What is the primary action of the vastus lateralis muscle?

• A. Flex hip
• B. Extend stifle
• C. Stabilize pelvis
• D. Rotate thigh

Answer: (B)

The vastus lateralis is one of the four muscles that make up the quadriceps femoris group, which is primarily responsible for knee extension. Its primary action is to extend the stifle, or knee joint, by contracting and pulling the patella (kneecap) and tibia. This muscle plays a crucial role in activities that require strong knee extension such as walking, running, and jumping.

When the vastus lateralis contracts, it works synergistically with the other quadriceps muscles to extend the leg at the knee joint, contributing both to the power and stability of the lower limb during various movements. Understanding this muscle's role is essential in anatomy and physiology, particularly in relation to locomotion and sports. The specific focus on extending the knee helps clarify why the chosen answer is the most accurate depiction of the vastus lateralis's primary function.