Large involuntary forces consistent with plateau-like behavior of human motoneurons

When electrical stimulation is applied over human muscle, the evoked force is generally considered to be of peripheral origin. However, in relaxed hum ans, stimulation (1 msec pulses, 100 Hz) over the muscles that plantarflex the ankle produced more than five times more force than could be accounted for by peripheral properties. This additional force was superimposed on the direct response to motor axon stimulation, produced up to 40% of the force generated during a maximal voluntary contraction, and was abolished during anesthesia of the tibial nerve proximal to the stimulation site. It theref ore must have resulted from the activation of motoneurons within the spinal cord. The additional force could be initiated by stimulation of low-thresh old afferents, distorted the classical relationship between force and stimu lus frequency, and often outlasted the stimulation. The mean firing rate of 27 soleus motor units recorded during the sustained involuntary activity a fter the stimulation was 5.8 +/- 0.2 Hz. The additional force increments we re not attributable to voluntary intervention because they were present in three sleeping subjects and in two subjects with lesions of the thoracic sp inal cord. The phenomenon is consistent with activation of plateau potentia ls within motoneurons and, if so, the present findings imply that plateau p otentials can make a large contribution to forces produced by the human ner vous system.