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 respon ses to a target. We recently found that C-5/C-6 tetraplegic subjects, who lacked voluntary control of their triceps muscle, were less accura te than control subjects in producing fast flexion movements to a targ et. 2. Two hypotheses are proposed to account for these larger errors: 1) the ability of tetraplegic subjects to compensate for errors arisi ng early in the motor response is impaired because of the lack of anta gonist muscle activation; or 2) tetraplegic subjects lack antagonist(b raking) force, so they must use much smaller accelerative forces when they move, in order to avoid overshooting their target. Because studie s have shown that low levels of force are produced with less relative accuracy than larger forces, this relative inaccuracy of force generat ion by the motor control system at low force levels is responsible for the inaccuracy of tetraplegics' movements. To test these two hypothes es, we compared the variability of ''fast and accurate as possible'' f orce pulses in four control subjects and four C-5/C-6 tetraplegic subj ects to targets at 15, 30, and 45% of maximum voluntary contraction. M ultiple regression analyses were performed to look for patterns of ago nist or antagonist muscle activation consistent with compensatory adju stments for early trajectory errors in both groups of subjects. 3. For ce rise time was significantly prolonged in tetraplegic subjects, alth ough there was some overlap between groups. At similar levels of effor t, there were no significant differences in constant and variable erro rs of control and tetraplegic subjects. We also found no consistent st atistical 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 the se experiments. 4. Together with our observation that lower force targ ets are indeed associated with larger relative variable errors, in bot h control and tetraplegic subjects, the above results lead us to concl ude that the second hypothesis Listed above is more likely correct. Th e antagonist muscle clearly enables the production of briefer force pu lses. In addition, the antagonist indirectly contributes to the accura cy of isotonic movements because antagonist braking allows larger agon ist forces to be used. These larger agonist forces are less variable, and produce more accurate movements, than the smaller forces used by t etraplegic subjects.