Two joints ballistic arm movements.

Fast planar arm movements involving two joints have been analyzed in humans. The EMG activity associated with the drawing of straight lines or geometric figures was characterized by sequences of bursts in the agonist and antagonist muscles of constant duration and different amplitude. The shape of each trajectory is defined by a particular sequence of burst activity in the four muscles studied. A pattern of one burst in the agonist and one in the antagonist is the basic building block for different kinds of ballistic trajectories. The time of execution increased linearly with the number of sides of the geometric figures by steps of about 210 ms and did not increase linearly with their size.