Slight contraction of which muscle engages the patellar locking mechanism?

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

Answer: (A)

Engaging the patellar locking mechanism relies on how the patella and its ligament interact with the knee’s bony ridges when the limb moves into a near-extended position. A small contraction of the vastus medialis pulls and stabilizes the patella medially, guiding the medial patellar ligament to catch on the medial trochlear ridge. This interlock holds the stifle in extension with little ongoing muscle effort, allowing the horse to rest.

The other muscles don’t promote this lock as directly: the vastus lateralis tends to pull the patella outward, which can oppose locking; rectus femoris mainly assists in extending the knee and hip without specifically enabling the lock; and the biceps femoris acts as a flexor of the stifle, not a stabilizer that promotes the locking mechanism.