What does it mean when your leg buckles in pain when walking up and down the stairs ?

Muscle inhibition: Sometimes, when pain is perceived in an area, the body inhibits the muscles in the region to try to avoid activity. Your symptoms could be coming from anywhere- from the knee, hip, ankle, or back. See your primary care doc so they can start the appropriate work up and you to a physiatrist or orthopedist, if necessary.

Weakness/ knee prob?: I would recommend orthopedic exam the giving out could be related to either leg weakness especially the quads. Other possibiltiy might involve some internal derangement in the knee.