Rotational constraints and motion invariants in multi-joint arm movements in three dimensions

Date of Completion

January 2000


Biology, Neuroscience|Psychology, Experimental|Biophysics, General|Psychology, Physiological




Three experiments investigated the kinematic structure of arm movement trajectories in a 3D reaching task with specific position and orientation requirements placed both on the initial and final posture of the hand. While translation parameters such as movement duration, relative timing of velocity components, and linearity of hand translation varied systematically as the position-orientation of the target and arm-load were manipulated, the manner in which the hand changed orientation was found to remain invariant and constrained to 2D planar motion in a 3D orientation-parameter space. When the task was performed rhythmically (with various movement speeds dictated by metronome) similar variation in translation parameters was observed, but the planar constraint on the manner of orientation-change remained. It was suggested that this planar constraint is an organizational principle which (a) reduces the effective biomechanical degrees of freedom of the arm to six (rendering articulatory and task spaces of commensurate dimensions), and (b) helps manage the geometric problems of generating 3D trajectories in which axes of rotation do not remain constant. ^