Leg pain and weakness

  My sister is 29 years old, she has been having pain and weakness in her legs. This pain started last year.  She was 20 pounds over weight and started dieting and walking approximatly 10 miles a week.  She lost the 20 lbs + an additional 10.  She started having knee pain with walking up hills.  Her family doctor and her Orthopedist diagnosed her with Patella-Femoral Syndrome.  She did the exercises and took Naprosyn, but it just kept getting worse until she had to stop her walking and exercises.  Within the past 5-6 months the pain has spread up through to her hips and low back.  She describes it as feeling like she exercised for hours.  There's alot of weakness in her legs also.  She is not even able to walk from a grocery store parking lot to the store without the pain.  Walking very slow seems to help.  Her doctors are now unsure of what is going on.  One suggested Lumbar Stenosis.  The Orthopedist doesn't have a clue.  Do you know of anything we should be looking for?  It's very upsetting to see her unable to take her kids for a walk.  
  Thank you.  
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Thanks for your question.  The symptoms described in your message are suggestive
of either a Radiculopathy (i.e. problems with the nerve roots as they exit
the spine) or a Lumbar Stenosis (i.e. a narrowing of the terminal portion
of the spinal canal, in the lumbar region).  In particular, the fact that
pain and weakness were "triggered" by exercise and upright posture, which
can worsen both of the above clinical conditions.  Another possible symptom
of lumbar stenosis - not mentioned in your message - is pain and weakness
in the same distribution by simply standing upright for a number of minutes,
such as when one is just brushing the teeth in the morning, or other similar
activity.
A number of steps can be taken towards the diagnosis, including imaging
studies of the lumbar spine (preferably a MRI of lumbar spine), and electrophysiological
studies, namely an Electromyography (EMG).  The latter is usually performed
by a neurologist, and can detect possible signs of nerve compression/lesion
by studying the interaction and activity of nerves and muscles.
I hope this information is helpful.  Best of luck.
This information is provided for general medical education purposes only.
Please consult your doctor regarding diagnostic and treatment options.