Time course and temporal order of changes in movement kinematics during learning of fast and accurate elbow flexions

Learning of a motor task, such as making accurate goal-directed movements, is associated with a number of changes in limb kinematics and in the EMG ac tivity that produces the movement. Some of these changes include increases in movement velocity, improvements in end-point accuracy, and the developme nt of a biphasic/triphasic EMG pattern for fast movements. One question tha t has remained unanswered is whether the time course of the learning-relate d changes in movement parameters is similar for all parameters. The present paper focuses on this question and presents evidence that different parame ters evolve with a specific temporal order. Neurologically normal subjects were trained to make horizontal, planar movements of the elbow that were bo th fast and accurate. The performance of the subjects was monitored over th e course of 400 movements made during experiments lasting approximately 1.5 h. We measured time-related parameters (duration of acceleration, duration of deceleration, and movement duration) and amplitude-related parameters ( peak acceleration, peak deceleration, peak velocity), as well as movement d istance. In addition, each subject's reaction time and EMG activity was mon itored. We found that reaction time was the parameter that changed the fast est and that reached a steady baseline earliest. Time-related parameters de creased at a somewhat slower rate and plateaued next. Amplitude-related par ameters were slowest in reaching steady-state values. In subjects making th e fastest movements, a triphasic EMG patterns was observed to develop. Our findings reveal that movement parameters change with different time courses during the process of motor learning. The results are discussed in terms o f the neural substrates that may be responsible for the differences in this aspect of motor learning and skill acquisition.