What is patellofemoral pain syndrome? Patellofemoral pain syndrome (PFPS), is one of the most common causes of knee pain. It is commonly presented as a diffuse ache around the knee cap that is worse with knee movements, particularly with loaded activities such as climbing stairs or running. Sometimes, prolonged sitting can also cause pain, known as the “theatre sign”. Crackling or popping sounds may also suggest PFPS due to malalignment or ...