Patellofemoral stresses during open and closed kinetic chain exercise

Rehabilitation of the symptomatic patellofemoral joint aims to strengthen the quadriceps muscles while limiting stresses on the articular cartilage. Some investigators have advocated closed kinetic chain exercises, such as squats, because open kinetic chain exercises, such as leg extensions, have been suspected of placing supraphysiologic stresses on patellofemoral cartilage. 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) with 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 "benefit indices" (the ratio of the quadriceps muscle force to the contact stress). The study revealed that, throughout the entire flexion range, the open kinetic chain stresses were not supraphysiologic nor significantly higher than the closed kinetic chain exercise stresses. These findings are important 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 proximal patellar lesions preclude loading the patellofemoral joint in deeper flexion.