Which is an action of the vastus lateralis?

• A. Extend the knee
• B. Flex the knee
• C. Internal rotation of the hip
• D. Abduct the hip

Answer: (A)

The action being tested is what the vastus lateralis does at the knee. This muscle is one of the quadriceps and sits on the lateral thigh, crossing the knee joint. When it contracts, it straightens the knee, causing knee extension. That’s why extending the knee is the correct action for the vastus lateralis.

It doesn’t flex the knee—flexion is mainly produced by the hamstrings. It doesn’t rotate the hip internally or abduct the hip—those motions are handled by hip rotators and the gluteal muscles, respectively. The vastus lateralis works at the knee level to extend the leg, not at the hip.