The reaction time of single motor units in the human muscle.

The reaction time for activating a single motor unit (MU) with one impulse only, upon visual and auditory feed-back was studied in six healthy subjects. Use was made of MUs from m.abductor dig.V. The subjects were trained until they achieved more than 50% correct performances. One hundred responses were investigated in each series of experiments. The stimulus to which the subjects should have to respond was a beam presented on the screen of a "Disa" 14A30 electromyograph. Simultaneously with the beam the stimulator switched on a digital chronometer which was switched off by the MU impulse. Another two series of experiments were also performed in which the subjects responded with a train of impulses from the MU or with a burst of impulses from the whole muscle. It was been found that the mean reaction time for one impulse from one MU is longer (300 ms), for a train of impulses from one MU it is shorter (260 ms) and for a burst of impulses it is the shortest (200 ms). The histogram of distribution in the first series of experiments deviated from the normal distribution and showed a second maximum at 120 ms after the first maximum. The latter did not differ from the maxima in the distributions of the second and third series of experiments--about 200 ms after the visual stimulus presentation. The difference in the mean reaction time for the three series of experiments is due to the differences in the motor tasks connected with different velocity of increase of the muscle tension, as increasing velocity decreases the MU recruitment thresholds. The separate MUs might be activated in the same short periods as the whole muscle but they showed some specificities in the time distribution of the responses which might be due to some discrete mechanisms of the motor control system.

Impulse activity of a single motor unit upon the rhythm of the summated muscle activity.

The impulses of single motor units (MUs) are recorded using the method of selective leading off with surface bipolar electrodes with small interpolar distance. Parallel recording is made of the summated EMG and of its apparatus-transformed form-the mean voltage curve of the EMG. The correct position of the electrodes with regard to the MU is controlled by means of a vector-electromyographic (VEMG) method. The statistical parameters of the impulse series of the individual MUs and of the groups of impulses in the summated EMG are determined, as well as the time interrelations between the impulses of the selected MUs and the groups of impulses in the mean voltage curve of the EMG. The delay in the appearance of the impulses from the selected MUs with regard to the beginning of the group of impulses, proved a statistically distributed value according to a bimodal type which becomes more pronounced upon rating to the intervals.