Prospective participants, men and women with MS, will be recruited. To accomplish this study, each of the two groups of participants will need to complete 3 sessions at the INPL, each separated by 5-8 days. The duration of each session will be approximately one hour. The investigators expect data collection to last 6 months.

The study compromises 2 groups of subjects which will attend the lab for three sessions. In the first session subjects will be consented, complete the Patient Determined Disease Steps (PDDS) questionnaire, the Fatigue Severity Scale (FSS), and a 6MWT for baseline performance. The second session will involve a 6 MWT performed in association with 2 conditions. Group 1: DURING_tDCS, DURING_SHAM. Group 2: BEFORE_tDCS, BEFORE_SHAM. tDCS will be applied first follwed by SHAM in each group. Intensity of tDCS will be 2mA for both groups. Group 1 will receive the conditions during the 6 MWT. tDCS for 6 min has been shown to be sufficient to induce cortical excitability. Group 2 will receive 13 min of tDCS or sham which results in after effects lasting through the completion of the 6MWT. tDCS will be applied to the motor cortex (M1)corresponding to the more-affected leg either before or during the 6 min walk test. Leg strength, 6 MWT, and tDCS: Maximal voluntary contractions (MVCs) of the right and left knee extensors, knee flexors, hip flexors, and dorsiflexors will be performed to determine the more-affected leg. When leg strength difference is less than 10%, the more affected side will be based on self-report. For the 6 MWT, participants will be asked to walk as far as they can in 6 minutes. The 6 MWT is well established in MS research and, in order to measure fatigability as a secondary outcome, the literature suggest using a 6MWT rather than a 2MWT.Participants will walk in a cordoned off hallway between two cones placed approximately 30 meters apart. The primary outcome measure will be the distance covered in the 6 MWT. Since the investigators hypothesize that tDCS will alter the utilization of their more-affected leg, standard gait metrics during the 6MWT including gait speed, cadence, stride length and time, step length and time will be assessed with inertial sensors (OPAL system) for tDCS and SHAM (secondary outcomes). Furthermore, the investigators will calculate the distance walked index (DWI, distance Min 1 - distance Min 6), which is an objective measure of fatigability. A tDCS device (ActivaDose II) will deliver a small direct current through two sponge surface electrodes (5cm × 5cm,soaked with 15 mM NaCL). The positive electrode will be placed over the motor cortex representation of the more affected leg, and a second electrode will be placed on the forehead above the contralateral orbit. The following sessions will be performed in randomized order. Group 1 (During) - (A) The participant will receive tDCS or SHAM throughout the walking. In the tDCS trial the intensity will start at 0 mA and will be increased to 2mA over a 30 second period of time. At the 6:30 minute time point (immediately after walking) the current will gradually be reduced from 2 mA to 0 mA. (B) In the sham condition the participants will only receive the initial 30 seconds of stimulation, after which the current will be set to 0. Group 2 (Before) - (C) After a 30s ramp-up, tDCS will be delivered for 13 minutes at an intensity of 2 mA before the 6 min walk test. At the 13:00 minute time point the current will gradually be reduced from 2 mA to 0 mA. (D) Participants who undergo a sham condition will only receive the initial 30 seconds of ramp-up, after which the current will be set to 0 (D). In session 3, the condition not performed during session 2 will be performed. E.g., if a patient in Group 1 was randomly assigned DURING_SHAM for session 2, the patient will perform DURING_tDCS in session 3. All other testing conditions will be the same as session 2.

There will be no long-term follow up.