A Method for Identifying the Initiation of Reaching Movements in Natural Prehension

Identifying the start of a movement is critical for calculating variables such as movement duration, movement speed, or trajectory length and straightness. In most experiments, the problem is simplified by giving instructions to the subjects and by constraining the task. This is not possible in populations that are too young or disabled to understand and execute the task as instructed. In this article, a method is presented for reliably identifying the onset of reaches in infants, whose movements are embedded in their ongoing activities. A computer program prompts the user to choose reach initiation on the basis of comparison between hand kinematics and videotaped behavior and to code the reach as emerging from 1 of 4 movement contexts. The method is highly reliable in infants of all ages and captures expected age-related changes in reaching. It is suggested that researchers could use this or a similar method to study movements that are part of natural, ongoing activities in normal and in clinical populations