This 1984 cadaver study directly measured forces in the quadriceps tendon and patellar ligament across 30°–120° of knee flexion in six specimens. It tested whether the long-held assumption that these forces are equal is actually true. The answer is definitively no — the ratio changes substantially with flexion angle.
Before this paper, the patella was widely modeled as a simple pulley — most force calculations assumed FL = FQ at all angles, which shaped rehabilitation protocols and surgical planning for decades. This paper proved that assumption wrong. The patella is a lever, and the FL/FQ ratio swings from 1.27 to 0.70 depending on where you are in the arc of motion.
When a patient has patellar tendinopathy or early patellar ligament pathology, this is why we restrict open-chain extension to angles greater than 45°. Below 45°, every degree toward full extension amplifies patellar ligament load beyond quadriceps load.
For surgical planning: any procedure that shifts the patellofemoral contact area proximally (tibial tubercle advancement, patellar resurfacing in TKA, capsular procedures) will reduce the FL/FQ ratio at a given angle. Understanding this geometry is essential for predicting how extensor mechanism surgery changes load distribution.
This 1984 cadaver study directly measured forces in the quadriceps tendon and patellar ligament across 30°–120° of knee flexion in six specimens. It tested whether the long-held assumption that these forces are equal is actually true. The answer is definitively no — the ratio changes substantially with flexion angle.
Before this paper, the patella was widely modeled as a simple pulley — most force calculations assumed FL = FQ at all angles, which shaped rehabilitation protocols and surgical planning for decades. This paper proved that assumption wrong. The patella is a lever, and the FL/FQ ratio swings from 1.27 to 0.70 depending on where you are in the arc of motion.
When a patient has patellar tendinopathy or early patellar ligament pathology, this is why we restrict open-chain extension to angles greater than 45°. Below 45°, every degree toward full extension amplifies patellar ligament load beyond quadriceps load.
For surgical planning: any procedure that shifts the patellofemoral contact area proximally (tibial tubercle advancement, patellar resurfacing in TKA, capsular procedures) will reduce the FL/FQ ratio at a given angle. Understanding this geometry is essential for predicting how extensor mechanism surgery changes load distribution.