Why is ascending stairs commonly cited as producing knee pain? Relate this to patellofemoral joint ...

ANS:
Ordinarily, an active contraction of the knee extensors produces compression of the patella into the patellofemoral groove. With increased knee flexion, this compression increases proportionally due to the increased passive tension within the muscle. Similarly, increased flexion results in a greater compression component and a reduced extension component for the total muscle pull. Stair climbing leads to pain when the patellofemoral joint has some degeneration, because the quads are contracting heavily to lift the body on the fixed tibia from a flexed position. The compression is less severe in descending stairs, because the quads are contracting eccentrically rather than concentrically, reducing the number of motor units firing and reducing patellofemoral compression.