The Psoas is undoubtedly one of the most common back pain inducing muscles. If you're familiar with back pain, you likely already know your way around the Psoas and if not - you should learn. Your back will thank you.
Psoas trigger points and problems are very common in modern culture - humans decided to go to war with the Psoas by inventing chairs... and the Psoas is winning the battle. Since sitting is a hip-flexed position, the nature of sitting in a chair encourages Psoas trigger points and tightness. Psoas tightness is very common in desk job workers and students. Click these links for video tutorials on how to stretch the Psoas and massage the Psoas for low back pain relief.
Psoas Anatomy & Function:
As written by Travell and Simons: "The primary function of the psoas major muscle is flexion of the hip."  p. 1198. Psoas is hip flexor muscle that connects from the lumbar vertebrae to the femur. It functions in hip flexion (lifting the knee to the chest), and tends to tighten and develop trigger points from excess sitting. Psoas is used in swinging the leg forward during running, as well as any motion that brings the knee toward chest - for example during sit-ups and hanging leg raises.
There is also evidence to suggest that the Psoas functions in spinal stability. "Yoshio et al found that the psoas major muscle acts primarily as a spinal stabilizer when the hip is flexed 0° to 45°."  p. 1200. There are conflicting views on this topic, but it is safe to assume a strong, flexible, trigger-point free Psoas is ideal.
The Iliopsoas Muscles via Travell & Simons Myofascial Pain & Disfunction p. 1193. The Iliopsoas complex contains the Psoas Major, Psoas Minor, and Iliacus muscles.
Psoas Trigger Points:
"It’s a mistake to assume the problem is at the place that hurts."  p. 36. Trigger points can refer pain to other parts of the body, and Psoas trigger points are no exception. As stated by Clair and Amber Davies, it is a mistake to assume the problem is where your pain is.
Psoas Referred Pain Patterns - via Travell & Simons "Myofascial Pain and Disfunction". The bright red regions are where pain is physically felt from Psoas trigger points.
As shown in the image above, Psoas trigger points can cause pain in the low back region, as well as the front of the upper thigh. "Pain referred from TrPs (Trigger Points) in the iliopsoas muscles forms a distinctive vertical pattern ipsilaterally along the lumbar spine"  p. 1201. This quote is descriptive of the referred pain pattern in the left-most image, which is the broad lumbar spine pain that many patients describe.
Travell & Simons also mention that difficulty standing after sitting for a long time is a tell tale sign sign of Psoas trigger points . When a patient extends the spine to stand upright and feels pain, Psoas trigger points are likely at fault. Imagine an old uncle letting out an "Ope... AHHH" when gingerly standing up straight for the first time in 3 hours - that guy could benefit from a Psoas massage.
Psoas Release Using the QL Claw Device
 Donnelly, Joseph M. Travell, Simons & Simons Myofascial Pain and Dysfunction: the Trigger Point Manual. 3rd ed., Wolters Kluwer Health, 2019.
 Davies, Clair, and Amber Davies. The Trigger Point Therapy Workbook: Your Self-Treatment Guide for Pain Relief. 3rd ed., New Harbinger Publications, Inc., 2013.