Accuracy of motor responses in subjects with and without control of antagonist muscle

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: 1) 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 2) tetraplegic subjects lack antagonist (braking) force, so they must use much smaller accelerative forces when they move, in order to avoid overshooting their target. Because 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 two 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 electromyographic activity in either group of subjects. Subjects 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 listed 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.