What Causes Extreme Leg Weakness?

I have a friend who began experiencing extreme leg weakness a few months ago. After extensive lab tests, tyroid test, 3 MRI s, & spinal puncture, the 1st neurologist had no idea what her diagnosis was and the 2nd neurologist visited suspected parkinsons, however without the tremors and other symptoms and prescribed 25 mg. ropinirole which seemed to help, but no where near 100% - Later changed it to 50 mg. which had negative effect and he put her back to 29 mg. This lady is 65 yrs old, but was previously very active, danced regularly and correctly watches her diet. Also, has gone from from a very calm demeanor to very sensitive nature during this time.