Journal of Neuroscience, Vol 6, 3309-3318, Copyright © 1986 by Society for Neuroscience
Step-tracking movements of the wrist in humans. I. Kinematic analysis
DS Hoffman and PL Strick
We have examined the kinematics of the initial trajectory of step- tracking
movements performed by human subjects. Each subject tracked a target that
required 5-30 degrees of radial or ulnar deviation of the wrist. All
movements were to be performed as accurately as possible. Speed
instructions were given before each trial. When subjects performed
different amplitude movements following the same speed instruction, the
peaks of velocity, acceleration, and jerk were linearly related to peak
displacement. The peaks of velocity, acceleration, and jerk also changed
when the speed instruction was altered. Thus, for any given movement, the
peak values of the derivatives of displacement were dependent on both
movement amplitude and intended speed. As a result, the peak values of the
derivatives cannot be used by themselves to control or monitor peak
displacement. When subjects performed different amplitude movements
following the same speed instruction, movement duration tended to remain
constant. In contrast, movement duration changed when the speed instruction
was altered. Movements performed when subjects intended to move slowly had
longer durations than when subjects intended to move quickly. These results
suggest that subjects volitionally alter intended speed by selecting
different movement durations. When both movement amplitude and intended
speed were varied, the peak displacement of a step- tracking movement was
linearly related to the product of 2 kinematic variables: the initial peak
of a derivative of displacement (either velocity, acceleration, or jerk)
and movement duration. On the basis of our observations, we propose that
central commands generate step- tracking movements of different amplitudes
and intended speeds by adjusting both the magnitude and duration of a
derivative of displacement.
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