Identifying the control structure of multijoint coordination during pistolshooting

The question of degrees of freedom in the control of multijoint movement is posed as the problem of discovering how the motor control system constrain s the many possible combinations of joint postures to stabilize task-depend ent essential variables. Success at a task can be achieved, in principle, b y always adopting a particular joint combination. In contrast, we propose a more selective control strategy: variations of the joint configuration tha t leave the values of essential task variables unchanged are predicted to b e less controlled (i.e., stabilized to a lesser degree) than joint configur ation changes that shift the values of the task variables. Our experimental task involved shooting with a laser pistol at a target under four conditio ns. The seven joint angles of the arm were obtained from the recorded posit ions of markers on the limb segments. The joint configurations observed at each point in normalized time were analyzed with respect to trial-to-trial variability. Different hypotheses about relevant task variables were used t o define sets of joint configurations ("uncontrolled manifolds" or UCMs) th at, if realized, would leave essential task variables unchanged. The variab ility of joint configurations was decomposed into components lying parallel to those sets and components lying in their complement. The orientation of the gun's barrel relative to a vector pointing From the gun to the target was the task variable most successful at showing a difference between the t wo components of joint variability. This variable determines success at the task. Throughout the movement, not only while the gun was pointing at the target, fluctuations of joint configuration that affected this variable wer e much reduced compared with fluctuations that did not affect this variable . The UCM principle applied to relative gun orientation thus captures the s tructure of the motor control system across different parts of joint config uration space as the movement evolves in rime. This suggests a specific con trol strategy in which changes of joint configuration that are irrelevant t o success at the task are selectively released from control. By contrast, c onstraints representing an invariant spatial position of the gun or of the arm's center of mass structured joint configuration variability in the earl y and mid-portion of the movement trajectory, but not at the time of shooti ng. This specific control strategy is not trivial, because a target can be hit successfully also by controlling irrelevant directions in joint space e qually to relevant ones. The results indicate that the method can be succes sfully used to determine the structure of coordination in joint space that underlies the control of the essential variables for a given task.