The illiacus attaches on the upper portion of the femur and begins on the inside crest of the illium (inside of the pelvis), where the psoas attaches all the way through the transverse processes of the lumbar spine, even binding into the discs directly. The rectus femoris begins at the base of the anterior superior illiac spine, and attaches all the way down to the knee cap, whereas the sartorius starts in the same place as the rectus femoris, but attaches on the medial aspect of the knee, blending with the MCL and portions of the hamstrings.


Grade II (moderate): A larger tear in your muscle that makes it difficult to move and causes a moderate amount of pain, especially when you move the affected muscle, swelling, and tenderness. You may have 5 percent to 50 percent loss of function and you may be limping. You can't go back to sporting activities until the tear is completely healed. These injuries can take anywhere from a couple weeks to a few months to heal, depending on how bad they are.
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:

Fun fact: I was hoping I could rename this because I have a phobia of butterflies. But.. I want you to be able to easily recognize this gym class favorite, so here we are. When it comes to hip flexor stretches, this is hands-down the most embarrassing for me- your knees should be much lower to the ground than mine, but that takes time. Work in progress, friends.
Luckily, you don’t have to quit your day job or forgo spin class to loosen them up. Simply stretching those hips can get your body back in alignment, increase your mobility (and thus your exercise performance) and maybe even ease pesky back pain, Moore says. “Given the amount of time we sit [each] day and the stress we put our bodies under, hip-opening moves are a necessary party of our daily routine.”
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