How do the quadriceps and patellar tendon extend the knee?

• A. The quadriceps generate knee extension torque, and the patellar tendon increases the extension moment arm.
• B. The quadriceps cause knee flexion while the patellar tendon stabilizes the joint.
• C. The patellar tendon primarily flexes the knee while the quadriceps extend it.
• D. The quadriceps directly extend the knee without involvement of the patellar tendon.

Answer: (A)

The key idea is how the force from the quadriceps is converted into knee extension through a pulley-like mechanism. When the quadriceps contract, their force is transmitted via the quadriceps tendon to the patella, and from there through the patellar tendon to the tibia. The patella sits in front of the knee, so this setup redirects the force and increases the distance between the line of pull and the knee’s axis—that is, it lengthens the moment arm. Since joint torque is the product of force and moment arm, a longer moment arm means more extension torque for the same muscle force. That’s why knee extension relies on both the quadriceps generating force and the patellar tendon providing the leverage to produce that torque. The other statements don’t fit because the quadriceps primarily extend the knee, not flex it; the patellar tendon doesn’t primarily flex the knee; and the extension action depends on transmitting the quadriceps force through the patellar pathway to the tibia.