Flexion of knee joint is caused by
**Core Concept**
Flexion of the knee joint is a movement where the lower leg is brought towards the thigh, resulting in a decrease in the angle between the femur and tibia. This movement is primarily facilitated by the hamstring muscles and the action of the quadriceps is to prevent hyperextension.
**Why the Correct Answer is Right**
The primary muscles responsible for flexion of the knee joint are the hamstring muscles, specifically the semitendinosus, semimembranosus, and biceps femoris. These muscles contract and pull on the tibia, causing it to move anteriorly and flex relative to the femur. The action of the quadriceps muscle, on the other hand, helps to stabilize the knee joint and prevent hyperextension.
**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not specify the primary muscles responsible for knee flexion.
**Option B:** This option is incorrect because extension of the knee joint is the opposite movement of flexion.
**Option C:** This option is incorrect because the quadriceps muscle primarily helps to stabilize the knee joint and prevent hyperextension, rather than causing flexion.
**Clinical Pearl / High-Yield Fact**
It's essential to remember that the quadriceps muscle plays a crucial role in preventing hyperextension of the knee joint, rather than causing flexion. This is particularly important in clinical scenarios where knee injuries or instability are a concern.
**Correct Answer: D. Hamstring muscles.**