What causes the legs to weaken?

Leg weakness: There are so many causes, it would be helpful for us if you could narrow it down a bit. Is it both legs, was it sudden or gradual, do the legs hurt, do you have back pain? Vascular disease can cause weak legs by reducing blood flow, and a stroke can also cause weakness. Spinal problems like bulging discs or spinal stenosis (narrowing) can do it. Anemia and thyroid disease can also do it.