In the case of a weak or under-active psoas or iliacus, the femur may move above the level of the hip but it is not from the action of the psoas and iliacus but rather from the momentum created by the other three hip flexors. With this knowledge in hand, I believe that our knowledge of back pain, “hip flexor strains,” and ‘quad pulls” is drastically expanded. Before we discuss specific injuries let’s first look at how to assess the function of the psoas and iliacus.
I’m a runner and started experiencing some familiar tightness in my hip and started getting worse everyday I ran. It’s always gotten sore after running for a long extent for the last 4 years or so. I’m glad I found this page because all of these stretches helped me realize what needed to be stretched and how tight I really was! I hope this will fix my overwhelming soreness. Thank you!
If you’re lucky, you won’t notice your hips are tight until you’re trying to do the Half Pigeon pose in your yoga class. But if you’re not so fortunate, your tight hips are making themselves known every time you so much as walk to the bathroom or sit on the couch—expressing themselves in the form of lower back pain and muscle stiffness. Tight hips can even shorten your stride, slowing your 5K goal time!
Extension (as a definition) increases the angle between the bones in a joint.  When you extend your knee, you are straightening your knee from the bent position, increasing the angle between the femur and the shin bones.  When you are extending your hip, your leg is essentially moving backwards in space, say 10-20 degrees.  When you walk, run, or lunge, you have one hip passing through extension.  Now why the fuss about these two words?

The hip flexors are the group of muscles that allow you to lift your knees toward your chest and bend forward from the hips.  What is collectively referred to as the hip flexors is actually a group of muscles that includes the iliopsoas, the thigh muscles (rectus femoris, Sartorius and tensor fasciae latae), and the inner thigh muscles (adductor longus and brevis, pectineus and gracilis).
Great exercises and stretches that can be easily done throughout the day to strengthen and loosen my hip flexors. i have very tight hip flexors so it's very helpful for me knowing these exercises and stretches. For those that want more info about exercises and stretches for hip flexors, i recommend the "unlock your hip flexors". It is a program that will show you many more exercises and stretches you can do. So check it out here
Following the core strengthening, working on glute activation through various hip extension movements is the big finale. For one, the glutes main function of hip extension is an agonist to the hip flexors, and are also directly involved in low back stability, which means they help to pick up the slack for the core during movements, and helps reduce the impulse on the psoas, therefore reducing the “tightness.”
The sartorius supports the more powerful iliopsoas and rectus femoris in accomplishing hip flexion. This long, narrow muscle arises from the front of your hipbone, crosses your front thigh and inner knee and inserts at the top of your shinbone. The unique position of the sartorius enables it to support other leg motions, including knee extension, leg rotation and outward thigh movement. Sartorius strains, which commonly occur in runners and hurdlers, usually occur where the muscle arises at the hipbone. Pain and leg weakness are common symptoms. As with the other hip flexors, stretching and strengthening exercises serve as good preventive measures.
The only activity performed on a regular basis that fully extends the hip is walking and running. Hence as activity levels decrease so does the ability to extend the hip. This results in compensatory pelvic tilting and lumbar extension, with a reduction in the ability to accommodate uneven ground, negotiate obstacles, or attempt to change walking speed quickly. The compensatory pelvic tilt that accompanies tight hip flexors also predisposes the individual to  postural problems and back pain. Hip stretches done on a regular basis can help you maintain extension range of motion and thereby improve function.
How to: Lie on your back with your right knee bent and foot flat on the floor (a). With your left leg fully extended, press into your right foot to shift onto your left hip. This is your starting position (b). Then, squeeze your right glutes to press your left hip open until you feel a stretch, pause, then return to start. That’s one rep (c). Perform six to eight reps, then repeat on the opposite side.