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. 2020 Mar;8(5):e14397.
doi: 10.14814/phy2.14397.

The relationship between maximum tolerance and motor activation during transcutaneous spinal stimulation is unaffected by the carrier frequency or vibration

Gerome A Manson  1 , Jonathan S Calvert  1   2 , Jeremiah Ling  1 , Boranai Tychhon  1 , Amir Ali  1 , Dimitry G Sayenko  1
Affiliations

The relationship between maximum tolerance and motor activation during transcutaneous spinal stimulation is unaffected by the carrier frequency or vibration

Gerome A Manson et al. Physiol Rep. 2020 Mar.

Abstract

Transcutaneous spinal stimulation (TSS) is a useful tool to modulate spinal sensorimotor circuits and has emerged as a potential treatment for motor disorders in neurologically impaired populations. One major limitation of TSS is the discomfort associated with high levels of stimulation during the experimental procedure. The objective of this study was to examine if the discomfort caused by TSS can be alleviated using different stimulation paradigms in a neurologically intact population. Tolerance to TSS delivered using conventional biphasic balanced rectangular pulses was compared to two alternative stimulation paradigms: a 5 kHz carrier frequency and biphasic balanced rectangular pulses combined with vibrotactile stimulation. In ten healthy participants, tolerance to TSS was examined using both single-pulse (0.2 Hz) and continuous (30 Hz) stimulation protocols. In both the single-pulse and continuous stimulation protocols, participants tolerated significantly higher levels of stimulation with the carrier frequency paradigm compared to the other stimulation paradigms. However, when the maximum tolerable stimulation intensity of each stimulation paradigm was normalized to the intensity required to evoke a lower limb muscle response, there were no statistical differences between the stimulation paradigms. Our results suggest that, when considering the intensity of stimulation required to obtain spinally evoked motor potentials, neither alternative stimulation paradigm is more effective at reducing discomfort than the conventional, unmodulated pulse configuration.

Keywords: Russian current; carrier frequency; pain tolerance; transcutaneous spinal stimulation; vibration.

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Conflict of interest statement

None declared.

Figures

FIGURE 1
FIGURE 1
Schematics presenting (a) a single biphasic symmetric square‐wave pulse with the positive phase of 1 ms duration (pulsed‐current, unmodulated), as well as (b) burst‐modulated waveform of 1 ms duration with the carrier frequency of 5 kHz, which consisted of 5 biphasic pulses each of 200 μs duration (carrier frequency, modulated), during Single‐Pulse Stimulation (top panels) and Continuous Stimulation (bottom panels) protocols. Note the difference in the cumulative area of the positive phase (shown by the gray shaded area) during the Unmodulated and Modulated pulses
FIGURE 2
FIGURE 2
Examples of waveforms and recruitment curves obtained during Pulsed‐Current (PC), Pulsed‐Current + Vibration (PC + V), and Carrier Frequency (CF) stimulation paradigms in one representative participant (P5). (a) demonstrates the waveforms of spinally evoked motor potentials in different muscles recorded at stimulation intensity corresponding to the common motor threshold (left panel, 90 mA for PC and PC + V, and 260 mA for CF paradigms) as well as maximum tolerable intensity (right panel, 210 mA for PC and PC + V, and 480 mA for CF paradigms). Vertical dashed lines indicate the time windows in which the peak‐to‐peak amplitude of the responses was calculated. (b) demonstrates recruitment curves during each stimulation paradigm for each muscle: VL, vastus lateralis; MH, medial hamstring; TA, tibialis anterior; GM, medial gastrocnemius; SOL, lateral soleus. Note that the scale indicating stimulation intensity for CF is displayed in red at the top of the Figure. Vertical dashed lines indicate the stimulation intensities corresponding to the common motor threshold (MT) and maximum tolerable intensity (MTI)
FIGURE 3
FIGURE 3
Summary of results for the Single‐Pulse Protocol. In each panel, the stimulation paradigm is labeled on the X‐axis: Carrier Frequency (CF), Pulsed‐Current (PC) and Pulsed‐Current + Vib (PC + V). Individual participant data are represented by colored circles, the box represents the mean and the standard error of the mean, and the whiskers show the standard deviation. (a) depicts the maximum tolerable intensity. (b) shows the motor thresholds. (c) shows the percentage of motor threshold to the maximum tolerable intensity
FIGURE 4
FIGURE 4
Summary of results for the Continuous Stimulation Protocol. In each panel, the stimulation paradigm is labeled on the X‐axis: Carrier Frequency (CF), Pulsed‐Current (PC) and Pulsed‐Current + Vib (PC + V). Individual participant data are represented by colored circles, the box represents the mean and the standard error of the mean, and the whiskers show the standard deviation. (a) the maximum tolerable intensity. (b) the relationship between the maximum tolerable intensity in the continuous protocol normalized to the maximum tolerable intensity in the single pulse protocol. (c) the maximum tolerable intensity in the continuous protocol normalized to motor threshold in the single pulse protocol
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