When these patients are treated conservatively with quadriceps
exercises against resistance, by extending the knee from 90" to full
extension with a lead boot, most of them are unable to accomplish this
exercise because of retropatellar pain. They are able, however, to do
straight leg raising against the same resistance.
The explanation is given in Figure 4 where a maximum PFJR force
of 1.4 body weight is found at 36 degrees of knee flexion when using
a 9 kg boot.
If a heavier boot is used, the general shape of the curve obtained
remains the same, while the force dimension is proportionally scaled
upwards.
Figure 4 also illustrates that the PFJR can be kept at a significantly
lower level by doing the straight leg raising exercise.
When the knee is extended, the line of action of the patellar ligament
and quadriceps muscle forces are almost parallel, and because
of the small angle between those two forces their vector sum represents
a lower value.