Rieko Osu, Yoji Uno, Yasuharu Koike, Mitsuo Kawata
Examinations of Possible Explanations for Trajectory Curvature in Multi-Joint Arm Movements
Abstract:Although the straightness of hand paths is a widely accepted feature of human multi-joint reaching movements, detailed examinations have revealed slight curvatures in some regions of the workspace. A question therefore arises as to whether planned trajectories are straight or curved.
If they are in fact straight, at least three possible factors can explain the observed curvatures: (1) Imperfect control, (2) Visual perceptual distortion, or (3) Interaction between straight virtual trajectories and the dynamics of the arm. However, these three factors are not exhaustive, just the likeliest candidates. Nonetheless, rejecting them is equivalent to saying that planned trajectories themselves are likely to be curved. In Experiment 1, subjects instructed to generate straight movement paths from the side of the body to the front, produced much straighter movements than those generated spontaneously; this argues against (1). In Experiment 2, subjects generated spontaneously curved trajectories in the fronto-parallel plane where visual perceptual distortion was not expected; this argues against (2). In Experiment 3, EMG signals of six related muscles suggested that subjects generated straighter paths without increase in arm stiffness; this argues against (3). It thus follows that planned trajectories are likely to be curved. Because there is no theoretical reason to plan curved trajectories in extrinsic space, we conclude that trajectories are planned in intrinsic space while taking into account the dynamics of the musculoskeletal system.