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 cl
osed, 9 subjects performed series of fast and accurate movements from diffe
rent initial positions to the same target. For some series, the movements w
ere unconstrained and were therefore performed along an approximately strai
ght vertical line. For other series, an obstacle was positioned so that tra
jectory length was increased because of an increase in movement curvature.
Ellipses of variability obtained by means of principal component analysis a
pplied to the scatter of movement final positions enabled the authors to as
sess the pattern of movement variable errors. The results showed that the o
rientation of the ellipses was not affected by movement displacement or by
trajectory length, whereas variable errors increased with move ment displac
ement. An increase in trajectory length as a consequence of increased curva
ture caused no change in variable error. From the perspective of current mo
tor 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.