Evidence for Dynamic Primitives in a Constrained Motion Task

Abstract

Ten right-handed male subjects turned a crank (radius 10.29 cm) in two directions at three constant instructed speeds (fast, medium, very slow) with visual speed feedback. They completed 23 trials at each speed. With the hand constrained to move in a circle, non-zero forces against the constraint were measured. Assuming a plausible mathematical model of interactive dynamics, the peripheral neuromechanics could be `subtracted', revealing an underlying motion that reflected neural control. We called this data-driven construct the zeroforce trajectory. The observed zero-force trajectory was approximately elliptical. Its major axis, estimated by the principal eigenvector of the covariance matrix, differed significantly for the two movement directions. As peripheral neuromuscular compliance (i.e. low mechanical impedance) mitigates the consequences of imperfect execution, the required precision of motion commands is reduced. An oscillatory zeroforce trajectory that leads hand motion suffices to produce circular hand motions. Due to non-isotropic peripheral dynamics, that lead differs between degrees of freedom, resulting in an elliptical zero-force trajectory. The ellipses' orientations differ with direction of rotation, as observed in the experimental data. As elliptical motion is generated by two non-colinear sinusoids with non-zero phase difference, these results support the hypothesis that humans simplify this constrained-motion task by exploiting primitive dynamic actions, oscmations and impedance.