Positioning End Effectors

Although the final motor commands act on muscles to move limb segments, the goal of a movement often involves the positioning of an end-effector. For example, to sip from a coffee cup, most people attend to the cup and not to what the elbow is doing. Signatures provide another case in point. Everyone has a unique signature, and its distinctive character persists even if different joints and muscles perform the signing movement. This principle has been called motor equivalence. Its basis is that the pencil serves as an end effector regardless of which muscles move it. The preservation of unique elements in a signature indicates that the highest levels of the CNS represent voluntary movements not as a pattern of muscle activations but as a kinematic pattern, specifically the desired motion of an end effector.

Of course, signatures consist of complicated movement trajectories. In principle, an infinite number of possible hand trajectories can be made between two points—some straight, others curved to varying degrees. However, unless the goal includes a curved trajectory, people show a remarkable similarity in the movements that they produce in reaching from a given hand position to a target. The hand moves with a unimodal, smooth, and symmetric velocity over the time course of the action, and it takes a straight-line path. Even blind people show this feature in their arm movements. When an experimenter demonstrates the target to a blind person by moving his or her hand to the target position (later returning it to the original location), he or she makes straight and smooth reaching movements just like sighted people. This smoothness in hand trajectory contrasts sharply with the changes that occur in joint positions during the same movement. For example, consider a movement of the right arm that starts with the hand at the far left of the midline and reaches to a target at the far right. For most people, this movement would be a straight line in terms of hand position. However, examination of joint angles shows that the elbow initially flexes and then extends. Therefore, its velocity is not unimodal. Human arm movements generally appear simple when described in terms of hand positions and velocities but are complex when described in joint coordinates. This regularity remains when people perform movements with different end effectors. For example, movements remain smooth and simple when our hands hold a long stick. In this case, the end of the stick moves smoothly and in a straight line.

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