When pressed, a trigger point causes immediate local pain disproportionate to the finger pressure. In other words, lightly pressing on a muscle one inch from a trigger point might cause no discomfort, but the same pressure directly on the area of maximum sensitivity prompts immediate pain. In addition to pain, an active trigger point causes muscle weakness and restricted motion. A latent triggerpoint causes weakness and restriction, but not pain - at least not until it becomes activated. Normal, healthy muscles contain no TPs.
Muscles develop TPs following acute trauma, chilling or chronic overload.
Acute trauma may consist of falls, car accidents, joint sprains, a direct blow to the muscle or abnormal excessive exercise, such as packing when moving or engaging in a strenuous bout of weekend athletics. A cold draft such as that from air conditioning or open window may activate latent TPs in tired muscles. Trigger points also develop in muscles that are chronically overloaded by poor posture, especially while working, or repeating muscle movements as in typing, playing the violin, or even knitting. Structural discrepancies, like uneven legs and pelvic bones, or short arms on a long upper body, often produce a chronic mechanical stress that activates TPs. This is why Travell prescribed left heel lifts for all of Kennedy's shoes and custom designed seating for the president's personal quarters, his office, helicopter, planes, boats and limousine.