There are 4 main locations of the smaller muscles that make up the overall hip flexor muscles: the abdomen/hip region, the front of the thigh, the gluteus, and the inside of thigh/groin.
Hip Flexor Muscles (Illiopsoas)
The Psoas Major is a tapered muscle that runs along the front of your pelvic bone down to your hip joint, it is probably the most important of the hip flexor muscles. While it does help lift the leg up, it also greatly aids in external rotation (moving your leg away from your center of gravity). The Psoas Minor also helps in this role, but only about 50% of people have this muscle! This is also one of the highest risk muscles for a hip flexor injury due to its’ central position.
The Illiacus Muscle is located right beside the Psoas, but it is a much smaller, triangular muscle. The Illiacus is actually important for two types of movements, the first: open-chained (like when you are standing), it helps to raise the leg/knee forward. In closed-chained positions (think lying down), it can help lift the upper half of the torso up, like in a sit-up. Because of the heavy loads often placed on the Illiacus it is often a source of hip flexor pain, usually as a result of a hip flexor strain.
Muscles on Front of Thigh
This is what most people would call their quadriceps muscle, but it is actually part of the hip flexor muscles. It obviously is involved in hip flexion, but it also is the primary muscle when extending the leg(knee flexion) in a standing position, like kicking a soccer ball. Note that it is not the primary muscle for this same movement in a seated position when the hip is already in a flexed position, it has limited power, so other muscles are use.
The Sartorius is a long thin muscle that runs from the hip joint all the way down to the knee over top of the thigh. It aids in 4 major movements: Flexion of the knee and hip, rotation of your leg, and adduction (pushing your legs together). Because of its’ many facets of movement, it primarily plays a support role to other hip flexor muscles.
Tensor Fasciae latae
The Tensor Fasciae latae is primarily an adductor and stabilizing muscle. In many activities, walking, running, snow activities, and many more, it is heavily used to stabilize the leg and support the knee.
Inner Thigh Muscles
These 4 hip flexor muscles all have extremely similar functions for the scope of this article. The Pectineus, Adductor longus and brevis, and the Gracilis all are located in what most people would consider the groin region. They are responsible mainly for moving the leg laterally, although they support in the flexion of the hip as well.
As you can see, there are several hip flexor muscles that all work together to facilitate hip flexion while fulfilling other functions as well.