What I've learned about life, and yoga teacher training.
The hip flexors are a large muscle group on the anterior (front) portion of the hip joint. Quite frequently, due to the increased amount of sitting in our modern culture, this muscle group lacks length and flexibility. Contrary to much popular belief, it is a tightness of the hip flexors that contributes greatly to low back pain (not hamstring tightness). But stretching the hip flexors isn't as easy as it sounds.