The taut band “housing” the MTrP is found in an otherwise more relaxed muscle (Fig. 11.1). The band forms by tension generated from contracted and stretched sarcomeres relating to a more centrally located knot (Simons & Dommerholt 2006a). It is important to note that MTrPs will always be found in a taut band, but a taut band does not always contain a MTrP.

MTrPs are generally divided into active and latent MTrPs. Both active and latent MTrPs are normally painful upon compression. The pain referral zone is commonly projected to a distant area, sometimes without necessary spontaneous pain sensations at the actual site of the trigger point itself.

The exact etiology of MTrPs has yet to be fully established (Simons 2008), but different theories have been presented over the years.

It is suggested that some form of trauma initially damages the end sacs of the sarcoplasmic reticulum (SR) and the sarcolemma. The trauma is either acute or in the form of repetitive microtrauma caused by overuse or overload of local muscles, something heavily prevalent in the athletic community. This produces a substantial release, with increased intracellular concentration, of calcium ions (Ca²⁺), triggering contractions of local sarcomeres in the affected muscle cells (Fig. 11.2).

More recent research has formed the generally adopted theory named “the integrated trigger point hypothesis” (Simons et al. 1999; Fig. 11.3). It suggests that an abnormal release of acetylcholine (ACh) from the motor endplates, even during rest, produces a sustained depolarization of the affected muscle cells. ACh triggers the calcium channels in the SR to start releasing Ca²⁺. An impaired release of acetylcholinesterase is also found to play a role (Simons 2008).

The malfunctioning motor endplates form a “central MTrP” located at the innervation site of the muscle. MTrPs seems to begin in a region of multiple dysfunctional endplates with an abnormal release of ACh. The dysfunctional endplates are connected to a section of muscle fibers forming a contractile knot and taut band in an otherwise more relaxed muscle. The sustained contraction increases local metabolic demands and constricts local capillaries, which reduces available oxygen and nutrient levels in the area, limiting ATP production. An accumulation of metabolites triggers local vasoconstriction that further decreases blood circulation. This finally leads to a local energy crisis due to a lack of ATP. The calcium pump returning calcium into the end sacs of the SR is highly dependent on ATP. The sudden lack disrupts its function, potentially sustaining the localized contractile reaction (Simons et al. 1999). The energy crisis triggers release of substances, i.e. bradykinin, cytokines, serotonin, histamine, E-type prostaglandins, substance P, etc., causing increased pain sensation. It is thought likely that if the endplate dysfunction is allowed to continue for any length of time, chronic fibrotic changes may occur in the tissue (Hou et al. 2002).

Contributing additional factors to MTrP development are deficiencies in vitamins B1, B6, B12, C, D, and folic acid, magnesium, zinc, and iron (Dommerholt et al. 2006; Simons & Dommerholt 2006a) as well as hormonal dysfunctions like hypothyroidism, increased levels of reversed T3, and human growth hormone deficiency (Simons & Dommerholt 2006a). Depressive emotional states have also been shown to predispose people to MTrP formation (Simons et al. 1999).

Because MTrP pain does not project along regular dermatomes, the exact explanation model for referred MTrP pain is still somewhat uncertain, although particular neural explanation models are favored. Some correlations between MTrPs and acupoints have also been mapped, and more recent investigations suggest the possibility that MTrPs and Ah Shi points could be the same phenomenon (Birch 2003; Simons & Dommerholt 2006b). It has also been shown that drawings by patients describing their pain pattern coincided to a high degree with the pathways of acupuncture channels (Baldry 1993).

Reproduction of the local or referred pain is possible through mechanical stimulation of a MTrP, either by focal compression or stretching. The pain may appear immediately or after a 10–15s delay. Although distinctive referred pain patterns for each MTrP have been established, substantial variations exist (Dommerholt et al. 2006).

MTrPs are generally located by evaluation of the skin condition, presence of taut band, local or referred pain, reflex/reactions, and/or referred autonomic phenomena.