Which key muscle is associated with the function of hip flexion?

The key muscle associated with the function of hip flexion is primarily the iliopsoas muscle, which is innervated by the lumbar nerves, particularly L1, L2, and L3. Among the options provided, L2 specifically plays a crucial role in supplying innervation to this muscle.

The iliopsoas is responsible for flexing the hip joint and elevating the thigh towards the abdomen. As the flexor of the hip, the iliopsoas originates from the lower vertebrae and inserts onto the femur, allowing it to exert force during the hip flexion movement.

In the context of the options listed, L2 is the most directly involved in the innervation of the muscle responsible for hip flexion. The other levels mentioned, such as L3 and L4, although related to the hip region and contributing to movement, are less specific to the primary action of hip flexion than L2. S1 is primarily associated with functions related to hip extension and knee flexion through different muscle groups, further differentiating its innervation focus from that of hip flexion.