Current views of the control of complex, purposeful movements acknowledge that organizational processes must reconcile multiple concerns. The central priority is of course accomplishing the actor's goal. But in specifying the manner in which this occurs, the action plan must accommodate such factors as the interaction of mechanical forces associated with the motion of a multilinked system (classical mechanics) and, in many cases, intrinsic bias toward preferred movement patterns, characterized by so-called “coordination dynamics.” The most familiar example of the latter (...) is the symmetry constraint, where spatial trajectories and/or temporal landmarks (e.g., reversal points) of concurrentlymoving body segments (limbs, digits, etc.) exhibit mutual attraction. The natural coordination tendencies that emerge through these constraints can facilitate or hinder motor control, depending on the degree of congruency with the desired movement pattern. Motor control theorists have long recognized the role of classical mechanics in theories of movement organization, but an appreciation of the importance of intrinsic interlimb bias has been gained only recently.Although detailed descriptions of temporal coordination dynamics have been provided, systematic attempts to identify additional salient dimensions of interlimb constraint have been lacking. We develop and implement here a novel method for examining this problem by exploiting two robust principles of psychomotor behavior, the symmetry constraint and the Two-Thirds Power Law. Empirical evidence is provided that the relative spatial patterns of concurrently moving limbs are naturally constrained in much the same manner as previously identified temporal constraints and, further, that apparent velocity interference is an indirect, secondary consequence of primary spatial assimilation. The theoretical implications of spatial interference are elaborated with respect to movement organization and motor learning. The need to carefully consider the appropriate dimensions with which to characterize coordination dynamics is also discussed. (shrink)

Although the model proposed by Thelen and co-authors provides a detailed explanation for the processes underlying reaching, many aspects of it are highly speculative. One of the reasons for this is our lack of knowledge about transformation of a hand movement plan into joint movements. The leading joint hypothesis (LJH) allows us to partially fill in this gap. The LJH offers a possible explanation for the formation of movement and how it may be represented in memory. Our explanation converges with (...) the dynamic model described in the target article. (shrink)