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  • 1
    Electronic Resource
    Electronic Resource
    A comparison of models explaining muscle activation patterns for isometric contractions (1999)
    Springer
    Biological cybernetics 81 (1999), S. 249-261 
    Details
    ISSN: 1432-0770
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology , Computer Science , Physics
    Notes: Abstract. One of the main problems in motor-control research is the muscle load sharing problem, which originates from the fact that the number of muscles spanning a joint exceeds the number of degrees of freedom of the joint. As a consequence, many different possibilities exist for the activation of muscles in order to produce a desired joint torque. Several models describing muscle activation have been hypothesized over the last few decades to solve this problem. This study presents theoretical analyses of the various models and compares the predictions of these models with new data on muscle activation patterns for isometric contractions in various directions. None of the existing models fitted the experimental data in all aspects. The best fit was obtained by models based on minimization of the squared sum of muscle forces (∑ m φ2 m , which is almost equivalent to the Moore-Penrose pseudo-inverse solution), muscle stress σ (∑ m σ m 2) or muscle activation α (∑ m α m 2). Since muscle activation patterns are different for isometric contractions and for movements, it could well be that other models or optimization criteria are better suited to describe muscle activation patterns for movements. The results of our simulations demonstrate that the predicted muscle activation patterns do not depend critically on the parameters in the model. This may explain why muscle activation patterns are highly stereotyped for all subjects irrespective of differences between subjects in many neuro-anatomical aspects, such as, for example, in the physiological cross-sectional area of muscle.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/s004220050560
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  • 2
    Electronic Resource
    Electronic Resource
    Characterization of spatial and temporal properties of monkey LGN Y-cells (1981)
    Springer
    Biological cybernetics 40 (1981), S. 157-170 
    Details
    ISSN: 1432-0770
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology , Computer Science , Physics
    Notes: Abstract 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 of 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 at 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.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00453366
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  • 3
    Electronic Resource
    Electronic Resource
    Reconstruction of cone-system contributions to responses of colour-opponent neurones in monkey lateral geniculate (1982)
    Springer
    Biological cybernetics 44 (1982), S. 211-221 
    Details
    ISSN: 1432-0770
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology , Computer Science , Physics
    Notes: Abstract Responses of colour-opponent X-cells to intensity-modulation at various wavelengths were obtained in the lateral geniculate nucleus (LGN) of the anaesthetized (N2O/O2) rhesus monkey. The gaussian white noise (GWN) analysis method was used to describe the stimulus-response relationship. Two different methods were used to estimate sign and relative strength of the response contribution of each of the three known cone systems as a function of time. Both methods revealed that, in contrast to the wellknown variability in gain and sign, the time course of the cone-type contributions was remarkably stereotyped in all cells. Surround-mediated cone-type contributions appeared to have a consistently longer delay than centre-mediated inputs. Response contributions from different types of cone appeared to add linearly in LGN neurones. Apart from rectification, it was possible to predict the response of the same neurone to step-modulation of intensity at various wavelengths successfully with the first-order Wiener kernel. This demonstrates that the cells behaved linearly under our stimulus conditions, which justifies the use of the first-order kernel as a means to characterize the system we wished to study.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00344277
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  • 4
    Electronic Resource
    Electronic Resource
    A model of the motor servo: Incorporating nonlinear spindle receptor and muscle mechanical properties (1987)
    Springer
    Biological cybernetics 57 (1987), S. 217-231 
    Details
    ISSN: 1432-0770
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology , Computer Science , Physics
    Notes: Abstract A model for the stretch reflex is proposed incorporating a nonlinear description of muscle receptor behavior, a delay in the reflex loop and a model of muscle mechanical properties. The model adequately describes the nonlinear response properties of EMG and force to constant ramps in loading and unloading direction. The EMG responses during the ramp and at ramp plateau could be simulated adequately for all ramp velocities except for high stretch velocities where EMG activity appeared in bursts, presumably due to spinal nonlinearities. Force responses during ramp stretches could be simulated except at ramp plateau, where the measured force response decayed slower than the simulated responses. The model also explained that EMG and force responses during ramp stretches after a displacement of about 1 cm could be approximately described by a product relationship between a position-related term and a low-fractional power of velocity. During unloading ramps the model did not predict a clear velocity dependence in agreement with the data.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00338815
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  • 5
    Electronic Resource
    Electronic Resource
    Frequency dependence of the action-perception cycle for postural control in a moving visual environment: relative phase dynamics (1994)
    Springer
    Biological cybernetics 71 (1994), S. 489-501 
    Details
    ISSN: 1432-0770
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology , Computer Science , Physics
    Notes: Abstract When standing human subjects are exposed to a moving visual environment, the induced postural sway displays varying degrees of coherence with the visual information. In our experiment we varied the frequency of an oscillatory visual display and analysed the temporal relationship between visual motion and sway. We found that subjects maintain sizeable sway amplitudes even as temporal coherence with the display is lost. Postural sway tended to phase lead (for frequencies below 0.2 Hz) or phase lag (above 0.3 Hz). However, we also observed at a fixed frequency, highly variable phase relationships in which a preferred range of phase lags is prevalent, but phase jumps occur that return the system into the preferred range after phase has begun drifting out of the preferred regime. By comparing the results quantitatively with a dynamical model (the sine-circle map), we show that this effect can be understood as a form of relative coordination and arises through an instability of the dynamics of the action-perception cycle. Because such instabilities cannot arise in passively driven systems, we conclude that postural sway in this situation is actively generated as rhythmic movement which is coupled dynamically to the visual motion.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00198467
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  • 6
    Electronic Resource
    Electronic Resource
    Frequency dependence of the action-perception cycle for postural control in a moving visual environment: relative phase dynamics (1994)
    Springer
    Biological cybernetics 71 (1994), S. 489-501 
    Details
    ISSN: 1432-0770
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology , Computer Science , Physics
    Notes: Abstract.  When standing human subjects are exposed to a moving visual environment, the induced postural sway displays varying degrees of coherence with the visual information. In our experiment we varied the frequency of an oscillatory visual display and analysed the temporal relationship between visual motion and sway. We found that subjects maintain sizeable sway amplitudes even as temporal coherence with the display is lost. Postural sway tended to phase lead (for frequencies below 0.2 Hz) or phase lag (above 0.3 Hz). However, we also observed at a fixed frequency, highly variable phase relationships in which a preferred range of phase lags is prevalent, but phase jumps occur that return the system into the preferred range after phase has begun drifting out of the preferred regime. By comparing the results quantitatively with a dynamical model (the sine-circle map), we show that this effect can be understood as a form of relative coordination and arises through an instability of the dynamics of the action-perception cycle. Because such instabilities cannot arise in passively driven systems, we conclude that postural sway in this situation is actively generated as rhythmic movement which is coupled dynamically to the visual motion.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/s004220050108
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  • 7
    Electronic Resource
    Electronic Resource
    Identification of the nonlinear state-space dynamics of the action-perception cycle for visually induced postural sway (1996)
    Springer
    Biological cybernetics 74 (1996), S. 427-437 
    Details
    ISSN: 1432-0770
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology , Computer Science , Physics
    Notes: Abstract Human subjects standing in a sinusoidally moving visual environment display postural sway with characteristic dynamical properties. We analyzed the spatiotemporal properties of this sway in an experiment in which the frequency of the visual motion was varied. We found a constant gain near 1, which implies that the sway motion matches the spatial parameters of the visual motion for a large range of frequencies. A linear dynamical model with constant parameters was compared quantitatively with the data. Its failure to describe correctly the spatiotemporal properties of the system led us to consider adaptive and nonlinear models. To differentiate between possible alternative structures we directly fitted nonlinear differential equations to the sway and visual motion trajectories on a trial-by-trial basis. We found that the eigenfrequency of the fitted model adapts strongly to the visual motion frequency. The damping coefficient decreases with increasing frequency. This indicates that the system destabilizes its postural state in the inertial frame. This leads to a faster internal dynamics which is capable of synchronizing posture with fast-moving visual environments. Using an algorithm which allows the identification of essentially nonlinear terms of the dynamics we found small nonlinear contributions. These nonlinearities are not consistent with a limit-cycle dynamics, accounting for the robustness of the amplitude of postural sway against frequency variations. We interpret our results in terms of active generation of postural sway specified by sensory information. We derive also a number of conclusions for a behavior-oriented analysis of the postural system.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00206709
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  • 8
    Electronic Resource
    Electronic Resource
    Activation of human arm muscles during flexion/extension and supination/pronation tasks: A theory on muscle coordination (1989)
    Springer
    Biological cybernetics 61 (1989), S. 1-9 
    Details
    ISSN: 1432-0770
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology , Computer Science , Physics
    Notes: Abstract Generally the number of muscles acting across a joint exceeds the number of degrees of freedom available to the joint. This redundancy raises a problem regarding the ratio in which these muscles are activated during a particular motor task. In this paper we present a theory to explain the activation patterns of muscles used during voluntary and reflex induced contractions. The basic assumptions underlying the theory are that 1) coordination of muscles is based on synergistic muscle activities, 2) the synergisms involved satisfy certain transformations of muscle spindle signals to muscle activation signals and 3) muscle spindle output is proportional to the ratio of muscle stretch and muscle length in lengthening muscles, and is zero in shortening muscles. The theory is used to predict the recruitment threshold of motor units in six arm muscles during voluntary isometric contractions. All theoretical predictions are in reasonable agreement with the experimentally observed behavior of a large population of motor units within each muscle. However, within a single muscle sometimes motor-unit populations have been found to have different types of recruitment behavior. This deviating behavior is discussed in the light of the theory presented here.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00204754
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  • 9
    Electronic Resource
    Electronic Resource
    Viscoelastic properties of the wrist motor servo in man (1984)
    Springer
    Annals of biomedical engineering 12 (1984), S. 599-620 
    Details
    ISSN: 1573-9686
    Keywords: Stretch reflex ; Muscle ; Elasticity ; Viscosity
    Source: Springer Online Journal Archives 1860-2000
    Topics: Medicine , Technology
    Notes: Abstract Viscoelastic properties play an important role in posture and movement. Such properties arise from muscle mechanics and from stretch-reflex actions. We describe experiments designed to characterize both linear and nonlinear elastic and viscous properties of the wrist motor servo in human subjects. First, we describe a trial comparison method for the identification of reflex responses that are unmodified by triggered reaction-time movements. Elastic properties were studied by applying step changes in load force that stretched or released the wrist flexor and extensor muscles. The properties were basically spring-like, but there was a short-range enhancement of stiffness that gave rise to a prominent hysteresis. Viscous properties were studied by applying ramp stretches at different velocities. Both EMG and force responses showed a weak fractional-power dependence on velocity similar to that described recently for muscle spindle receptors. Consideration is given to the possible advantages of this type of nonlinear feedback in the damping of postural responses and movements.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF02371452
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  • 10
    Electronic Resource
    Electronic Resource
    Equipment for the quantification of motor performance for clinical purposes (1987)
    Springer
    Medical & biological engineering & computing 25 (1987), S. 311-316 
    Details
    ISSN: 1741-0444
    Keywords: Clumsiness ; Motor control ; Motor disorder ; Reflex ; Stiffness ; Tracking behaviour
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology , Chemistry and Pharmacology , Medicine
    Notes: Abstract 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 movements the mass and friction of the mechanical part of the equipment are eliminated by feedback of the force exerted at the handle by the subject. The computer program that controls the apparatus gives a choice of four different tests that characterise different aspects of the motor system: the reflex organisation, the regulation of viscoelastic properties mediated in part by reflex activity, the control of fast goal-directed movements, and the performance in a tracking task. The results of a pilot study to the tracking behaviour of clumsy children show that the group of clumsy children differs from a group of normal children in the latency of the tracking response, in the ability to track high-frequency components and in the fact that clumsy children introduce relatively more frequency components in the response that are not present in the tracking signal.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF02447430
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