[Systematic positioning errors and electromyographic activity of muscles in the human single-joint movements]

Accuracy of positioning was studied on the elbow joint movements in healthy adult humans. Individual trials consisted of the conditioning and test movements (CM, TM) produced with visual guidance and without it, correspondingly. The two types of CMs included a phase of slow movement (flexion - P1, or extension - P2) to the same target position (50 degrees in the most experiments). P1 and P2 CMs started, correspondingly, from the full extension position (0 degrees), or after preliminary flexion (100 degrees). Subjects were asked to remember the target position during CMs and to reproduce it in TM. The experiments have shown that position errors depend on the type of CMs. The subjects overestimated the target position, the errors were more significant in TMs produced after P2 CMs as compared with P1 CMs. Full-wave rectified, averaged EMGs were compared for the two types of CMs. In flexors the dynamic phases ofEMG activity were statistically greater in P1 than in P2 CMs (a group of 25 subjects), while the stationary components of EMG did not differ. Possible mechanisms of the position errors are discussed. It is suggested that after-effects in the activity of muscle spindles may be the main source for the position errors.