Living with MS means sometimes having to modify your exercise routine to accommodate your abilities. Because I have issues with balance related to my MS (not general clumsiness I swear) I find myself unable to do lunges with proper form. Not having proper form while exercising can lead to injury and can be a waste of time if you are not properly working the muscle you set out to work.
Lunges are a great quad and glute exercise, so as a substitute for that I do a split squat with the support of a weight bench. I rest one hand on the barbell on the weight bench for balance (being careful not to put any weight on that arm), holding a dumbbell in the other hand, I sit back and squat down.
The main muscles worked here are the quadriceps but you are also recruiting your glutes, hamstrings and calves, I love this exercise for strengthening my lower body, while working on my balance.