Muscle activation patterns in point-to-point and reversal movements in healthy, older subjects and in subjects with Parkinson's disease.

When young, healthy subjects perform rapid point-to-point and reversal movements over a range of distances, the patterns of muscle activation associated with accelerating the limb toward the target are modulated in the same way for both movement tasks. Differences in patterns of muscle activation for these two movement types are not observed until the deceleration phase of the movements. In this study, we first test the hypothesis that healthy, older subjects and subjects with Parkinson's disease will modulate the pattern of muscle activation in the same way during the acceleration phase of point-to-point and reversal elbow movements. Second, we test the hypothesis that healthy, older subjects and subjects with Parkinson's disease exhibit the same relationship in muscle activation patterns between the two movement types that have been observed for the young in the deceleration phase of the movements. Subjects performed point-to-point and reversal movements initiated in the direction of flexion over three distances (36, 54 and 72 degrees) "as fast as possible". Angle, velocity, acceleration and surface EMGs from biceps and triceps were recorded. With respect to the first hypothesis, the EMG, kinetic, and kinematic measures related to the acceleration phase of the movements were modulated in the same way for both movement types in the healthy older subjects. In the Parkinson's disease group, the kinematic and kinetic measures during the acceleration phase of the movements were the same in both movement types; however, the flexor and extensor EMG activation was smaller during reversal movements than during point-to-point movements. With respect to the second hypothesis, in contrast to that found in young subjects, in healthy older subjects, there was no significant difference between the movement types in the flexor EMG activity immediately after the time of peak velocity. This difference between younger and older subjects may be attributed to the fact that older subjects perform both movement types more slowly than do younger subjects. Although subjects with Parkinson's disease also move slowly, the flexor EMG shuts off more abruptly and more completely just after the time of peak velocity during reversal movements than during point-to-point movements. These results show that (1) for healthy subjects, when the task requirements are the same for the two movement types (acceleration phase), muscle activation patterns are modulated in the same way, and (2) both age and disease alter the relationship of muscle activation, kinetics and kinematics between point-to-point and reversal movements.

Control of movement distance in Parkinson's disease.

Studies of electromyographic (EMG) patterns during movements in Parkinson's disease (PD) have often yielded contradictory results, making it impossible to derive a set of rules to explain how muscles are activated to perform different movement tasks. We sought to clarify the changes in modulation of EMG parameters associated with control of movement distance during fast movements in patients with PD. Specifically, we studied surface EMG activity during rapid elbow flexion movements over a wide range of distances (5-72 degrees) in 14 patients with relatively mild symptoms of PD and 14 control subjects of similar age, sex, height, and weight. The PD group exhibited several changes in EMG modulation including impaired modulation of agonist burst duration; increased number of agonist bursts; reduced scaling of agonist EMG magnitude in the more severely impaired subjects; and increased temporal overlap of the antagonist and agonist signals in the most severely impaired subjects. These findings suggest that progressive motor dysfunction in PD is accompanied by increasing deficits in modulating muscle activation. These results help clarify previous disparate and sometimes contradictory results of EMG patterns in subjects with PD.

Pallidotomy and bradykinesia: implications for basal ganglia function.

BACKGROUND AND OBJECTIVE: The scientific rationale for pallidotomy as a treatment for PD is that the lesion will reduce excessive tonic inhibition of the thalamus, thereby allowing movement to proceed more normally. If true, then PD patients who move slowly while on medication should increase movement speed following pallidotomy. To test this we used a simple motor task to determine if pallidotomy leads to an improvement in "on" motor performance when those movements are impaired before surgery. METHODS: Nine patients with PD performed elbow flexion movements "as fast as possible" while they were "on" before and 1 month after pallidotomy. Patients with mild PD and healthy control subjects were also tested. RESULTS: The clinical effects of pallidotomy were typical of those found in other studies. "Off" Unified Parkinson's Disease Rating Scale scores improved and dyskinesias were reduced. Although before surgery the patients were far slower while they were "on" than the groups of mild PD patients and healthy control subjects, there was no change in mean peak velocity while they were "on" after pallidotomy. There was no change in other mean "on" motor performance measures such as peak acceleration, peak deceleration, initiation time, and symmetry. There was a decrease in the variability of peak acceleration, symmetry, and initiation time. CONCLUSION: Despite the clinical efficacy of pallidotomy while patients were "off," bradykinesia of elbow flexion movements while patients were "on" is not affected by pallidotomy. Therefore, we conclude that the bradykinesia observed in this experiment is due to a mechanism other than excessive tonic inhibition of the motor thalamus. Our results are consistent with the idea that pallidotomy reduces the noise from the abnormally functioning basal ganglia.

Common principles underlying the control of rapid, single degree-of-freedom movements at different joints.

Studies of rapid, single degree-of-freedom movements have shown different changes in electromyographic patterns for movement tasks that appear very similar (e.g., movements over different ranges of distance). However, it is not clear whether these differences are a result of joint-specific control schemes or whether they are instead due to the limited range of task parameters studied relative to the mechanical constraints of each joint (e.g., short compared with long movements relative to the range of motion of a particular joint). In this study, we measured and compared the kinematic trajectories and electromyograms recorded during various movement tasks at the wrist, elbow, and ankle. Subjects performed movements over a wide range of distances "as fast as possible," "at a comfortable speed," and against two inertial loads (at the elbow only), and they performed movements over a fixed distance at three different speeds at the wrist and ankle. For fast movements we show that, in spite of some joint-specific differences, the basic pattern of electromyographic (EMG) modulation is similar at all three joints; for example, the agonist EMG burst transitions from a fixed duration to an increasing duration with increasing movement distance at all three joints. Moreover, the distance at which this transition occurs in one joint relative to the distance at which this transition occurs in the other two joints is consistent across subjects. The transition occurs at the shortest distance at the ankle and the longest distance at the wrist. In general we suggest that the data are consistent with a single set of control rules applied at all three joints, with the biomechanical constraints at each joint accounting for the differences in the EMG and kinematic patterns observed across joints.

Effect of stimulation in the ventral intermediate nucleus of the thalamus on limb control in Parkinson's disease: a case study.

This study focuses on upper extremity strength and movement control in a patient with Parkinson's disease who had stimulating electrodes surgically implanted in the ventral intermediate nucleus (VIM) of the left thalamus. We examined torque generation and control of movement distance in single degree-of-freedom elbow movements under three different stimulation conditions: (a) no stimulation, (b) high stimulation, in which tremor was minimized but there was also tingling and perceived weakness, and (c) moderate stimulation, in which tremor was partially reduced, but there was also a subjective sense of increased strength compared with the high-stimulation condition. The patient's ability to generate both steady torque and rapid movements was poorest with no stimulation. The patient generated the largest torques with moderate stimulation and performed the fastest movements with high stimulation. However, even with tremor minimized, the patient's electromyogram (EMG) burst patterns were not typical of those of neurologically healthy subjects, although the movements were clearly improved.

New models of the oculomotor mechanics based on data obtained with chronic muscle force transducers.

Several phenomenological models of the oculomotor mechanics that produce saccadic eye movements have been developed. These models have been based on measurements of macroscopic muscle and orbital tissue properties and measurements of eye kinematics during saccades. We recorded the forces generated by the medial and lateral recti during saccades in an alert, behaving monkey using chronically implanted force transducers. With this new data, we tested the ability of the classic saccade models to generate realistic muscle force profiles. Errors in the predictions of the classic saccade models led to a reexamination of the current models of extraocular muscle. Both a phenomenological, Hill-type muscle model and an approximation to Huxley's molecular level muscle model based on the cross-bridge mechanism of contraction (distribution moment model) were derived and studied for monkey extraocular muscle. Simulations of the distribution moment model led to insights suggesting (i) specific modifications in the lumped force/velocity relationship in the Hill-type model that resulted in this type of phenomenological model being able to generate realistic dynamics in extraocular muscle during saccades; (ii) the distribution of activity in the different fiber types in extraocular muscle may be central to the characteristics exhibited by the muscle during saccades; (iii) the transient properties of lengthening muscle such as yielding are not significant during saccades; and (iv) the series elastic component in active muscle may be predominantly generated by the elastic properties of the cross-bridges.