What action is performed by the vastus medialis muscle?

• A. Flex hip
• B. Extend stifle
• C. Adduct thigh
• D. Externally rotate leg

Answer: (B)

The vastus medialis muscle is part of the quadriceps group located in the thigh and is specifically responsible for extending the stifle, or knee joint. As a key muscle in the knee extension movement, it plays a significant role in activities such as walking, running, and jumping.

In terms of its function, the vastus medialis primarily works to stabilize the patella (kneecap) during knee movements, ensuring proper alignment and preventing issues such as patellar tracking disorders. The extension of the stifle is mainly achieved through the coordinated action of all quadriceps muscles, but the vastus medialis contributes uniquely by aiding in the final phase of extension and maintaining stability in the joint.

The other actions listed do not accurately correspond to the primary role of the vastus medialis. Flexing the hip involves other muscles such as the iliopsoas, while adducting the thigh and externally rotating the leg are movements associated with different muscle groups in the inner thigh and outer thigh, respectively. Thus, the identification of stifle extension as the primary action of the vastus medialis is fundamental in understanding its anatomy and functional relevance in locomotion.