Organization of goal-directed action at a high level of motor skill: The case of stone knapping in India

We analyzed the relationship between goal achievement and execution variability in craftsmen who have acquired the highest "ultimate" skills of stone knapping. The goal of a knapping movement is defined as the vector of the final velocity of a hammer, crucial for detaching a flake and, consequently, for the shape of the final product. The execution of the movement is defined by the kinematic pattern of the arm (i.e., by the coordination between the joint angles corresponding to the seven arm degrees of freedom). The results show that (a) the direction of final velocity is very stable for all craftsmen, whereas the amount of kinetic energy transmitted to the stone was craftsman specific and (b) the kinematic pattern of the arm was strongly individual and was a reliable sign of the level of skill-the highest level was characterized by the highest flexibility of movement kinematics. We stress the importance of conducting the experiment in natural conditions for better understanding of the relationship among the purpose (the final shape of the stone), the goal, and the execution of the movement.