The psoas and iliacus are hip flexors
There are several muscles that act as hip flexors. The psoas and iliacus are both hip flexors. They attach respectively to the lumbar spine and the inner surface of the hip bone. From there they attach to the inner aspect of femur, just below its neck.
If your hip flexor pain is just below the ASIC or anterior superior iliac crest, the "eye" of the hip bone, then it probably isn't the psoas or iliacus that is the problem.
Hip flexors that attach at or near the ASIC
Hip flexors that attach at or near the ASIC include the rectus femoris, sartorius and tensor fascia latae.
All three of these muscles cross the knee joint to attach to the upper portion of the lower leg bone.
Rectus femoris, tensor fascia latae and sartorius
The rectus femoris attaches to the knee cap and from there it's fibers continue to attach to the front of the tibia.
The tensor fascia latae attaches to the IT band and from there to the outer aspect of the tibia.
Meanwhile, the sartorius passes down the inner thigh and attaches to the inside of the tibia.
How these hip flexors affect the knee
Note that the rectus femoris can aid in controlling the amount of knee bend or at the very least be affected by it.
Sartorius and tensor fascia latae can help control rotation of the lower leg relative to the femur when the knee is bent. When the knee is straight these two muscles can help control rotation of the leg as a whole relative to the hip bone because the ability of the shin to rotate relative to the femur is limited when the knee is straight.
A simple fix for hip flexor and knee pain
Why mention all of this in an article about hip flexor pain while walking? Because if you get hip flexor pain and knee pain while walking it's possibly a problem with one of these three muscles.
And one very simple fix for any nagging muscle pain is to anchor the muscle (or muscles) in question.
Dealing with a combination of hip flexor and knee pain (on the same side), you may find that creating an upwards pull on the ASICs is sufficient to anchor these hip flexors.
Lacking an anchored end point, these muscles may be activated haphazardly in order to create stability. By creating an upwards pull on the ASICs, you give these three muscles a fixed end point to which they can pull towards, helping to effectively control the hip and knee (and possibly preventing hip flexor and associated knee pain).
Note, you could think of the ASIC as a "corner point" of the hip bone. Corner points in general can be thought of as important for providing leverage.
Creating an upwards pull on the ASICs to anchor the hip flexors
How do you create an upwards pull on the ASICs?
Lengthen your spine
One way to create an upwards pull on the ASICs is to lengthen your spine, in the process lifting your chest just enough to add tension to the abs, particularly the external obliques (though it wouldn't hurt to also add tension to the rectus abdominis).
This can then create an upwards pull on the ASIC's (and if the rectus abdominis is lengthened, then also the pubic bone.)
Note that you don't have to lift the chest maximally. Just enough so that you can notice an upwards pull on your ASIC's. If you habitually sit in front of a computer or walk with your chest slumped, this may be the simplest fix.
In other words, stop slouching!
For more on how to prevent slouching read slouch to zero slouch exercises to fix your posture.
Activate your transverse abdominis
Another method for generating an upwards pull on the ASICs is to pull your belly in using your transverse abdominis.
1. Transverse Abdominis relaxed.
2.Transverse Abdominis activation so that Belly is pulled in.
This can tend to activate the aforementioned abs automatically. However, it may take some practice. And here again, you may find that you don't have to pull your belly in fully.
Find an amount of inwards pull that creates a sufficient amount of upwards pull on your ASICs that you don't get any hip flexor pain or knee pain.
Anchoring the hip flexors via the opposite leg
Another means of anchoring the hip flexors is to use the hamstrings, superficial gluteus maximus and also tensor fascia latae of the opposite leg.
This method may be particularly relevant if you have foot problems, say a tendency towards a collapsed arch, on the side opposite to the one with the painful hip flexor.
On the opposite side to the side that hurts, you could create a downwards pull on your sitting bones. This movement can be transmitted via the SI joints to the opposite hip bone so that it too tilts backwards (or is subject to a backward tilting force) with the result being that the ASICs on that side (both sides actually) is subject to an upwards pull.
Creating a downwards pull on the PSICs
Additionally, you'll also want to create a downwards pull on the PSICs, which is the part of the hip crest just to the side of the sacrum.
Doing this, you'll help to create a downwards pull on that hip bone which causes the opposite hip bone to lift a slight amount. Both actions together will help to produce an upward pull on the ASIC of the opposite hip bone thus anchoring the hip flexors on that side.
Note, here the trick will be activating your hamstrings and/or posterior glutes just prior to the point in time where you normally experience hip flexor pain or knee pain.
Difficulty activating hamstrings
Note that if you have difficulty activating hamstrings you may need to make the foot and ankle stable. Fittingly, this can come from using your foot muscles to lift the arch of the foot and at the same time create a downwards pull on the fibula.
Note that the downwards pull on the fibula will help to anchor the outer hamstrings (biceps femoris). Lifting the arch tends to cause an external rotation of the shin relative to the foot and that in turn can help to anchor the inner hamstrings (semitendinosus and semimembranosus) since it resists internal rotation.
Note that the idea when shaping the arch of the foot is not to just activate muscles that cause external rotation but to use internal and external rotation muscles against each other to create rotational stability.
For more on shaping the foot read this article on foot exercises.
Once you've learned how to shape your foot, also take a look at this article on anchoring the biceps femoris so that it's easy to prevent a collapsed arch even while walking: anchoring biceps femoris short head.
Note that to create a downwards pull on the PSIC, you may be targeting the superficial fibers of the gluteus maximus, which tends to externally rotate the shin via the IT band. To balance this external rotation, the tensor fascia latae which attaches to the ASIC may also activate.
Activating the same-leg hamstrings
Note that depending on where and when you get hip flexor pain, you can also try activating the hamstrings and gluteus maximus on the same side that you get the hip flexor pain.
Here also the technique can be as simple as creating a downwards pull on the sitting bone. And you can experiment with also creating a downwards pull on the PSIC.
Adjusting for hip flexor and hamstring activation
For all of the recommended muscle activations, the muscles under consideration, whether hip flexors or hamstrings work on both the hip joint and the knee joint. And all of these muscles can affect, and be affected by, leg rotation relative to the hip. So if you still get nagging pain, one final tip is to adjust rotation of the leg relative to the thigh. This "adjustment" is akin to tuning the radio to get static free reception. You'll have to go back and forwards a few times, and the actual movements (rotating inwards or outwards) are slight.
Try adjusting after creating an upwards pull on your ASICs. Try adjusting after creating a downwards pull on your sitting bones and or PSIC.
Resisting downwards hip compression
One additional point to consider that I'll mention here is that the above mentioned activations of the hip flexors, gluteus maximus and hamstrings tend to pull the hip bone down onto the ball of the femur.
You could think of this as being roughly equivalent to a cars suspension bottoming out. However, rather than bottoming out because you've gone over a bump, in this case, it's bottoming out because there's too much weight.
So you need to jack-up the suspension.
With the hip joints supporting the body in an upright position, you can accomplish this via activation of muscles like the obturator internus, gemellus superior and inferior as well as the obturator externus.
Note that you can use the lower corner points of the hip bones or the top of the femur, as references for activating these muscles.
Note that all of the above is based on my personal experience using muscle control to fix various problems including hip flexor pain, knee pain and collapsed arches.
I've tried to extract the essence of that experience in my courses on muscle control. Basically, I've divided all of the above muscle control elements into simple exercises that allow you to improve your ability to feel and control your body via your muscles.
The nice thing is, the lessons you learn from these muscle control courses don't end at fixing hip flexor pain.
You can use muscle control to continually dive deeper into experiencing and understanding your own body. You can use it to move with more awareness and control and with greater efficiency if you choose.
Contact me using the intercom bubble on this page (blue chat icon thingy at the bottom of the screen) Neil Keleher: learn to understand to find out more about muscle control options.
You can also scan the list of currently available courses and prices.
If you are on a tight budget (or simply want free lessons) then check out my youtube channel. I'm currently working on a series of uploads, starting with the spine. If you are self starter, you can ideally learn a lot from these videos.
Simple hip flexor activation drills so you can stretch them, feel them, use them.
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