78 Year Old Female Who Is On No Other Medications

78 year old female who is on no other medications nor does she have any other illnesses or health conditions. Looking for possible cause of terribly weak legs( almost can't lift legs after walking short distances). There is also lower back pain. Has been getting progressively worse from when it began to now (over the past year). Prior to that, was very active. We have been to several neurologists, neurosurgeons, ortho. We are going in a circle.