Here is a fairly recent review which in the background covers the causative aspects of PFS

PEDro - Detailed Search Results

One of the sports physios on this site may explain this more eloquently but I will do my best. Certain muscles that cross one joint in the body have a very important stabilizing role for the joint. The VMO plays a crucial role in ensuring the patella tracks in the groove between the two femoral condyles while the quadriceps is working during knee movement and when the VMO is weak the patella tends to be pulled laterally due to the knee angle, and structures such as the ilio tibial band. These muscles like the VMO tend to be vulnerable to dysfunction. For example even quite low levels and swelling at the knee is accompanied by reduced and abnormal firing of VMO. Therefore minor trauma or injury can induce this type of weakness and it appears to be neurologically mediated. Biomechanical factors such as tight lateral structures (eg the ilio tibial band) increased Q angle (this is to do with the line of pull of the quadriceps) and small or misshapen patellae can also predispose this problem.

So in the case of runners, even though they may hqve muscles with good srength and endurance generally, the small VMO could become dysfunctional quite readily through factors listed above. presumably this is often a case of a vicious cycle: once the muslce is "switched off by trauma the abnormal tracking producers trauma resulting in pain, inflammation to the anterior structures of the knee which in turn switch off the VMO more and so the syndrome arises. Muscles like the VMO readily atrophy following the neurogenically mediated weakness.

Hope this is of help and good luck with your report