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. 2021 Jul 25;10(15):3278.
doi: 10.3390/jcm10153278.

Transcutaneous Electrical Neuromodulation of the Cervical Spinal Cord Depends Both on the Stimulation Intensity and the Degree of Voluntary Activity for Training. A Pilot Study

Hatice Kumru  1   2   3 , María Rodríguez-Cañón  1   4 , Victor R Edgerton  1   5 , Loreto García  1   2   3 , África Flores  4 , Ignasi Soriano  1   2   3 , Eloy Opisso  1   2   3 , Yury Gerasimenko  6   7   8 , Xavier Navarro  1   4 , Guillermo García-Alías  1   4 , Joan Vidal  1   2   3
Affiliations

Transcutaneous Electrical Neuromodulation of the Cervical Spinal Cord Depends Both on the Stimulation Intensity and the Degree of Voluntary Activity for Training. A Pilot Study

Hatice Kumru et al. J Clin Med. .

Abstract

Electrical enabling motor control (eEmc) through transcutaneous spinal cord stimulation offers promise in improving hand function. However, it is still unknown which stimulus intensity or which muscle force level could be better for this improvement. Nine healthy individuals received the following interventions: (i) eEmc intensities at 80%, 90% and 110% of abductor pollicis brevis motor threshold combined with hand training consisting in 100% handgrip strength; (ii) hand training consisting in 100% and 50% of maximal handgrip strength combined with 90% eEmc intensity. The evaluations included box and blocks test (BBT), maximal voluntary contraction (MVC), F wave persistency, F/M ratio, spinal and cortical motor evoked potentials (MEP), recruitment curves of spinal MEP and cortical MEP and short-interval intracortical inhibition. The results showed that: (i) 90% eEmc intensity increased BBT, MVC, F wave persistency, F/M ratio and cortical MEP recruitment curve; 110% eEmc intensity increased BBT, F wave persistency and cortical MEP and recruitment curve of cortical MEP; (ii) 100% handgrip strength training significantly modulated MVC, F wave persistency, F/M wave and cortical MEP recruitment curve in comparison to 50% handgrip strength. In conclusion, eEmc intensity and muscle strength during training both influence the results for neuromodulation at the cervical level.

Keywords: cervical spinal cord; hand training; intensity effect; muscle strength effect; neuromodulation; transcutaneous spinal cord stimulation.

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Conflict of interest statement

V.R.E. and Y.G. hold shareholder interest in NeuroRecovery Technologies and hold inventorship rights on intellectual property licensed by The Regents of the University of California to NeuroRecovery Technologies and its subsidiaries. Y.G. also holds shareholder interest in Cosyma Inc., St. Petersburg, Russia. The funders had no role in the design of the study, in the collection, analyses, or interpretation of data, in the writing of the manuscript, or in the decision to publish the results.

Figures

Figure 1
Figure 1
Schematic representation of the experiment conditions, order of intervention and the evaluation time points during each experiment, and the functional and motor strength assessment and neurophysiological assessments performed.
Figure 2
Figure 2
Functional and motor outcomes assessed by the Box and Block test and MVC. (A) percentage changes in the number of blocks moved in the Box and Block test with respect to baseline. Intervention simple effect shows significant differences at follow of 90% (** p = 0.004) and 110% eEmc (**** p < 0.0001) with respect to 80% eEmc; (B) percentage changes in MVC with respect to baseline. Intervention simple effect showed differences of 90% eEmc compared with 80% (** p = 0.0026) and 110% eEmc at post (* p = 0.0211) and follow (*** p = 0.0108; * p = 0.0002) time points.
Figure 3
Figure 3
F wave persistency and F/M wave ratio. (A) percentage changes in F wave persistency with respect to baseline. Intervention simple effect showed significant differences of 90% eEmc with respect to 80% (** p = 0.0079) and 110% eEmc (*** p = 0.0005) at post and at follow (* p = 0.0304; ** p = 0.0020); (B) percentage changes in F/M ratio with respect to baseline. Intervention simple effect showed significant differences at post of 90% and 80% eEmc (** p = 0.0032); and at follow of 80% eEmc and 90% (* p = 0.0491) and 110% eEmc (** p = 0.0051).
Figure 4
Figure 4
Cortical excitability outcomes. (A) percentage changes in cortical MEP evoked by TMS 120% of RMT with respect to baseline. Intervention simple effect showed significant differences at follow of 110% eEmc with respect to 80% (* p = 0.0126) and 90% eEmc (*** p = 0.0005); (B) difference in cortical MEP amplitude between post and pre time points. Intensity simple effects showed significant differences of 80% eEmc with respect 90% and 110% of eEmc at 1.4 (* p = 0.0390, + p = 0.0133) and 1.5 (** p = 0.0042, +++ p = 0.0008) RMT cortical MEP; (C) difference in cortical MEP amplitude between follow and pre time points. Intensity simple effects showed significant differences of 80% and 110% eEmc at 1.2 (+ p = 0.0162), 1.4 (++++ p < 0.0001) and 1.5 RMT (++++ p < 0.0001); and of 90% and 110% eEmc at 1.2 (# p < 0.0225) and 1.4 RMT multiple (## p = 0.0015); and of 80% and 90% eEmc at 1.5 (**** p < 0.0001) RMT multiple.
Figure 5
Figure 5
Changes in MVC, F wave persistency and F/M wave ratio. (A) percentage changes in grip force during MVC according to baseline. Intervention simple effect showed differences at post (* p = 0.0144) and follow (** p = 0.0011); (B) percentage changes in F wave persistency according to baseline. Intervention simple effects showed significant differences at post (** p = 0.0073) and follow (* p = 0.0191); (C) percentage changes in F/M ratio according to baseline. Intervention simple effect showed significant differences at post (** p = 0.0016).
Figure 6
Figure 6
Cortical excitability outcomes. (A) percentage changes in cortical MEP from 120% of RMT according to baseline. Intervention simple effect showed significant differences at follow; (B) difference amplitude between post and pre cortical MEP recruitment. Intensity simple effects showed significant differences at 1.5 RMT multiple (** p = 0.0059); (C) difference amplitude between follow and pre MEP recruitment. Intensity simple effects showed significant differences at 1.5 RMT multiple (** p = 0.0063).
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