Two types of motor modulation underlying human stepping evoked by spinal cord electrical stimulation (SCES)

The EMG-activity during stepping-like movements evoked by spinal cord electrical stimulation (SCES) was studied in paraplegic patients. Both typical locomotor EMG-pattern with rhythmically, bilaterally alternating flexor-extensor activity ("locomotor stepping") as well as a rhythmic, bilaterally alternating modulation of short latency stimulus coupled reflexes ("reflexogenic stepping") and mixed pattern were observed. While the frequency of "locomotor stepping" could be independent from the stimulus frequency and the stepping was continuing after the end of stimulation for several cycles, the "reflexogenic stepping" was largely somehow coupled to the stimulus frequency and ended with the end of stimulation. It is assumed that both types of rhythmic motor activity reflect the activity of a spinal locomotor generator, but that the activity of the locomotor generator is subthreshold for fully activating the motoneuronal pools in cases with "reflexogenic stepping".