For About 4 Yrs Now My Legs Are So Weak

For about 4 yrs now my legs are so weak when standing still--OK when walking. Getting worse with trembling with extension to whole body shaking if I am not able to find a place to sit or lean on a object. Had MRI of brain and spinal cord and evaluated by a neurologist and a rhematologist with no diagnosis. Can u help Please. Carolyn I.