Chapter 14 New aspects of human muscle coordination as revealed by motor-unit studies
Natural and Artificial Control of Hearing and Balance, 1993
Increasing evidence has become available against the view that individual muscles should be regar... more Increasing evidence has become available against the view that individual muscles should be regarded as functional units in the control of force by the motor apparatus. The relative contribution of various motoneurone pools (belonging to a single muscle) to muscle force appears to depend on the direction of torque exerted by the corresponding limb. This clearly indicates the presence of more than one control or activation parameter for the total motoneurone pool of a single muscle. Windhorst et al. (1989) have even proposed a combined sensory and neuromuscular partitioning, such that segmental control mechanisms are based on subdivisions of motoneurone pool--muscle complexes. Size-related recruitment occurs within these subdivisions rather than in the motoneurone pool as a whole. Moreover, motoneurones can operate in various modes (different relative contributions of the recruitment mechanism and the firing frequency mechanism to muscle force) related either to movement or to force. This indicates that muscle coordination is organised differently in force tasks and in movement tasks. These properties of motoneurone activation mentioned above are difficult to incorporate in existing models on sensorimotor organisation. Some of these discrepancies will be reviewed in order to highlight future directions for modelling.
Too Early to Explain All Experimental Data with a Single Model
Motor Control, 2000
The target article by Prilutsky gives an excellent overview of the predictions of the Crowninshie... more The target article by Prilutsky gives an excellent overview of the predictions of the Crowninshield and Brand model and about the relevant literature about muscle coordination. However, we do not agree with the claim that the Crowninshield and Brand model can explain the coordination between one-joint and two-joint muscles. In this commentary we will make three claims: (a) The Crowninshield and Brand model cannot explain all aspects of muscle coordination, (b) there is good experimental evidence that different constraints and models may be necessary to explain muscle coordination in different motor tasks, and (c) the reason for the lack of quantitative fits between predictions about muscle force and experimental data is that it is hard to measure muscle force in man. As a compromise one has to rely on EMG activity as a measure of muscle force. Because of the complex relationship between EMG and muscle force, a quantitative test of models is difficult.
The Activation of Motor Units in Coordinated Arm Movements in Humans
Physiology, 1990
The motoneuron pool of muscles consists of several subpopulations, each of which receives a diffe... more The motoneuron pool of muscles consists of several subpopulations, each of which receives a different but homogenous activation during isometric and reflex-induced contractions. Studies on the dynamics of motor unit activation of skeletal muscle reveal important characteristics that are not reflected in electromyograms.
Comparison Between Psychophysical and Saccadic Responses to Near Threshold Visual Stimuli
Eye Movements from Physiology to Cognition, 1987
Publisher Summary This chapter makes a comparison between psychophysical and saccadic responses t... more Publisher Summary This chapter makes a comparison between psychophysical and saccadic responses to near threshold visual stimuli. There exists an inverse relationship between the contrast detection threshold and stimulus duration for stimuli up to a certain duration. The interval of this linear summation has been shown to be about 60 ms for luminance contrast stimuli and about 200 ms for chromatic stimuli equiluminant to the visual field. For saccadic measurements the observer has to respond by making a saccade to the stimulus at every stimulus presentation. The contrast detection threshold is defined as the point at which 80% of the responses are correct. The probability of evoking a correct saccade depends on the duration of a stimulus both for luminance contrast stimuli and for chromatic stimuli. The spatiotemporal properties of visual stimuli affect saccadic responses in the same way as for psychophysical responses.
Equipment for the quantification of motor performance for clinical purposes
Medical & Biological Engineering & Computing, 1987
An apparatus is described for the quantitative assessment of important parameters that characteri... more An apparatus is described for the quantitative assessment of important parameters that characterise motor performance in normal subjects and in patients with different types of motor disorders. The apparatus has a handle that can be moved along a straight horizontal track either by the subject (to study voluntary movements) or by a torque motor (to study reflex activity). During voluntary
Differences and similarities in electrical muscle activity for the abductor pollicis longus muscle divisions
Journal of Electromyography and Kinesiology, 1995
The abductor pollicis longus (APL) has to be viewed as an important muscle for moving and stabili... more The abductor pollicis longus (APL) has to be viewed as an important muscle for moving and stabilizing the human thumb(10). This muscle has two divisions, a distal superficial division and a more proximal deep one. The electromyographic (EMG) signals from these divisions were measured for several motor tasks in order to investigate differences in activation and function. EMG signals were recorded with intramuscular wire electrodes in isometric as well as in dynamic contractions in different directions, both for the thumb and for the hand. The EMG signals of the right hand of eight subjects were scaled relative to the mean EMG value at the maximum voluntary isometric contraction (MVC) in order to compare relative muscle activity in various directions for different subjects. In 18 out of the 22 directions the EMG activity in the two divisions of the APL was modulated differently, indicating a different activation. The differences were most prominent in dorsal and palmar flexion of the hand. The results suggested that the APLdeep is activated to stabilize the carpal joint. The APLdeep is a direct mover of the carpal joint in dorsal flexion of the hand.
Characterization of spatial and temporal properties of monkey LGN Y-cells
Biological Cybernetics, 1981
LGN Y-cells in 3 anaesthetized (N2O/O2) and paralyzed rhesus monkeys were investigated with stimu... more LGN Y-cells in 3 anaesthetized (N2O/O2) and paralyzed rhesus monkeys were investigated with stimuli, intensity modulated by gaussian white noise, and with moving and counterphase modulated spatial sine wave gratings. The results support the model, postulated on the base of electrophysiological recordings in the retina cat and mudpuppy, which consists of a linear centre and surround mechanism whose responses are modified in a frequency-selective multiplicative way by a nonlinear mechanism in the receptive field. This nonlinear mechanism is also held responsible for the second-order harmonic responses, which are the defining characteristic of Y-cells. The temporal and spatial characteristics of these mechanisms were determined. The responses obtained with the GWN stimulation and with modulated spatial sine wave gratings both indicate that the optimal temporal frequency of the linear mechanisms is near 7 Hz and 70 td and near 5 Hz for the nonlinear mechanism. The optimal spatial frequency for the linear mechanism is between 0.5--2 cycles/deg and between 6--12 cycles/deg for the nonlinear mechanism.
How do neuronal and muscle-mechanical properties contribute to the performance of the “delta lognormal” model?
Behavioral and Brain Sciences, 1997
Plamondon & Alimi's model will gain substantially in credibility when it is able to come up w... more Plamondon & Alimi's model will gain substantially in credibility when it is able to come up with predictions for new (rather than old) experimental results that discriminate between various models. Moreover, the present model is nothing more than a descriptive “black box,” not an explanation for motor performance. A link to the contribution of various neuronal mechanisms involved in motor control and of muscle properties to the performance of the model is crucial.
Automatic, unsupervised classification of dyskinesia in patients with Parkinson’s Disease
One of the characteristic symptoms of patients with Parkinson Disease (PD) is a rigidity of movem... more One of the characteristic symptoms of patients with Parkinson Disease (PD) is a rigidity of movement. These symptoms disappear after administration of Levodopa. However, the long-term use of levodopa causes involuntary movements (dyskinesia). A proper diagnosis requires an automatic, unsupervised method for the detection and classification of levodopa induced dyskinesia. The main problem, however, is to distinguish dyskinesia from voluntary movements. The aim of this study was to train a neural network for the classification and rating of dyskinesia and to use the trained neural networks to extract parameters, which are important to distinguish between dyskinesia and normal voluntary movements. The neural network has a performance near 97% correct, which greatly improves upon previous methods (typically 75% correct).
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