Which muscle's contraction is involved in the patellar locking mechanism?

• A. Vastus medialis
• B. Vastus lateralis
• C. Rectus femoris
• D. Gastrocnemius

Answer: (A)

The patellar locking mechanism is driven by how the patella is held in place within the femoral groove as the knee moves into full extension. When the knee nears full extension, the vastus medialis contracts to pull the patella medially, helping align it with the trochlear groove and press it firmly into place. This medial stabilization, especially from the vastus medialis, centers the patella so the surfaces stay in contact and the knee can lock into extension smoothly.

If the patella were pulled laterally, by the vastus lateralis for example, the patella could drift out of position, making locking less stable or more likely to fail. Rectus femoris contributes to knee extension but doesn’t specifically enforce the locking alignment of the patella, and the gastrocnemius doesn’t play a direct role in patellar tracking or locking. So the muscle most responsible for the patellar locking mechanism is the vastus medialis, which provides the essential medial pull to center and secure the patella.