The use of exercise as an adjunct to other disease modifying treatments in MS has found increasing support over the past decade. There is ample evidence that exercise therapy can improve physical measures of disability in MS, in addition to being safe and well-tolerated. However, there is a paucity of data on whether exercise is equally effective for those with primary progressive MS as it is in relapsing remitting MS. In addition, studies of exercise therapy have traditionally excluded MS patients with severe disability. There are also currently no treatments for primary progressive MS, so finding a safe disease-modifying therapy is of the utmost importance. The investigators are therefore conducting a pilot trial to evaluate the effects of exercise in the form of supported treadmill walking for patients with primary progressive MS with severe functional impairments.