The vastus intermedius is a close associate of the vastus medialis and lateralis. It lies between them and beneath the rectus femoris. It is difficult to see this muscle separate from the medialis and often the two are continuous for part of their length.
The vastus intermedius originates on the anterior and lateral sides of the femur, running almost the entire length of the shaft of the femur.
The vastus muscles run almost the entire length of the femur and converge into the quadriceps femoris tendon, which attaches to the patella bone (kneecap).
Knee joint extension.