Moving effortlessly in three dimensions: does Donders' law apply to arm movement?

Donders' law, as applied to the arm, predicts that to every location of the hand in space there corresponds a unique posture of the arm as defined by shoulder and elbow angles. This prediction was tested experimentally by asking human subjects to make pointing movements to a select number of target locations starting from a wide range of initial hand locations. The posture of the arm was measured at the start and end of every movement by means of video cameras. It was found that, in general, the posture of the arm at a given hand location does depend on the starting location of the movement and that, consequently, Donders' law is violated in this experimental condition. Kinematic and kinetic factors that could account for the variations in arm posture were investigated. It proved impossible to predict the final posture of the arm purely from kinematics, based on the initial posture of the arm. One hypothesis was successful in predicting final arm postures, namely that the final posture minimizes the amount of work that must be done to transport the arm from the starting location.