The Rectus Femoris muscle is part of the Quadriceps muscle group. It is the only muscle of the group which crosses the hip joint and is a powerful knee extensor when the hip is extended, but is weak when the hip is flexed.
Vastus Intermedius is one of four quadricep muscles, located deep in the thigh underneath the Rectus Femoris muscle.
Vastus Lateralis is the most lateral (outer) of the four quadriceps muscles and is felt on the outside top of the thigh.
Vastus Medialis is the most medially (inner) located of the quadricep muscles. The portion of the muscle just above the knee is known as VMO (vastus medialis oblique). This is important in stabilising the knee joint and often becomes inhibited following injury.