There are hip flexors whose only role is to flex the hip. Those are called the prime movers.
Then, there are hip flexors that assist and facilitate the flexion of the hip, but they also perform other actions like hip rotation, adduction, and adbuction. Those are known as synergists.
The prime movers in hip flexion are:
1. Rectus Femoris (one of the Quads) whose only role is hip flexion;
2. Psoas whose main role is hip flexion but also facilitating hip rotation;
3. Iliacus (same as Psoas);
5. Sartorius (the longest muscle in the body) whose role is to flex the hip, as well as to externally rotate the hip.
The synergists or facilitators of hip flexion are:
1. Gluteus Medius, whose main role is hip abduction = lifting the leg to the side, but whose anterior fibers are right next to TFL, and assist in hip flexion;
2. Gracilis, one of the 5 Addictors, whose main role is to adduct the hip = squeeze the thighs together, but it assists in hip flexion;
3-5. Adductor Magnus, Longus, and Brevis, whose main roles are to adduct the hip (see Gracilis) and facilitate hip flexion to a certain extent;
6. Pectineus, another adductor, the smallest one and closest to the groin whose role is just like all the other 5.
When someone says "My hip flexor is hurt", what do you think we think as massage therapists? ... "Let's find out which one exactly."
It's like being a detective to eliminate all the healthy ones and determine the real culprit of any discomfort or pain. Any one of these 11 muscles can be the cause.