My sister and I are both soccer and volleyball players. We have the same body type but she suffers greatly from patellofemoral pain syndrome. When we look in a mirror, our knees look the same. How come she has this problem and I don’t?

New information about patellofemoral pain syndrome (PFPS) has come to light in the last 10 years. But scientists are still scratching their heads over the issue of why one person gets it and another (with similar anatomical structure) doesn’t.

There is one theory that may explain why the same load causes patellofemoral pain for one person but not another. It’s called the envelope of function or the envelope of load acceptance theory.

This theory says that loads transmitted across the knee vary for a wide range of reasons. The person’s center of gravity may be slightly different from someone else’s. The center of gravity changes with each position and movement of the knee. The rate at which the load is applied to the joint may make a difference. Then there’s the angle of joint flexion and rotation to consider.

Different activities and different levels of physical condition can also affect the patellofemoral joint’s load envelope. And an injury may change the load acceptance causing pain with activities that never hurt before.

More study is needed to sort out these kinds of issues. It’s especially important because variances may make a difference in treatment. Rehab for individuals with PFPS is tailored for each individual person and sport.