Psoas (pronounced, 'so-az') muscle!
I love talking about the psoas, iliacus...AKA, hip flexors. If you suffer from low back pain, sit a lot through out the day, have tight quads, and stiff knees, then you probably have tight hip flexors.
Stretch these muscles daily and you will discover less pain.
Here is one great example of psoas/hip flexor stretch with release of trigger points prior to the stretch: