Which muscle is responsible for flexion of the hip joint?

The rectus femoris is responsible for hip flexion due to its anatomical positioning and function. It is one of the four muscles that comprise the quadriceps femoris, and it crosses both the hip and knee joints. As such, when it contracts, it aids in moving the thigh toward the torso, effectively flexing the hip joint. This action is important for various activities, including walking, running, and kicking.

In contrast, while the sartorius does assist in hip flexion, its primary roles are to flex, abduct, and laterally rotate the thigh. The deltoid muscle primarily functions in the shoulder joint, responsible for arm abduction, and has no involvement with hip flexion. The biceps femoris, part of the hamstrings group, mainly functions in knee flexion and extension of the hip but is not the primary muscle for flexing the hip itself.