89 Yr Old Mom Is Having Difficulty Walking Recently. Can's

89 yr old Mom is having difficulty walking recently. Can's seem to lift right leg. She has to use her arms to raise the leg. Was in hospital and stroke rules out. She had a MRI of brain and spine and they didn't find anything. She is using a walker but really struggling to walk and get out of chair. She has fallen backwards a few times. Very concerned about what is happening to her and so suddenly.