Author(s): SORDET, J., MARTIN, A., AMIEZ, N., VITRY, F., QUENOT, J.P., PAPIORDANIDOU, M. , Institution: INSERM U1093, Country: FRANCE, Abstract-ID: 1226

INTRODUCTION:
Transcutaneous spinal cord stimulation (tSCS) has experienced a clinical interest over the last years especially in patients undertaking locomotor recovery (1). This stimulation technique elicits responses in all muscles of the lower limb, called Posterior Root Muscle (PRM) reflexes (2). Although these responses seem of reflex nature, the electrode placement during tSCS can influence the type of recruited fibers (3). The purpose of this study was to evaluate the homosynaptic post-activation depression mechanism (HPAD) when tSCS was applied at two different levels of the spinal cord.
METHODS:
20 subjects participated in one experimental session and 10 out of them accepted to take part in a second experiment. For the first experiment tSCS was delivered at the lumbar level (L1-L2), while for the second experiment PRM reflex was elicited by stimulating at the thoracic level (T11-T12). For both experiments, these responses were compared with the H-reflex, elicited by stimulating the posterior tibial nerve. EMG activity was recorded from the right soleus (SOL) and vastus lateralis (VL) muscles. Recruitment curves in paired-pulse stimulations were built for the two types of stimulation. The amplitude of PRM and H-reflexes were matched and the HPAD phenomenon was tested by delivering paired-pulse stimulations at 1s inter-stimulus interval, by changing the test and the conditioning stimulation. The degree of HPAD was assessed by the amplitude of the second response expressed as a percentage of the amplitude of the first response (HCOND/HTEST, PRMCOND/HTEST, HCOND/PRMTEST and PRMCOND/PRMTEST).
RESULTS:
For both experiments, results of the SOL recruitment curves showed that the amplitude of the second PRM-reflex was significantly decreased (P<0.05) compared to the first one for all stimulation intensities. Concerning the VL muscle, the amplitude of the second response was not significantly decreased during the first experiment while during the second one, a decrease of the second response was observed, indicating VL afferents solicitation during tSCS. Regarding the conditioning stimulations of the first experiment, no difference was observed between the different conditioning ratios (P=0.99), suggesting that the degree of HPAD was identical between tSCS and nerve stimulation. During the second experiment, where the VL afferents were solicited by tSCS, the PRMCOND/PRMTEST was significantly lower (P<0.05) compared to the other conditioning ratios.
CONCLUSION:
Although PRM reflex and H-reflex seem to be sensitive in the same way to the HPAD phenomenon, the influence of the heteronymous contribution of VL afferents on SOL PRM reflex seems to have an impact on the degree of HPAD that can be evoked by tSCS. The electrode placement plays an important role in the nature of the elicited responses and should be taken into account when comparing the responses evoked by tSCS and peripheral nerve stimulation.

1. Hofstoetter et al., 2013
2. Minassian et al., 2007
3. Sayenko et al., 2015