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The main idea is that the patellofemoral joint mainly acts to increase the mechanical advantage of the knee extensor mechanism. The patella sits in the femoral trochlear groove and works like a pulley, changing the angle at which the quadriceps tendon pulls on the tibia. By doing this, it lengthens the effective lever arm (moment arm) of the quadriceps, so a given muscle force produces a larger knee-extension torque. This makes knee extension more efficient, especially near straightening of the leg. The patellofemoral joint also helps distribute compressive loads between the quadriceps tendon and the femoral groove and guides patellar tracking during movement.

It doesn’t stabilize the proximal tibiofibular joint, which is supported by its own ligaments and membranes. It isn’t the primary hinge for knee flexion—that role lies with the tibiofemoral joint. And while it bears some load during gait, it does not transmit all body weight; weight transfer is mainly through the tibiofemoral articulation and the surrounding structures.