Effects of anode position on the responses elicited by transcutaneous spinal cord stimulation

The purpose of this study was to investigate the effect of anode position on the spinal reflex responses evoked by transcutaneous spinal cord stimulation. Healthy males participated in two experiments. In Experiment 1 (Exp. 1, n = 3), we investigated the effect of anode position on the spinal reflex responses in multiple lower-limb muscles. Experiment 2 (Exp. 2, n = 8) focused on the effect of anode position on the spinal reflex response in the vastus medialis (VM) muscle. In each experiment, electromyographic signals were recorded in the right leg muscle(s). The cathode was placed over the area between the spinous processes of the L1 and L2 vertebrae. The anode was placed over (1) the abdomen (ABD), (2) the ipsilateral (right) anterior superior iliac spine (iASIS), or (3) the contralateral (left) anterior superior iliac spine (cASIS). A double pulse stimulation test was conducted to confirm that the response was due to activation of sensory fibers (i.e. spinal reflex). The results showed that the anode position was critical for inducing the spinal reflex in the VM (Exp. 1). The ratio of second to first responses was smaller when the anode was placed over the ABD or cASIS than when the anode was placed over the iASIS (Exp. 2). In addition, the onset latency of the first response was longer when the anode was placed over the ABD or cASIS than when the anode was placed over the iASIS (Exp. 2). These results showed that the anode should be placed over the ABD or cASIS to effectively elicit spinal reflexes in lower-limb muscles.