Constraints in Coordinating Limb Movements

When coordinating movements of the limbs or fingers, constraints emerge that impose limits on our ability to perform different actions concurrently. For example, it is a common observation that one action pattern tries to impose its spatiotemporal characteristics on the other, or that both actions mutually affect each other. This has been denoted as structural or specific interference within information processing accounts. It has been argued to result from a fundamental limitation in the division of attention. Rather than a limitation, action theoretical accounts regard this phenomenon, called entrainment, as evidence for an economical control model in which an optimal solution to the degrees of freedom problem is explored. Entrainment is considered an important feature of coordinative structures. In acquiring new skills, humans often attempt to overcome the constraints imposed by entrainment. We discuss experimental evidence showing that practice enables subjects to inhibit or repress the resulting superfluous muscle activity. Moreover, in coordinating differing movements, the human control system manages to delay the initiation of one activity with reference to the other (possibly through inhibitory processes), thereby preventing the occurrence of interference. This delay mechanism allows functionally-related effector systems to perform concurrent actions largely independently. The generality of this mechanism needs to be further explored.