Since the multifidus muscle helps to stabilize the lumbar spine, the investigators hope to learn if there is dysfunction and/or atrophy of this muscle in MS patients who also have back pain. The main objective is to study if it is possible to do a home-exercise program in patients with MS younger then 50 years old and who do not need any assistance to walk. Another objective is to further characterize the multifidus function in those patients. This will be by by measuring the size of muscle at the beginning and end of the study, and also by physical exam. As an exploratory aim, the investigators would like to evaluate the relation between compliance with the home-exercise program and the change in the size of the multifidus muscle and the change in pain.
