There are many potential causes of Patellofemoral pain, and therefore many treatments. In some instances physiotherapists may recommend certain stretches, and below is an explanation of why:
Quadriceps, (front of thigh). If these are tight, when the knee is bent it causes excessive compression of the knee cap against the bone underneath.
Calf. If the calf is tight it encourages the foot to excessively roll in, (pronate). This leads to rotation of the shin bone, which in turn causes asymmetrical loading of the patellofemoral joint, (PFJ).
Hamtrsings, (back of thigh). Tightness here can also cause a compressive effect through the PFJ.
ITB, (strip down the outer side of the leg from the pelvis to the knee). The ITB itself can’t be stretched, but it’s muscular attachments at the top can be. Any tightness in this complex can tilt the knee cap and once again cause excessive pressure. (To understand more on the source of pain in PFPS look at my blog on why does my knee hurt when my scan is normal?)