Although muscle needling techniques have been used for thousands of years in the practice of acupuncture, there is still much uncertainty about their underlying mechanisms. The acupuncture literature may provide some answers,however,due to its metaphysical and philosophical nature, it is difficult to apply traditional acupuncture principles to the practice of using acupuncture needles in the treatment of MPS.
Mechanical Effects Dry needling of an MTrP may mechanically disrupt the integrity of the dysfunctional motor end plates. From a mechanical point of view, needling of MTrPs may be related to the extremely shortened sarcomeres. It is plausible that an accurately placed needle provides a localized stretch to the contracted cytoskeletal structures, which may disentangle the myosin filaments from the titin gel at the Z-band. This would allow the sarcomere to resume its resting length by reducing the degree of overlap between actin and myosin filaments. If indeed a needle can mechanically stretch the local muscle fiber,it would be beneficial to rotate the needle during insertion. Rotating the needle results in winding of connective tissue around the needle,which clinically is experienced as a ‘needle grasp.’ Comparisons between the orientation of collagen following needle insertions with and without needle rotation demonstrated that the collagen bundles were straighter and more nearly parallel to each other after needle rotation. Langevin and colleagues report that brief mechanical stimulation can induce actin cytoskeleton reorganization and increases in proto-oncogenes expression, including cFos and tumor necrosing factor and interleukins.36 Moving the needle up and down as is done with needling of a MTrP may be
sufficient to cause a needle grasp and a resultant LTR. As a result of mechanical stimulation, group II fibers will register a change in total fiber length, which may activate the gate control system by blocking nociceptive input from the MTrP and hence cause alleviation of pain. The mechanical pressure exerted via the needle also may electrically polarize the connective tissue and muscle. A physical characteristic of collagen fibers is their intrinsic piezoelectricity,a property that allows tissues to transform mechanical stress into electrical activity necessary for tissue remodeling, possibly contributing to the LTR.
Neurophysiologic Effects In his arguments in favor of neurophysiological explanations of the effects of dry needling, Baldry concludes that with the superficial dry needling technique, A-nerve fibers (group III) will be stimulated for as long as 72 hours after needle insertion. Prolonged stimulation of the sensory afferent A-nerve fibers may activate the enkephalinergic inhibitory dorsal horn interneurons, which would imply that superficial dry needling causes opioid mediated pain suppression.Another possible mechanism of superficial dry needling is the activation of the serotonergic and noradrenergic descending inhibitory systems,which would block any incoming noxious stimulus into the dorsal horn. The activation of the enkephalinergic, serotonergic, and noradrenergic descending inhibitory systems occurs with dry needle stimulation of Anerve fibers anywhere in the body Skin and muscle needle stimulation of A-  and C- (group IV) afferent fibers in anesthetized rats was capable of producing an increase in cortical cerebral blood flow, which was thought to be due to a reflex response of the afferent pathway, including group II and IV afferent nerves and the efferent intrinsic nerve pathway,including cholinergic vasodilators. Superficial needling of certain acupuncture points in patients with chronic pain showed similar changes in cerebral blood flow.Gunn’s and Fischer techniques of needling both the paraspinal muscles and peripheral muscles belonging to the same myotome,appear to be supported by several animal studies. For example,Takeshige and Sato determined that both direct needling into the gastrocnemius muscle and into the ipsilateral L5 paraspinal muscles of a guinea pig resulted in significant recovery of the circulation, after ischemia was introduced to the muscle using tetanic muscle stimulation. They also confirmed that needling of acupuncture points and non-acupuncture points involved the descending pain inhibitory system, although the actual afferent pathways were distinctly different. Acupuncture analgesia involved the medial hypothalamic arcuate nucleus of the descending pain inhibitory system, while non-acupuncture analgesia involved the anterior part of the hypothalamic arcuate nucleus. In both kinds of needle stimulation, the posterior hypothalamic arcuate nucleus was involved. There is no research to date that clarifies the role of the descending pain inhibitory system with needling of MTrPs.
Chemical Effects The studies by Shah and colleagues demonstrated that the increased levels of various chemicals, such as bradykinin, CGRP, substance P, and others, at MTrPs are immediately corrected by eliciting a LTR with an acupuncture needle. Although it is not known what happens to these chemicals when a needle is inserted into the MTrP, there is now strong albeit unpublished data that suggest that eliciting a LTR is essential.