Biomechanical structure of the arm predicts kinematic invariants of hand movements

It has been noticed that during arm movements the hand trajectory is characterized by a number of kinematic invariants that are observed during various direction, amplitude, speed, and load conditions. For instance, the hand trajectory is usually close to a straight line and the velocity profile of the hand has a bell shape during point-to-point movements. During drawing movements the two-third power law is observed that predicts variations in the hand velocity depending on the trajectory curvature. These and other consistent kinematic characteristics of hand motion are usually interpreted as preplanned features of movement that the central nervous system (CNS) pursues for some particular reason, such as decreasing the number of degrees of freedom. The present study demonstrates that the geometrical structure of the arm and the organization of muscular control at each joint predict the kinematic invariants. These predictions are derived analytically with use of the small approximation approach for description of the hand movement as a function of sinusoidal movements of small amplitude at the shoulder and elbow. Our experimental data support the predictions derived for small movements and demonstrates that these predictions hold for movements usually studied in human motor control.