Upper Body Coordination and Movement Control in Unconstrained Visually Guided Three-Dimensional Reach Movements.

Abstract

Reaching is a basic component of human movements requiring the coordination of the eyes and multiple body segments including the hand, forearm, arm and torso. Although this movement has been studied extensively, the theory bridging the explicit reaching behavior (coordinated movement of body segments) and the implicit reaching strategy (control mechanisms) is limited. Hence, modeling unconstrained reach movements as a result of coordination remains a difficult task. The aims of the present study were to investigate the relationships defining the coordination pattern, control mode composition and movement phase transition in order to develop a model of coordinated reach movements. This work focuses more particularly on the characterization of body segment kinematics in movement phases and control mode transition in relation to visual information. A novel approach to determine control mode transition is proposed by using changes in curvature of the elbow swivel angle (ESA) combined with the content of visual information. The results show that this approach seems to be a good indicator of control mode transition in reach movements. The relative durations of movement control modes were therefore determined and modeled as a function of reaching requirements. In addition, the use of the swivel angle enables the reduction of the degrees of freedom and contributes to a simplification of arm movement models. Two strategies of movement execution were observed as a function of the availability of the visual information. In absence of vision, the movement variability was significantly reduced in order to constrain the system degrees of freedom. Furthermore, the orientation of the movement errors strongly support that in the present context, movements are planned in a local coordinate system and the head is the origin of that frame of reference. A coordination model was developed to describe the timing and kinematics of three-dimensional reach movements. This model also includes the relationship between the eyes and body segment movements. With a generalized hand trajectory, the proposed model generates the sequence of movement phases and drives a multi-linkage system as a function of target locations.