"There seems to be a reasonable consensus on what causes a trigger point, but not on what it is. According to Travell, trigger points are characterized by increased concentration of nuclei and increased interstitial fluid in muscle tissue, whereas other studies have not found any pathological changes in the muscle tissue. It has been shown that the pain threshold is decreased over a trigger point, and that this threshold is increased after acupuncture treatment. The mode of action of acupuncture is not fully understood. Nevertheless, enkephalin is believed to be involved at the spinal level, and endorphin and serotonin at the central level.

Trigger points are believed to cause a contraction of the muscle and thereby shortening of the muscle fibres. This muscle contraction is considered to cause a compression of the vessels within the muscle and thereby a localised ischemia and an accumulation of lactic acid. However, muscular ischemia on its own is not a cause of pain: it has been shown that an ischemic muscle during rest does not cause pain, although contraction of an ischemic muscle does, probably due to release of neurotransmitters such as bradykinin, prostaglandins, potassium ions, etc. Furthermore, it has been found that acupuncture increases the regional blood flow, probably via a decrease in the sympathetic tone. It is therefore likely that acupuncture, in addition to a pain relieving effect, normalizes the haemodynamic conditions in the muscle."

* see Bibliography reference #3