Illustration showing three of the heads of quadriceps. The fourth head - vastus intermedius - is not seen here as it lies underneath the rectus femoris.
Quads contraction, showing how the rectus femoris takes much of the load as the knee extends.
The quadriceps and the extensor mechanism
The quads is linked to the tibia via the patella and the patellar tendon, as well as via fibrous tissue around the patella. The whole structure moves in concert when the knee is extended - and is known as the extensor mechanism.