Time relations between the impulses of two motor units from symmetrical muscles upon voluntary control

The activity of two motor units (MUs) belonging to two symmetrical muscles of both hands in six well trained subjects was investigated. Bipolar selective surface electrodes were used to lead off the electrical activity of mm interossei dig. dors. I or mm opponentes pollices. The "Medelec" electromyograph insured the visual and auditory feed-back from the firing MUs. The continuous or intermittent activity of both MUs was controlled voluntarily and simultaneously by the subjects. The probability of appearance of an impulse (the first or the last impulse in an intermittent train) from one unit when the other was firing continuously was studied. Alternation of the probability at 20 ms steps was found. At the same time the rhythm of the continuously firing MU was slightly affected. The data suggest a joint supraspinal influence or segmental contralateral outflow from the symmetrical motor pool. The hypothesis of the alphamotoneuron firing as a random process could not be used to explain the cases of joint activity.