A systematic review
Thomas Edwards and Lara A. Pilutt had that very same question. They looked over the existing research on the topic, and submitted their results to the Multiple Sclerosis and Related Disorders Journal, which published the paper on June 14, 2017.
They engaged in a systematic review, which means they gained access to a large number of published papers on the subject, and thoroughly pored through the data to find links between papers and to reach conclusions the individual researchers could not reach.
They looked through four electronic databases of research papers. These were Pubmed, EMBASE (Excerpta Medica dataBase), OvidMEDLINE, and PsychINFO. The articles were published from the beginnings of the database up until October 2016.
As the two researchers speak only English, only English language papers were used.
They used papers which included the effect of exercise training on people with multiple sclerosis and who had a severe mobility disability, even if that was not the direct purpose of the paper. To qualify, the participants needed to require assistance while walking, or had an EDSS (which stands for Expanded Disability Status Scale) score of 6.0 or above.