There's actually been some research to suggest that the pain of patellofemoral pain syndrome (PFPS) is misinterpreted as stiffness. There isn't any real mechanical stiffness at all.

A study comparing patients who had PFPS with normal subjects showed no difference in the actual amount of mechanical stiffness present. Mechanical stiffness was defined as resistance of a body part to changes in shape or position.

The joint capsule, muscles, tendons, skin, and ligaments around the joint affect mechanical stiffness in the knee. Since there was no link between mechanical stiffness and pain, it was suggested that patients think the sensation of pain is stiffness.

Treating the pain may reduce stiffness. More study is needed to sort this all out.