FIG. 3.
Characteristics of spinal stimulation to induce self-assisted standing. (A) EMG activity in the leg muscles during standing without (black traces), in the presence of sham (olive traces and font, show intensities from 40 to 110 mA) and effective (red traces and font, show intensities from 10 to 80 mA) tSCS delivered over the L1 at 15 Hz, recorded in a representative participant (P2). “L Knee” and “R Knee,” readings from the pressure sensors placed above the participant's knees quantifying assistance provided by the trainer. “Self-assisted” label indicates both knees and hips extension independent of trainer assistance. (B) Pooled effects of the amount of assistance applied to the participants' knees during standing without stimulation (n = 15, black font), during sham stimulation (n = 11, olive font), and during standing enabled by tSCS (n = 15, red font). Without and during sham stimulation, all participants required trainers' assistance applied to the knees and hips, whereas in the presence of tSCS, the incidence and required force of knee assistance were reduced; hip assistance was not required in eight participants (also see Table 1). “LKs” and “RKs” indicate the left and right knee being supported, respectively; “LKi” and “RKi” indicate the left and right knee being independent, respectively; “NS” indicates standing with no stimulation (gray and black); “sham” indicates sham stimulation (light and dark yellow); “stim” indicates standing during tSCS (pink and red). (C) The mean muscle EMG amplitude during standing with no stimulation (n = 15), in the presence of sham stimulation (n = 11), and sitting and standing (n = 15) in the presence of tSCS of the same intensity during the first experimental session. (D) Recruitment curves of motor evoked potentials in the left leg muscles obtained at different frequencies of spinal stimulation delivered over the L1 during standing in a representative participant (P3). Vertical dashed lines indicate the stimulation intensity sufficient to generate self-assisted standing at indicated frequency. Note the decrease in the evoked potentials' magnitude at higher stimulation frequencies. Lower panels present the center of pressure (COP) oscillations in the anteroposterior (AP) direction, as well as the frequency spectrum corresponding to the AP COP. With tSCS frequency of 5 Hz, the dominant frequency of the COP oscillations corresponds to that of the stimulation (e.g., 5 Hz). During tSCS of higher frequencies, the COP oscillations are independent from the stimulation frequency. (E) Pooled effects (n = 6) of tSCS delivered over the T11 and L1 at 5, 15, and 25 Hz on EMG responses in leg muscles. The magnitude of responses was normalized to the maximum amplitude of each muscle during stimulation delivered at 0.2 Hz. EMG, electromyogram; tSCS, transcutaneous electrical spinal cord stimulation.