Hip flexor pain can manifest in one of three hip flexors—psoas major, sartorius, or rectus femoris.
Pain in the psoas (my favorite muscle) can take different forms; it is often a dull ache deep in the groin; sometimes a wrapping pain at the front of the body from the inner thigh to the outer hip; and it might also show up as lower back pain.
The sartorius is the longest muscle in the body and pain here shows up in the outer hip, sometimes trailing down across the thigh to the inner knee.
A common and prevalent form of hip flexor pain concerns the rectus femoris muscle, one of the four quadriceps muscles. The four quadriceps muscles all meet to insert into the tibia of the shin. But the rectus femoris is the only one of these muscles that connects to the pelvis, which means it is the only quadriceps muscle involved in hip flexion.
The rectus femoris muscle is the culprit here because poor posture usually involves the leg bones sinking forward of the pelvis pushing into the tendon of the rectus femoris.
So when people talk about hip flexor issues that are common in yoga, the gym, and life, they are most often talking about stress in the tendon of rectus femoris.
You know if this is you because when you attempt an action, no matter what muscles are supposed to do the work, the rectus femoris takes over, with the tendon popping like a steel cable being pulled taut (It is crazy to watch this happen!).
Good posture relates to an alignment of the bones that bear and transferweight through the skeleton with as little extra effort as possible. The bones sit one on top of the other (relatively) and muscles with balanced tone support the good alignment of the bones.
Poor posture that lets the thighs lean forward puts undue pressure on the rectus femoris muscle asking it to help in holding us upright when it has better things to do. All too often this leads to overworked muscles that are constantly aching and cranky.
Changing our walking and standing patterns can alleviate hip flexor pain, which starts by getting the legs to fall directly under the hip sockets and pelvis, freeing the rectus femoris and psoas, allowing us to move from our deep core, which can have an amazing effect on many pain issues.