What Causes Weak Legs And Difficulty Walking?

i am 51 years old female very healthy working full time workingout allthe sudden 2 years back my legs get so weak so hard for mr to walk or stand i can not walk without help isee doctor neourologist he said my nerves and my mucles are fine ahg he want do mri on me fo0r my back and for neck and headplease can you give me some answer