Low-back pain is now afflicting the nation! And if you bike, run, or sit too much, constricted hips could be adding to the pain around your spine. A major hip flexor muscle, the psoas, not only pulls the thigh toward the abdomen, but is also involved to the five lowest vertebrae of the spine. When the psoas shortens and tightens from activities like sprinting or cycling, it can pull on the vertebrae, generating a sensation of tightness in the low back. The good news is that stretching out the hip flexors can relieve a lot of lower back pain. Once you are warmed up, and after every workout, you should stretch your psoas.
More images and information at http://www.popsugar.com/fitness/Back-Pain-Relief-Stretch-Your-Hip-Flexors-3368118