Trigger points are described as hyperirritable spots in skeletal muscle that are associated with palpable nodules in taut bands of muscle fibers. The palpable nodules are said to be small contraction knots and a common cause of pain. Compression of a trigger point may elicit local tenderness, referred pain, or motor dysfunction.
Trigger points have been a subject of study by a small number of doctors for several decades although this has not become part of mainstream medicine. The existence of tender areas and zones of induration in muscles has been recognized in medicine for many years and was described as muscular rheumatism or fibrositis. However, there was little agreement about what they meant. Important work was carried out by J. H. Kellgren at University College Hospital, London, in the 1930s and, independently, by Michael Gutstein in Berlin and Michael Kelly in Australia (the latter two workers continued to publish into the 1950s and 1960s). Kellgren conducted experiments in which he injected saline into healthy volunteers and showed that this gave rise to zones of referred extremity pain.
Today, much treatment of trigger points and their pain complexes are handled by massage therapists, physical therapists, osteopaths, occupational therapists, myo-therapists, some naturopaths, chiropractors and acupuncturists , and other hands-on somatic practitioners who have had experience or training in the field of neuromuscular therapy (NMT).
Janet G. Travell, MD
It was, however, an American physician, Janet G. Travell, who was responsible for the most detailed and important work. Her work treating US President John F. Kennedy's back pain was so successful that she was asked to be the first female Personal Physician to the President.  She published more than 40 papers between 1942 and 1990 and in 1983 the first volume of The Trigger Point Manual appeared; this was followed by the second volume in 1992. In her later years Travell collaborated extensively with her colleague David Simons. A third edition is soon to be published by Simons and his wife, both of whom have survived Travell.
The trigger point concept remains unknown to most doctors and is not generally taught in medical school curricula. Other health professionals, such as physiotherapists, osteopaths, naturopaths, chiropractors, physiatrists, massage therapists and structural integrators and some veterinarians are generally more aware of these ideas and many of them make use of trigger points in their clinical work.
Travell and Simon's seminal work on the subject, Myofascial Pain and Dysfunction: The Trigger Point Manual  , states the following:
around 75% of pain clinic patients have a trigger point as the sole source of their pain.
The following conditions are often diagnosed (incorrectly) when trigger points are the true cause of pain: carpal tunnel syndrome, bursitis, tendinitis, angina pectoris, sciatic symptoms, along with many other pain problems.
Arthritis is often cited as the cause for pain even though pain is not always concomitant with arthritis. The real culprit may be a trigger point, normally activated by a certain activity involving the muscles used in the motion, by chronically bad posture, bad mechanics, repetitive motion, structural deficiencies such as a lower limb length inequality or a small hemipelvis, or nutritional deficiencies.
Myofascial pain syndrome
The main innovation of Travel’s work was the introduction of the myofascial pain syndrome concept (myofascial referring to the combination of muscle and fascia). This is described as a focal hyperirritability in muscle that can strongly modulate central nervous system functions. Travell and followers distinguish this from fibromyalgia, which is characterized by widespread pain and tenderness and is described as a central augmentation of nociception giving rise to deep tissue tenderness that includes muscles.
Qualities of trigger points
Trigger points have a number of qualities. They may be classified as active/latent and also as key/satellites and primary/secondary.
An 'active trigger' point is one that actively refers pain either locally or to another location (most trigger points refer pain elsewhere in the body along nerve pathways). A latent trigger point is one that exists, but does not yet refer pain actively, but may do so when pressure or strain is applied to the myoskeletal structure containing the trigger point.
A key trigger point is one that has a pain referral pattern along a nerve pathway that activates a latent trigger point on the pathway or creates it. A satellite trigger point is one which is activated by a key trigger point. Successfully treating the key trigger point often will resolve the satellite and return it from being active to latent or completing treating it too.
In contrast, a primary trigger point in many cases will biomechanically activate a secondary trigger point in another structure. Treating the primary trigger point does not treat the secondary trigger point.
Activation of trigger points may be caused by a number of factors, including acute or chronic muscle overload, activation by other trigger points (key/satellite, primary/secondary), disease, psycho-emotional disorders, homeostatic imbalances, direct trauma to the region, radiculopathy, infections and health choices such as smoking.
Trigger points can appear in many myofascial structures including muscles, tendons, ligaments, skin, joint capsule, periosteal, and scar tissue. When present in muscles there is often pain and weakness in the associated structures. These pain patterns in muscles follow specific nerve pathways and have been readily mapped to allow for identification of the causative pain factor. Many trigger points have pain patterns that overlap, and some create reciprocal cyclic relationships that need to be treated extensively to remove them.
Diagnosis of trigger points is by examining signs, symptoms, pain patterns and manual palpation. Usually there is a taut band in muscles containing trigger points, and a hard nodule can be felt. Often a twitch response can be felt in the muscle by running your finger perpendicular to the muscle's direction; this twitch response often activates the "all or nothing" response in a muscle that causes it to contract. Pressing on an affected muscle can often refer pain. Clusters of trigger points are not uncommon in some of the larger muscles, such as the Gluteus group (Gluteus Maximus, Gluteus Medius, Gluteus Minimus). Often there is a heat differential in the local area of a trigger point, and many practitioners can sense that.
In order for a medical sign to be diagnostically useful, independent examiners must be able to agree on its presence.
Misdiagnosis of Pain
The misdiagnosis of pain is the most important issue taken up by Travell and Simons. Referred pain from trigger points mimics the symptoms of a very long list of common maladies; but physicians, in weighing all the possible causes for a given condition, have rarely even conceived of there being a myofascial source. The study of trigger points has not historically been part of medical education. Travell and Simons hold that most of the common everyday pain is caused by myofascial trigger points and that ignorance of that basic concept could inevitably lead to false diagnoses and the ultimate failure to deal effectively with pain. 
There have been several theories about trigger points. It was once believed that trigger points were scars or inflammation in the muscle. This was disproved when biopsies showed no abnormalities. 
More recently it has been proposed that trigger points are spasms or contractures of voluntary muscle, possibly caused by an abnormality at the neuromuscular junction where the nerves controlling muscles connect to the muscle fibers (Travell & Simon). This theory seems unlikely because no contractions of voluntary muscle have been identified by traditional EMG and because the trigger points are often not in the location of the neuromuscular junction.
The most recent theory is that trigger points are muscle spindles, made over-active by adrenalin stimulation. These very short muscle fibers, only about 1 cm in length, are called intrafusal muscle fibers to distinguish them from the voluntary muscle fibers which are called extrafusal muscle fibers. Only the intrafusal muscle fibers inside the spindle are activated by adrenalin via the sympathetic nervous system which also controls heart rate, blood pressure and other internal regulatory functions. The “sympathetic spindle spasm” theory of trigger points proposes that when spindles are over-activated by adrenalin they become painful. A clinical research trial is being conducted and should be completed by the end on 2006 by Dr. David Hubbard in San Diego, California. Paul Svacina, Engineer and bodyworker also in California, believes that this theory supports the idea that stress and decrease of moderate physical activity in modern lives has increased the occurrence of myofascial pain and trigger points.
Current theories include:
Travell’s Initial Trauma Theory
Integrated Trigger Point Hypothesis
Muscle Spindle Hypothesis
Fibrotic Scar Tissue Hypothesis
Prior to treatment commencement, the therapist should be sure be that the pain patterns they are treating lend themselves to Trigger point therapy. If the patient presents with swelling, discoloration, or neurological symptoms, it is always advisable to refer to another health/medical care provider regarding further examination and/or investigation. The therapist should be aware of his/ her professional limitations.
Treatment of trigger points may be by manual massage (deep pressure as in Bonnie Prudden's approach or tapotement as in Dr. Griner's approach), mechanical vibration, pulsed ultrasound, electrostimulation , ischemic compression, injection (see below), dry-needling, "spray-and-stretch" using a cooling (vapocoolant) spray, and stretching techniques that invoke reciprocal inhibition within the musculoskeletal system. Use of elbows, feet or various tools to direct pressure directly upon the trigger point often occurs, to save practitioner's hands.
A successful treatment protocol relies on identifying trigger points, resolving them and, if all trigger points have been deactivated, elongating the structures affected along their natural range of motion and length. In the case of muscles, which is where most treatment occurs, this involves stretching the muscle using combinations of passive, active, active isolated (AIS), muscle energy techniques (MET), and proprioceptive neuromuscule facilitation (PNF) stretching to be effective. Fascia surrounding muscles should also be treated, possibly with myofascial release, to elongate and resolve strain patterns, otherwise muscles will simply be returned to positions where trigger points are likely to re-develop.
The results of manual therapy are related to the skill level of the therapist. If trigger points are pressed too short a time, they may activate or remain active; if pressed too long or hard, they may be irritated or the muscle may be bruised, resulting in pain in the area treated. This bruising may last for a 1-3 days after treatment, and may feel like, but is not similar to, delayed onset muscle soreness (DOMS), the pain felt days after overexerting muscles. Pain is also common after a massage if the practitioner uses pressure on unnoticed latent or active trigger points, or is not skilled in Trigger Point Therapy. Further pain after one to three treatment sessions by a Trigger Point practitioner should be referred to a medical professional. Evidence based medicine researchers have concluded evidence for the usefulness of trigger points in the diagnosis of fibromyalgia is thin. Fibromyalgia patients generally have multiple, reoccurring trigger points, typically in a quadrant or more of the body.
Injections provide more immediate relief and can be effective when other methods fail. Various injections can be used including saline, local anesthetics such as procaine hydrochloride (Novocain), steroids, and Botox. Injection with a low concentration, short acting local anesthetic (Procaine 0.5%) without steroids or adrenalin is recommended. High concentrations or long acting local anesthetics as well as epinephrine cause muscle necrosis. Use of steroids can cause skin atrophy. Dry needling can be just as effective but causes more post-injection soreness. Botox is rarely indicated. 
Despite the concerns about long acting agents , a mixture of lidocaine and Marcaine is often used.  A mixture of 1 part 2% lidocaine with 3 parts 0.5% Marcaine provides 0.5% lidocaine and 0.375% Marcaine. This has the advantages of immediate anesthesia with lidocaine during injection to minimize injection pain while providing a longer duration of action with a lowered concentration of Marcaine. Sarapin can be used for trigger point injection.
There are a number of ways to self-treat trigger points and these methods are described in numerous texts. It should be noted that due to the controversy related to this field, as well as its relative newness, that no single guide should be taken as the sole or final truth. Underlying any attempts at self-treatment should be a working knowledge of the area to be treated, especially with regard to the musculature, nerves, glands and vessels.
Trigger points in the male or female pelvis, such as found in chronic pelvic pain syndrome (CPPS), should be treated by medical doctors or osteopaths trained in the use of intra-rectal trigger point and myofascial release techniques.
Caution: Self-treatment does have some inherent dangers of damaging soft tissue and other organs. The trigger points in the upper Quadratus Lumborum, for instance, are very close to the kidneys and poorly administered self-treatment may lead to kidney damage. Likewise, treating the masseter muscles may damage the salivary glands superficial to this muscle. Furthermore, and some experts believe trigger points may develop as a protective measure against unstable joints. A qualified professional should eliminate this cause prior to beginning a self-treatment program.
^ a b c d Travell, Janet; Simons, David; Simons, Lois (1999). Myofascial Pain and Dysfunction: The Trigger Point Manual (2 vol. set, 2nd Ed.). USA: Lippincott Williams & Williams. ISBN 0-683-08363-5.
^Simons,D.G.CardiologyandMyofascialTriggerPoints:JanetG.Travell's Contribution . Tex Heart Inst J. 2003; 30(1): 3–7.
^GerwinRD,ShannonS,HongCZ,HubbardD,GevirtzR.Interrater reliability in myofascial trigger point examination. Pain. 1997 Jan;69(1-2):65- 73.
^Davies,Clair;Davies,Amber(2004).TheTriggerPointTherapyWorkbook (2nd Ed.). USA: New Harbinger Publication, Inc., 323. ISBN 1-57224-375-9.
^TheImmediateEffectivenessofElectricalNerveStimulationandElectrical Muscle Stimulation on Myofascial Trigger Points American Journal of Physical Medicine & Rehabilitation. 76(6):471-476, November/December 1997. Hsueh, Tse-Chieh MD, MS 2; Cheng, Pao-Tsai MD, MS; Kuan, Ta-Shen MD, MS; Hong, Chang-Zern MD
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