What Could Constant Shaking Of Legs In Elderly Suggest?

I think my previous attempt disappeared into the ether (without my permission). I wanted to ask about my Mother-in-law's condition of shaking legs especially when she is standing. She's 84 but has had this condition for some years. It's slowly getting worse. I don't believe it is Parkinson's as she does not display that condition in arms or head, but when she's standing, the condition makes her 'bob' up and down seemingly uncontrollably.