Rehabilitation of the symptomatic patellofemoral joint aims to strengthen t
he quadriceps muscles while limiting stresses on the articular cartilage. S
ome investigators have advocated closed kinetic chain exercises, such as sq
uats, because open kinetic chain exercises, such as leg extensions, have be
en suspected of placing supraphysiologic stresses on patellofemoral cartila
ge. We performed computer simulations on geometric data from five cadaveric
knees to compare three types of open kinetic chain leg extension exercises
(no external load on the ankle, 25-N ankle load, and 100-N ankle load) wit
h closed kinetic chain knee-bend exercises in the range of 20 degrees to 90
degrees of flexion. The exercises were compared in terms of the quadriceps
muscle forces, patellofemoral joint contact forces and stresses, and "bene
fit indices" (the ratio of the quadriceps muscle force to the contact stres
s). The study revealed that, throughout the entire flexion range, the open
kinetic chain stresses were not supraphysiologic nor significantly higher t
han the closed kinetic chain exercise stresses. These findings are importan
t for patients who have undergone an operation and may feel too unstable on
their feet to do closed chain kinetic chain exercises. Open kinetic chain
exercises at low flexion angles are also recommended for patients whose pro
ximal patellar lesions preclude loading the patellofemoral joint in deeper
flexion.