Trouble walking, abnormal EMG

I have trouble walking and abnormal EMG tests...please help...what is causing it?

1-I have constant stiffness and soreness in my sacroiliac area, getting worse since 2001!

2-I also get burning stiff heaviness in my legs every time I stand or walk, that goes away after resting.  (Sometimes it takes a day to get better, if I haveto walk longer than an hour...)  I do not drive and ytake the bus and  do all my daily chores and errands that requires standing and walking.)  

I havd severe trouble getting up from a squatting position or out of the tub, because my upper thigh muscles burn with heavinessand my knees are sore weak!  I can not climg more than a few steps and can not stand on my toes or one leg.  And the doctor will not tell me why!  (One doctor said I have unstable knee caps.  What is that?)
My leg function has been getting worse for 11 years and no doctor takes me seriously!
I also have abnormal EMG tests and...and need help!

These are the abnormal results: "diminished amplitude of tibial motor response bilaterally...absent medial and
plantar bilaterally," and my doctor says it is normal.  

Question: What nerves affect the knees, and would further testing show what is wrng?   I have another EMG test coming up in a few weks...what nerves should the doctor test?