KINEMATIC DESCRIPTION OF VARIABILITY OF FAST MOVEMENTS - ANALYTICAL AND EXPERIMENTAL APPROACHES

Analysis of variability of fast aimed movements predicts the propertie s of trajectory variance. The analysis is based on a kinematic model w ith nonlinear changes in ''internal time''. The purpose of the work wa s to identify different sources of variability and their influence on the trajectory variance. An analytical expression for the speed-accura cy trade-off is introduced. Experiments were performed with subjects m aking single-joint elbow flexion movements over different distances as fast as possible with their eyes closed to memorized targets. Standar d deviation of movement trajectory increased during the first part of the movement and subsequently decreased. The variance peaked after the time of peak velocity, close to the time of peak deceleration. A depe ndence of the trajectory variance on movement distance (speed-accuracy trade-off) was seen during the movement (at times of peak velocity an d peak deceleration) but not after the movement termination. We conclu de that the previously reported drop in the variability of movement tr ajectory during the deceleration phase does not necessarily mean a com pensation by the control system but may result from purely kinematic p roperties of the movement. The importance of the time of measurement f or analysis of the speed-accuracy trade-offs is emphasized.