This project will evaluate a new therapy for patients with a severe form of MS known as progressive MS. These patients are severely limited in their ability to walk and function in everyday life. The exercise therapy we are proposing is a recumbent cross trainer which allows patients to be seated and move their arms and legs at the same time against resistive foot pedals and arm levers. The recumbent cross trainer is safe and easy to use for people with disabilities and is more cost-effective compared with other rehabilitation equipment. Before the recumbent trainer can become part of mainstream MS therapy, it is important to determine if it is as effective as other exercise therapies in improving functional performance and quality of life. In this study we will compare the effects of the recumbent cross training with supported treadmill walking on functional and psychological outcomes. Further, since this is a new therapy for progressive MS patients, we also want to determine if it is safe and enjoyable to use.