 patients with symptoms but no signs of radiculopathy, EMG was abnormal in 5 (31%) and CRS was abnormal in 9 (56%).

Kinematic models and human elbow flexion movements: Quantitative analysis

SummaryThe 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 (< 300 ms duration) were fairly symmetric in that the durations and amplitudes of acceleration and deceleration were approximately equal; slower movements (> 300ms) 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.

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.

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 Parkinsons 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.

Orthosis for Improvement of Arm Function in C5/C6 Tetraplegia

People with spinal cord injury at the C5/C6 motor complete level typically have relatively well-preserved biceps function but minimal or no voluntary control of triceps. The authors previously have shown the lack of voluntary control of triceps results in deficiencies in speed and accuracy of elbow movements as well as reduction in reachable workspace. The authors also have shown these deficiencies can be corrected by the addition of constant extensor torque at the elbow. The purpose of this article is to describe a prototype constant-torque elbow orthosis and illustrate the improvements in function it affords people with C5/C6 tetraplegia.

Design of a triceps orthosis for C5/C6 quadriplegics

Persons with spinal cord injury at the C5/C6 cervical level typically have relatively well preserved biceps function, but minimal or no voluntary control of triceps. Our previous work has showed that this results in deficiencies in speed and accuracy of elbow movements. Our goal is to build a simple mechanical orthosis, worn at the elbow, to correct these deficiencies. In this study, we have further defined the mechanical components of the orthosis by using a mathematical model and experiments with C5/C6 quadriplegics in which mechanical springs and damping elements were electronically emulated.