Overuse of quads..
The quadriceps muscles are the ones on top of the thigh, that connect your pelvis to your knee; underlying bone is the femur (longest in body).
Most common cause of what you are describing are not well conditioned quads that are then overused (long runs, stairs..). The quads are key muscles to develop for any lower extemity activity. Weak quads r one of the most common causes of falls in elderly.