FIG. 4.

The locomotor networks for spinal stepping are stimulation frequency dependent. (A) Schematic of progression of a spatiotemporally-independent monopolar stimulation to electrically-enabled motor control (SIM-eEmc) trial. (B) The inter-stimulation pulse interval between L2 and S1 is such that the L2 pulse follows S1 (S1+L2) or precedes S1 (L2+S1; data not shown). (C) The modulation of evoked potentials generated for each stimulation pulse in an medial gastrocnemius (MG) electromyographic (EMG) burst in a S1+L2 trial at four different frequencies. For all SIM-eEmc trials, the S1 pulse is at time “0” and an intervening blue dot indicates the start time of each L2 pulse. (D) The sum of the rectified signal from all S1+L2 pulses within a single MG EMG burst increases with an increase in S1 stimulation frequency. (E) Given the greater number of S1 stimulation pulses, there was an increase in the pulse-by-pulse cumulative integrals of the interaction-evoked responses with increase in S1 stimulation frequency. Concurrently, the sum of the rectified EMG (F) and cumulative integral (G) of the isolated L2 pulse decreased with an increase in S1 stimulation frequency. The numbers in (E) and (G) in red and blue indicate the total number of S1 stimulation pulses and L2 isolated pulses, respectively, that are summated in each burst for each trial. The dotted green line represents a theoretical linear slope if the cumulative energies from consecutive pulse were linear; the magenta lines indicate the actual slope. Mean (± SD) cumulative integrals in the MG (H) and tibialis anterior (TA; I) from the “S1+L2” and “isolated” L2 evoked responses and the total cumulative integral for each burst. *Significantly different from trial 5. Color image is available online at www.liebertpub.com/neu