Trigger point injection (TPI) is used to treat extremely painful areas of muscle. Normal muscle contracts and relaxes when it is active. A trigger point is a knot or tight, ropy band of muscle that forms when muscle fails to relax. The knot often can be felt under the skin and may twitch involuntarily when touched (called a jump sign). 

The trigger point can trap or irritate surrounding nerves and cause referred pain — pain felt in another part of the body. Scar tissue, loss of range of motion, and weakness may develop over time.

TPI is used to alleviate myofascial pain syndrome (chronic pain involving tissue that surrounds muscle) that does not respond to other treatment, although there is some debate over its effectiveness. Many muscle groups, especially those in the arms, legs, lower back, and neck, are treated by this method. TPI also can be used to treat Fibromyalgia and tension headaches.