THE REPRESENTATION OF GRAVITATIONAL FORCE DURING DRAWING MOVEMENTS OFTHE ARM

The purpose of the present experiment was to study the way in which th e central nervous system (CNS) represents gravitational force (GF) dur ing vertical drawing movements of the arm. Movements in four different directions: (a) upward vertical (0 degrees), (b) upward oblique (45 d egrees), (c) downward vertical (180 degrees) and (d) downward oblique (135 degrees), and at two different speeds, normal and fast, were exec uted by nine subjects. Data analysis focused upon arm movement kinemat ics in the frontal plane and gravitational torques (GTs) exerted aroun d the shoulder joint. Regardless of movement direction, subjects showe d straight-line paths for both speed conditions. In addition, movement time and peak velocity were not affected by movement direction and co nsequently changes in GT, for both speeds tested. Movement timing (eva luated through the ratio of acceleration time to total time) changed s ignificantly, however, as a function of movement direction and speed. Upward movements showed shorter acceleration times when compared with downward movements. Concerning the four directions, movements made at 0 degrees and 45 degrees differed significantly from those made at 135 degrees and 180 degrees. Drawing movements executed at rapid speed pr esented similar acceleration and deceleration times compared with move ments executed at normal speed, which showed greater acceleration than deceleration times. In addition, the form of velocity profiles (asses sed through the ratio of maximum to mean velocities), was significantl y modified only with movement speed. Results from the present study su ggest that GF is efficiently incorporated into internal dynamic models that the brain builds up for the execution of arm movements. Furtherm ore, it seems that GF not only is a mechanical parameter to be overcom e by the motor system but also constitutes a reference (vertical direc tion), both of which are represented by the CNS during inverse kinemat ic and dynamic processes.