3. Quadriceps exercises by extending the knee from 90" flexion against
resistance with a 9 kg lead boot yield a PFJR force of 1.4 body
weight at 36" of knee flexion. This explains why some patients
with PF derangements complain of retropatellar pain during this
exercise.
The straight leg raising exercise against the same resistance
results in a PFJR force at only 0.5 body weight so that they are able
to accomplish this exercise.