title: Effects of Displacement and Trajectory Length on the Variability Pattern of Reaching Movements.
abstract: The design of the present study enabled the authors to distinguish between the possible effects of movement displacement and trajectory length on the pattern of final positions of planar reaching movements. With their eyes closed, 9 subjects performed series of fast and accurate movements from different initial positions to the same target. For some series, the movements were unconstrained and were therefore performed along an approximately straight vertical line. For other series, an obstacle was positioned so that trajectory length was increased because of an increase in movement curvature. Ellipses of variability obtained by means of principal component analysis applied to the scatter of movement final positions enabled the authors to assess the pattern of movement variable errors. The results showed that the orientation of the ellipses was not affected by movement displacement or by trajectory length, whereas variable errors increased with movement displacement. An increase in trajectory length as a consequence of increased curvature caused no change in variable error. From the perspective of current motor control theory, that finding was quite unexpected. Further studies are required so that one can distinguish among the possible effects of various kinematics, kinetics, and other variables that could affect the pattern of variable errors of reaching movements.
authors.affiliation: Department for Musculoskeletal Research, National Institute for Working Life, Box 7654, S-907 13 Umeå, Sweden. jaric@niwl.se
authors.forename: Slobodan
authors.initials: S
authors.lastname: Jaric
authors.forename: Sandra M.S.
authors.initials: SM
authors.lastname: Ferreira
authors.forename: Charli
authors.initials: C
authors.lastname: Tortoza
authors.forename: Nadia F.
authors.initials: NF
authors.lastname: Marconi
authors.forename: Gil L.
authors.initials: GL
authors.lastname: Almeida