ACUTE EFFECTS OF TRANSCUTANEOUS SPINAL DIRECT CURRENT STIMULATION ON CORTICOSPINAL EXCITABILITY AND MOTOR PERFORMANCE OF THE LOWER EXTREMITIES

Abstract

Physical function is strongly determined by the ability to generate force. Force production is the result of neuromuscular factors such as the activation of cortical areas, recruitment of corticospinal neurons, mechanisms of nerve conduction, and skeletal muscle properties. Transcutaneous spinal direct current stimulation (TSDCS) has received attention due to its potential to modulate motor pathways and, consequently, physical performance. Nevertheless, TSDCS can be applied using a variety of configurations. PURPOSE: The purpose of this dissertation was to investigate the effects of TSDCS on corticospinal excitability and motor performance of the dorsiflexor muscles using different electrode arrangements and polarities. METHODS: Thirty-six subjects were recruited. From them, 25 were tested applying TSDCS with a longitudinal electrode arrangement, whereas 21 subjects received the transversal arrangement. A subset of 10 participants underwent both electrode arrangements. The TSDCS active electrode was placed around the 10th thoracic vertebra for both arrangements. One reference electrode was placed around the shoulder for the longitudinal arrangement, and two reference electrodes were placed on the iliac crests for the transversal arrangement. For polarity, participants received 15 minutes of anodal or cathodal TSDCS (charge density: 0.018 mAh/cm2) that were compared to a sham stimulation. The conditions were tested in separate days in randomized order. Participants reported their sensitivity to TSDCS using a 10-point Visual Analog Scale (VAS). Each day, maximal voluntary contraction (MVC), Rate of Force Development (RFD), and motor-evoked potentials (MEP), using transcranial magnetic stimulation (TMS), were quantified. RESULTS: Cathodal TSDCS delivered with the longitudinal electrode arrangement increased MEP amplitude compared to anodal or sham. TSDCS delivered with the transversal arrangement did not produce significant changes. TSDCS did not alter MVC and RFD regardless of the polarity and arrangement used. VAS was similar across polarities and arrangements indicating the participants could not differentiate the conditions received. CONCLUSIONS: Cathodal, but not anodal, TSDCS applied using a longitudinal arrangement and targeting lower extremity muscles can modulate corticospinal excitability without changing motor performance. This phenomenon may be related to how the TSDCS-induced electric field is distributed along the spinal cord based on different electrode locations.