Accuracy of motor responses IN SUBJECTS WITH AND WITHOUT CONTROL OF
ANTAGONIST MUSCLE.
Wierzbicka, M. Margaret and Allen W. Wiegner.
Spinal Cord Injury Service, Brockton/West Roxbury VA Medical Center and
Department of Neurology, Harvard Medical School, Boston, MA 02132.
APStracts 2:0370N, 1995.
SUMMARY AND CONCLUSIONS
1. The aim of this study was to investigate the role of the antagonist muscle
in determining the accuracy of fast, single-joint motor responses to a target.
We recently found that C5/C6 tetraplegic subjects, who lacked voluntary
control of their triceps muscle, were less accurate than control subjects in
producing fast flexion movements to a target. 2. Two hypotheses are proposed
to account for these larger errors: a) The ability of tetraplegic subjects to
compensate for errors arising early in the motor response is impaired because
of the lack of antagonist muscle activation; or b) Tetraplegic subjects lack
antagonist (braking) force, so they must use much smaller accelerative forces
when they move, in order to avoid overshooting their target. Since studies
have shown that low levels of force are produced with less relative accuracy
than larger forces, this relative inaccuracy of force generation by the motor
control system at low force levels is responsible for the inaccuracy of
tetraplegics' movements. To test these 2 hypotheses, we compared the
variability of "fast and accurate as possible" force pulses in four control
subjects and four C5/C6 tetraplegic subjects to targets at 15%, 30% and 45% of
maximum voluntary contraction. Multiple regression analyses were performed to
look for patterns of agonist or antagonist muscle activation consistent with
compensatory adjustments for early trajectory errors in both groups of
subjects. 3. Force rise time was significantly prolonged in tetraplegic
subjects, although there was some overlap between groups. At similar levels of
effort, there were no significant differences in constant and variable errors
of control and tetraplegic subjects. We also found no consistent statistical
evidence for the presence of compensatory EMG activity in either group of
subjects. S ubjects who lacked the ability to make corrections involving the
triceps muscle performed as well as subjects with normal triceps strength.
This suggests that a corrective mechanism involving the triceps must have a
weak role, if any, in these experiments. 4. Together with our observation that
lower force targets are indeed associated with larger relative variable
errors, in both control and tetraplegic subjects, the above results lead us to
conclude that the second hypothesis in paragraph 2 above is more likely
correct. The antagonist muscle clearly enables the production of briefer force
pulses. In addition, the antagonist indirectly contributes to the accuracy of
isotonic movements because antagonist braking allows larger agonist forces to
be used. These larger agonist forces are less variable, and produce more
accurate movements, than the smaller forces used by tetraplegic subjects.
Received 14 February 1995; accepted in final form 11 December 1995.
APS Manuscript Number J106-5.
Article publication pending J. Neurophysiol.
ISSN 1080-4757 Copyright 1995 The American Physiological Society.
Published in APStracts on 23 December 95