Neural Network Modelling of Voluntary Single Joint Movement Organization I. Normal Conditions

Voluntary movements are goal-directed movements triggered either by internal or external cues. Voluntary movements can be improved with practice as one learns to anticipate and correct for environmental obstacles that perturb the body. Single-joint rapid (ballistic) movements are goal-directed movements performed in a single action, without the need for corrective adjustments during its course. They are characterized by a symmetric bell-shaped velocity curve, where the acceleration (the time from the start to the peak velocity) and deceleration (the time from the peak velocity to the end of movement) times are equal (Britton et al. 1994). Similar velocity profiles have also been observed in multi-joint movements (Camarata et al. 1992). The electromyographic (EMG) pattern of single-joint rapid voluntary movements in normal subjects is also very characteristic. It is characterized by alternating bursts of agonist and antagonist muscles (Hallett et al. 1975a). The first agonist burst provides the impulsive force for the movement, whereas the antagonist activity provides the braking force to halt the limb. Sometimes a second ago-nist burst is needed to bring the limb to the final position (Berardelli et al. The combination of the agonist-antagonist-agonist bursts is known as the triphasic pattern of muscle activation (Hallett et al. 1975a). An excellent review on the properties of the triphasic pattern of muscle activation and the produced movement under different experimental conditions can be found in Berardelli and colleagues (1996). The origin of the triphasic pattern and whether it is controlled by the nervous system has been long debated (Stein 1982). In a review paper by Ber-ardelli and colleagues (1996), three conclusions were made: (1) the basal ganglia output plays a role in the scaling of the first agonist burst size, (2) the corticospinal tract has a role in determining spatial and temporal recruitment of motor units, and (3) the proprioceptive feedback is not necessary to the production of the triphasic pattern, but it contributes to the accuracy of both the trajectory and the end-point of ballistic movements. That means that the origin of the triphasic pattern of muscle activation may be a central one, but afferent inputs can also modulate the voluntary activity.