Understanding the unique functional contributions of the psoas and iliacus illustrates how a weak or under-active muscle can be a factor in both back pain and in quadriceps strains. With back pain, inability to flex the hip past ninety degrees will often cause many clients or athletes to flex the lumbar spine to give the illusion of flexing the hips. Watch how many of your clients or athletes will immediately flex the lumbar spine when asked to bring the knee to the chest. There is a clear distinction between bringing the knee to the chest and bringing the chest to the knee. Attempting to bring the knee toward the chest and above the level of the hip forces the athlete or client to use, or attempt to use, the psoas and iliacus. If they are unable to do this one, or all, of three things happen:

Kneel with a wall or pillar behind you, knees hips-width apart and toes touching the wall. Arch your back to lean back while keeping your hips stacked over your knees. Take your arms overhead and touch your palms into the wall behind you. This bend does not need to be extremely deep to feel a great stretch in the hips and strength in the lower back.


The rectus femoris is one of the quadriceps muscles. The rectus femoris arises from the front of your hipbone, runs through the middle region of the front thigh and attaches to the top of the kneecap. In addition to hip flexion, the rectus femoris straightens, or extends, your knee. This dual function increases the vulnerability to strain injuries. Stretching exercises to maintain flexibility and balanced training to equalize your quad and hamstring strength reduce the likelihood of rectus femoris strains.
Well, most of us work the hip flexors (including the psoas and iliacus) most of the time- sitting, practicing while seated, cycling, driving...but only in a limited range, i.e. knees and hips bent to 90 degrees.  We need to balance out the movements of the hips a bit more- add more extension and more varieties of flexion.  For example, sitting cross legged, sitting on the floor, squatting, kneeling, etc. all require more varieties of hip movement.  To get more hip extension in your life, you can add some restorative exercises like standing apanasana, lunges (lots of lunges!) and go walk (not on a treadmill).  That way, you don't lose your capacity to move those joints to their full capacity, and you will have loaded the tissues in more diverse ways.
An active warm-up is essential to achieve good form and maximum efficiency, especially if you train in the evening, advises Jason Fitzgerald, founder of StrengthRunning.com. A series of dynamic, prerun movements will lubricate the joints, improve your active range of motion, and wake up muscles that have been dormant all day, helping you to stay upright and extend out the back. For this, try Gary Gray’s celebrated lunge matrix.
If you can set aside time apart from your workouts, try Starrett’s couch stretch: In front of a couch or wall, sit on all fours. Place the shin of one leg parallel against the wall or couch, then bring one leg up into a kneeling position with your knee above your foot. Straighten your torso and fire your glute, as if to slide your two legs together. Hold for two minutes per side, contracting and releasing as you wish.

Athletes with marked weakness of the hip abductors will exhibit the classic Trendelenburg gait pattern. Hallmarks of the Trendelenburg gait pattern are depression of the swing phase pelvis (as the stance phase hip abductors cannot resist the pull of gravity on the unsupported side of the body).4,8,13 Athletes often find ways to compensate for a relative weakness, such as with a compensated Trendelenburg gait pattern. With this pattern the athlete exhibits increased deviation of the body in the frontal plane toward the stance leg. This causes a decrease in the moment arm of gravitational forces pulling on the unsupported half of the body and a relative decreased load on the stance phase hip abductors (Table 12-1).8,13
Then, consider that where there is tightness there might also be weakness somewhere near by. With 11 muscles contributing to the gross movement of hip flexion, it’s possible that some of the muscles are stronger than others. If some are stronger and work harder than others they might get overly tight. Identifying which hip flexors are weak and strengthening them is another way to approach hip flexor tightness.
The main work of your hip flexors is to bring your knee toward your chest and to bend at the waist. Symptoms associated with a hip flexor strain can range from mild to severe and can impact your mobility. If you don’t rest and seek treatment, your hip flexor strain symptoms could get worse. But there are many at-home activities and remedies that can help reduce hip flexor strain symptoms.
Of course, you know what it feels like to have a tight muscle. But tight hips aren't just uncomfortable—they can lead to all sorts of other aches and pains, especially in the lower back. "People focus on the hips and say their hips are tight, but we don't always think about the fact that the lower back connects to our legs at the hip," Charlee Atkins, C.S.C.S., instructor at Soul Annex in New York City and creator of Le Stretch class, tells SELF. Tight hip flexors make it harder for your pelvis to rotate properly, which can cause your lower back to overcompensate, "and this can be a setup for lower-back injury," Teo Mendez, M.D., an orthopedic surgeon at NY Orthopedics who focuses on operative and non-operative management of sports-related injuries, musculoskeletal injuries, and arthritis, tells SELF.
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