There is some convincing evidence that altered kinematics is a major factor in patellofemoral pain syndrome (PFPS). Kinematics refers to patterns of movement — specifically how the patellofemoral joint and the knee joint rotate and glide in relation to one another during motion.
The patellofemoral joint occurs where the patella (kneecap) glides up and down over the femur (thighbone). Increased pressure from contact between the patella and the femur can lead to PFPS. This is called retropatellar stress — it means behind the kneecap.
Stress on the patellofemoral joint is made worse by rotations of the lower leg during weight-bearing activities. And repetitive actions with weight-bearing load during running and jumping increase retropatellar stress. The result is PFPS.
The mechanical factors that cause PFPS are likely present when you do the particular activities you are practicing. So doing more of them can make the problem worse. Altered hip and knee motion during weight bearing activities seems to be the main culprit.
Studies show that in athletes with PFPS (especially female athletes), there is an external rotation movement that occurs in the knee during single-leg squats and jumps. Evidently, this motion is not present in athletes who don’t have PFPS.
A closer look at the dynamics of movement has shown that when it comes to PFPS, changes in hip motion may be just as important as what the knee is doing. It appears that athletes who adduct the hip (knee moves toward the other knee) during single-leg squat motions have a greater tendency to develop PFPS. PFPS is more likely to occur if the knee externally rotates at the same time. The overall pattern of motion is one of medial collapse, sometimes referred to as dynamic knee valgus.
Training to stabilize the hip and retrain the knee to reduce the amount of external rotation during weight-bearing activities may be the answer. Physical therapists are actively studying this problem and looking for a solution. Someday, it may even be possible to identify athletes at risk for PFPS and put a stop to it before it even starts.