Trigger points are taut bands of muscle that send pain to other areas of the body. Oftentimes people with trigger points might suffer pain in one area of the body, like the lower back, and not realize that the pain is actually originating on the other side of the body (the psoas, in this case). The field of trigger point therapy was pioneered by Dr. Janet Travell, personal physician to John F. Kennedy. After Travell's 40-plus years of work, she co-wrote a (very expensive) two-volume, 1600 page book detailing over 700 trigger point locations and their effects on the human body.
For those of you who don't want to spend a few hundred dollars on Travell's books, Real Bodywork has made a universal iOS app called Muscle Trigger Points that details trigger points for over 70 muscles and their pain referral patterns. The app itself is an excellent reference guide that lets you search for trigger points by specific muscle, or by pain zones on your body. Best of all, the app is cheap compared to Travell's books.
For those of you who are into trigger point therapy, I highly recommend Muscle Trigger Points just for the fact that it provides an interactive reference that you can carry with you on your iPhone or iPad. That being said, the app does have some drawbacks. It doesn't feature all the muscles or trigger points in the human body, and it does leave out some major ones (Extensor hallucis longus, anyone?). The app also doesn't show you how to deactivate your trigger points, though it does talk you through self-treatment in a four-minute audio guide. (For those who want detailed instructions about self-treatment, Claire Davies' respected Trigger Point Therapy Workbook is the best.)
Muscle Trigger Points is a universal app, and it's available for US$2.99 on the App Store.