For decades people with MS were told to conserve energy and avoid exertion, on the theory that activity accelerated the disease. That advice has not survived contact with the evidence. NICE guideline NG220, Multiple sclerosis in adults: management — published in June 2022, replacing CG186 — recommends exercise for people with MS, and is clear that there is no evidence exercise is harmful or worsens the condition.
This matters practically, because the deconditioning that follows years of avoiding activity produces its own weakness, stiffness and fatigue on top of the MS. A good deal of what physiotherapy achieves in MS is unpicking that second layer.