Rapid One-Joint Movements: A Qualitative Model and its Experimental Verification
One of two versions of the equilibrium-point hypothesis is used to provide a qualitative explanation for the control of fast one-joint movements. It is suggested that discrete unidirectional movements are produced by a rapid gradual shift in an equilibrium point to a final position, in combination with another gradual command which produces agonist-antagonist coactivation and ensures the stiffness of the joint. The monotonie central commands and the reflex effects occurring in the course of movement give rise to the characteristic three-burst pattern, as is qualitatively illustrated using the concept of muscle activation area. This form of central command also makes it possible to explain the observable correlation between the duration of the first agonist burst and movement amplitude. Two predictions of the model are experimentally corroborated in the present study: (1) movement trajectories with terminal oscillations can be transformed into aperiodic trajectories by means of transitory perturbations; (2) the onset time, magnitude and duration of the first agonist burst are reflex-dependent. The results are consistent with the assumption that neither the form of movement trajectory nor the form of EMG bursts (time periods and amplitudes) are centrally programmed.
KeywordsOnset Time Movement Trajectory Central Command Agonist Burst Antagonist Burst
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