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!
Top 5 hamstring stretch to prevent injury during workouts. You may also want to do this stretch sitting down as passive stretching is more effective than active stretching as in this diagram of an active hamstring stretch.