Six patients with primary progressive MS with high disability level will be recruited to participate in this study. Participants will exercise using body-weight supported treadmill training (BWSTT; Woodway Loco-system) which allows a portion of an individual's body-weight to be counterbalanced by an overhead pulley system. This system allows patients with limited mobility to walk upright on the treadmill with therapist assistance. Therapists trained in using the BWSTT system will guide the patients' legs through a proper gait motion and assist with lower limb control. Participants will complete 72 training sessions (30min/session, 3x/week) over the course of 24 weeks. Participants will be evaluated at baseline, 12 weeks (half-way through the training program) and 24 weeks (end of intervention). A follow-up assessment will also be performed 12 after participants have completed the training program (36 weeks). Outcome measures are described below.