Unlocking of knee is done by which muscle:September 2009, March 2013 (d, e, h)
**Core Concept**
The unlocking of the knee joint involves the relaxation of the ligaments and the contraction of specific muscles that straighten the knee, allowing for flexion and extension movements. This process is essential for normal knee function and is regulated by various muscles and ligaments around the knee joint.
**Why the Correct Answer is Right**
The correct answer involves the quadriceps femoris muscle, which is a group of four muscles (rectus femoris, vastus lateralis, vastus medialis, and vastus intermedius) that work together to straighten the knee joint. When the quadriceps contract, they pull the patella (kneecap) upwards and straighten the knee, effectively unlocking it. This action is mediated by the patellar tendon, which connects the patella to the tibia.
**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because the biceps femoris muscle, part of the hamstring group, flexes the knee and is not responsible for unlocking it.
* **Option B:** This option is incorrect because the gastrocnemius muscle, part of the calf muscles, primarily flexes the foot and ankle and is not involved in knee unlocking.
* **Option D:** This option is incorrect because the popliteus muscle does play a role in knee rotation, but it is not primarily responsible for unlocking the knee joint.
**Clinical Pearl / High-Yield Fact**
The quadriceps femoris muscle is responsible for 50% of the total knee extension force, making it a crucial muscle for normal knee function and movement.
**Correct Answer: C. Quadriceps femoris muscle.