A trigger point is a taut nodule within the muscle tissue that healthy muscle tissue does not have. Your muscles are made of many bundles of long muscle fibers when healthy these muscle fibers squeeze or contract during physical movement or activity and return to their normal resting length afterwards. But this cannot happen when you have active trigger points. Read more here http://rpm-therapy.com/2011/trigger-points-and-muscle-pain/
Learn About Psoas Muscle Pain & Psoas Release Techniques. A great read for anyone who suffers from, or knows anyone suffering from low back pain.