Unlocking of knee is caused by-
**Core Concept**
The unlocking of the knee joint refers to the movement from a flexed position to extension, involving the coordination of multiple muscle groups and joint structures. This process is crucial for normal gait and mobility. The unlocking of the knee is primarily facilitated by the quadriceps femoris muscle, which extends the knee joint.
**Why the Correct Answer is Right**
The quadriceps femoris muscle, composed of four muscles (rectus femoris, vastus lateralis, vastus medialis, and vastus intermedius), plays a pivotal role in knee extension. When the quadriceps contract, they pull the patella (kneecap) upward, causing the tibiofemoral joint to extend. This action is essential for weight-bearing activities, such as walking or running. The quadriceps muscle's role in knee extension is also critical in maintaining knee stability and preventing excessive flexion.
**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because the quadriceps femoris muscle, not the hamstrings, is primarily responsible for knee extension.
* **Option B:** This option is incorrect because the iliopsoas muscle primarily flexes the hip joint and assists in knee flexion, but it is not the primary muscle responsible for knee extension.
* **Option C:** This option is incorrect because the gastrocnemius muscle primarily plantarflexes the foot and assists in knee flexion, but it is not the primary muscle responsible for knee extension.
**Clinical Pearl / High-Yield Fact**
The quadriceps femoris muscle is often tested clinically by assessing the knee extension lag (or "quadriceps lag") in patients with quadriceps weakness or paralysis, which can be a result of various conditions, such as stroke or spinal cord injury.
**Correct Answer:** A. Quadriceps femoris muscle.