Cervical and thoracolumbar transspinal stimulation at 0.3 Hz and 30 Hz facilitates neuronal excitability in healthy humans: a randomized cross-over study.

Effective neuromodulation protocols for upper motor neuron lesions are in great need. Based on the significant role of intraspinal neuronal pathways in motor control, we investigated here the effects of multisite cervical and thoracolumbar transspinal stimulation on arm and leg corticospinal and spinal reflex excitability. This was a randomized cross-over study in which 10 healthy subjects received either 0.3–30 Hz multisite transspinal stimulation on different days. Stimulation at either frequency was delivered at alternating subthreshold and suprathreshold intensities in blocks of 10 min for 40 min. Before and after each intervention, the right flexor carpi radialis and tibialis anterior motor evoked potentials following transcranial magnetic stimulation, and the right soleus M-wave and H-reflex recruitment curves were assembled. Multisite cervical and thoracolumbar transspinal stimulation at 0.3 and 30 Hz increased corticospinal excitability for both arms and legs in both targeted and non-targeted muscles. The main similarity between 0.3 and 30 Hz was that corticospinal excitability increased in a cross bilateral pattern between the extremities. A shift to the left was evident for the soleus H-reflex recruitment curve with multisite transspinal stimulation at 30 Hz, which supports changes in the recruitment order of soleus motoneurons or excitability thresholds of group Ia afferents. These findings suggest that multisite transspinal stimulation along the neural axis increases the excitability state of distributed corticospinal neuronal networks and could potentially be beneficial for recovery of weak corticospinal control of arms and legs as is the case in upper motoneuron lesions in humans.