Facet joints tend to get larger as they degenerate. This process is the body’s attempt to decrease the stress per unit area across a degenerated joint. Unfortunately, as the joint enlarges, it can place pressure on the nerves as they exit the spine (see Figure 1).
Standing upright further decreases the space available for the nerve roots, and can block the outflow of blood from around the nerve. Congested blood then irritates the nerve and the pain travels into the legs.
Generally, patients with spinal stenosis are comfortable if they are sitting, but have more pain down their legs when they walk and the pain increases with more walking ("neurogenic claudication"). Walking while leaning over a supporting object (such as a walker or shopping cart) can help ease the pain, and sitting down will cause the pain to recede.