Figure 1. Knee/Leg Extensors (Quadriceps)

The knee/leg extensors are a group of four muscles located in the anterior thigh region on both sides of the body. Their names are:

1) Vastus Rectus (a.k.a. Rectus Femoris)
2) Vastus Medialis
3) Vastus Lateralis
4) Vastus Intermedialis

They are commonly known as the quadriceps or quads.

These muscles are shown in figure 1 above.

The quadriceps main function is to extend the leg at the knee. This occurs during activities such as running, jumping, kicking and walking. This is illustrated in Figures 2a and 2b

Figure 2a.
Neutral Position
 

Figure 2b.
Knee Extension with Hip Fexion