Trigger points are caused by muscle injury. Muscles can be injured suddenly in accidents, or damage can occur slowly, due to repeated movements or poor posture. A muscle is composed of tiny fibers, which contract and relax in response to messages from the brain. When muscle fibers become injured or over stimulated, they cannot relax and form contraction knots.
A trigger point consists of many contraction knots, where individual muscle fibers contract and cannot relax. Fibers, extending from the trigger point to the muscle attachments shorten, and form a tight band. The persistent contraction of muscle fibers compresses blood vessels and decreases their blood supply, leading to oxygen starvation and the accumulation of waste products. This irritates nerves and causes pain. Activated pain receptors generate specific referred pain patterns, depending on nerve passage and muscle anatomy.