What action does the vastus intermedius perform?

The vastus intermedius is one of the four muscles that make up the quadriceps femoris group, which is predominantly involved in the extension of the leg at the knee joint. This muscle is located deep to the rectus femoris and primarily acts to extend the leg during activities such as walking, running, and jumping. The quadriceps muscle group, including the vastus intermedius, works collectively to extend the knee, particularly when the body transitions from a flexed position to a straightened leg.

In contrast, actions like flexing the thigh, laterally rotating the thigh, or adducting the thigh involve different muscle groups. The hip flexors, such as the iliopsoas, are responsible for thigh flexion, while muscles like the gluteus medius assist in abduction and lateral rotation. Therefore, the primary and correct action of the vastus intermedius is to extend the leg, affirming that this is the accurate choice in the context of its function.