Effects of biomechanical and task constraints on the organization of movement in precision aiming

Nine participants performed a reciprocal precision aiming task under different experimental conditions. Due to the anisotropy of the work space, varying the direction of motion ( from 0degrees to 315degrees in steps of 45degrees) allowed exploration of the effects of biomechanical constraints that were found to affect the duration of movement but not the shape of the kinematic pattern. Varying the size of the targets to be attained ( W: 2.5, 1.25, and 0.625 cm, for a constant intertarget distance of 10 cm) and the nature ( linear or non-linear) of the mapping between effector space ( motion of a handheld stylus on a graphics tablet) and task space ( motion of a pointer between targets on a computer screen) also led to changes in movement duration. However, the latter type of constraint gave rise to systematic changes in the pattern of movement, with progressively more difficult tasks being characterized by progressively less harmonic motion patterns. We conclude that in contrast to ( biomechanical) constraints at the level of the effector, ( informational) constraints at the level of the task affect the processes underlying movement organization. For the range of values studied, the effects of these two types of constraint can be considered to be independent.