Psoas muscle attaches to your lower spine and to your thigh bone. If it's tight (sitting too much?) it pulls your lower back. It's one of the common causes of lower BACK PAIN! If you can stretch this regularly then your lower back may feel a lot better!
Using a towel or strap during shoulder stretches helps you feel a deeper stretch. Pulling downward with the right hand stretches the left shoulder region while pulling upward with your left hand will stretch the right shoulder region.