J Manipulative Physiol Ther. 2007 (Oct); 30 (8): 578–583 ~ FULL TEXT
Mariana Ruiz-Sáez, PT, DO, César Fernández-de-las-Peñas, PT, PhD,
Cleofás Rodríguez Blanco, PT, DO, Raquel Martínez-Segura, PT, DO,
Rafael García-León, PT, DO
Escuela de Osteopatía de Madrid (EOM), Spain.
OBJECTIVE: This study analyzed the immediate effects on pressure pain threshold (PPT) in latent myofascial trigger points (MTrPs) in the upper trapezius muscle of a single cervical spine manipulation directed at the C3 through C4 level.
METHODS: Seventy-two volunteers (27 men and 46 women; mean age, 31 years; SD, 10 years) participated in this study. Subjects underwent a screening process to establish both the presence of MTrPs in the upper trapezius muscle as described by Simons et al (Myofascial pain and dysfunction: the trigger point manual, vol 2. 3rd ed. Baltimore: Williams & Wilkins, 1999. p. 23-34) and the presence of intervertebral joint dysfunction at the C3 through C4 level by the lateral gliding test for the cervical spine. Subjects were divided randomly into 2 groups: manipulative group, which received a cervical spine manipulation directed at the C3 through C4 level, and a placebo group, which received a sham manual procedure. The outcome measure was the PPT on the MTrP in the upper trapezius muscle ipsilateral to the side of the joint dysfunction, which was assessed pretreatment and 1, 5, and 10 minutes posttreatment by an assessor blinded to the treatment allocation of the subject.
RESULTS: The analysis of variance showed a significant effect for time (F = 5.157; P = .02) but not for side (F = 0.234; P = .63). Furthermore, an interaction between group and time was also found (F = 37.240; P < .001). The experimental group showed a trend toward an increase in PPT levels after the manipulative procedure, whereas the control group showed a trend toward a decrease in PPT. Positive within-group effect sizes ranging from medium to small were found in the manipulative group (0.1 <d < 0.5), whereas negative within-group effect sizes ranging from large to medium were found in the placebo group (0.3 <d < 1).
CONCLUSIONS: Our results suggest that a cervical spine manipulation directed at the C3 through C4 segment induced changes in pressure pain sensitivity in latent MTrPs in the upper trapezius muscle. Different therapeutic mechanisms, either segmental or central, may be involved at the same time.
Key Indexing Terms: Pressure Pain Threshold, Trigger Points, Joint Dysfunction
From the Full-Text Article:
Myofascial trigger points (MTrPs) are claimed to be a common source of musculoskeletal pain in people presenting to manual therapists for treatment. Simons et al  defined MTrPs as hypersensitive tender spots associated with a taut band of a skeletal muscle that is painful on compression and on stretch and gives rise to a typical referred pain pattern. From a clinical viewpoint, active MTrPs cause pain and their local and referred pain is responsible for patient complaints. Latent MTrPs have similar clinical features (tender spot within a taut band, local twitch response) as active MTrPs, but they are not responsible for pain symptoms; however, they can elicit referred pain, not recognized by patients as their usual pain, with muscle contraction or mechanical stimulation of the tissues.  An active MTrP is distinguished from a latent one when the referred pain elicited by exploration of the MTrP is recognized as familiar. [1, 2] This clinical distinction has been substantiated by histochemical findings because higher levels of chemical mediators (ie, bradykinin, substance P, or serotonin) have been found in active MTrPs as compared with latent MTrPs or control points. 
Although the etiology of MTrPs is not completely known, recent studies have hypothesized that the pathogenesis of MTrPs results from injured or overloaded muscle fiber.  This could lead to endogenous and involuntary shortening, loss of oxygen supply, loss of nutrient supply, and increased metabolic demand on local tissues.  The most credible etiologic suggestion for trigger point pathogenesis is the integrated hypothesis, which proposes that abnormal depolarization of motor end plates and sustained muscle contraction give rise to a localized “ATP energy crisis” associated with sensory and autonomic reflex arcs sustained by central sensitization. [5, 6] The integrated hypothesis is based on the 2 most widely accepted theories (energy crisis theory and motor end plate) and, when combined, provide a plausible explanation for MTrPs.n  There is a third theory, yet to be experimentally verified, which suggests the primary pathologic site of MTrPs to be the spinal nerve, with secondary muscle changes occurring after.  Nevertheless, the more widely accepted theory is centered on the muscle cell and motor end plate as the primary pathologic sites.  However, these events have not been completely proven, and more research is needed.
A clinical relationship between MTrPs and joint impairments has been suggested by some authors. [1, 10] Fernández-de-las-Peñas et al  found a relationship between the presence of MTrPs in the upper fibers of the trapezius muscle and the presence of cervical impairment (hypomobility considered when an abnormal end feel, an increased resistance, and a decreased joint gliding were found) at the C3 through C4 vertebrae. Several theories on muscle-joint interrelationship have been proposed. Perhaps the increased tension of the taut bands and facilitation of motor activity can maintain displacement stress on the joint such that a trigger point would provoke the joint dysfunction.  On the other hand, an abnormal sensory input from the joint dysfunction may reflexively activate the trigger point. 
It may be hypothesized that the treatment of an intervertebral joint dysfunction could provoke a therapeutic effect in MTrPs located in those muscles innervated by the same segment. In clinical practice, therapists use a therapy approach that includes different techniques directed at both muscle and joint impairments. No previous controlled studies have investigated changes on MTrP pain sensitivity after the application of a cervical spine manipulation. The aim of the present study was to analyze the immediate effects on pressure pain sensitivity in latent MTrPs in the upper trapezius muscle of a single cervical spine manipulation directed at the C3 through C4 level.
Our study is the first to analyze changes on PPT levels in latent MTrPs in the upper trapezius muscle after a cervical thrust manipulation.  We found that the application a single cervical spine manipulation directed at the C3 through C4 segment evoked changes in pressure pain sensitivity in latent MTrPs in the upper trapezius muscle. Subjects who received the manipulative intervention showed a trend toward an increase in PPT vs those subjects receiving a sham manual procedure who showed a trend toward a decrease in PPT levels (Fig 3). One possible explanation for the decreased PPT levels found in our control group could be a peripheral sensitization of muscle nociceptors stimulated by the repeated measures over the same point.
Our results are similar to those reported by Vernon et al  who demonstrated that cervical manipulation produces significantly greater increases in PPT of tender points (which do not elicit referred pain) surrounding a cervical dysfunction. The neurophysiologic mechanisms by which spinal manipulative therapy may evoke changes in pressure pain sensitivity in MTrPs remain to be fully elucidated. Both peripheral, for example, reduction of chemical algogenic mediators  or activation of segmental inhibitory pathways,  and central descending inhibitory pathway activation  mechanisms have been suggested. For example, one possible segmental mechanism could be that the manipulation may induce a reflex muscle relaxation by modifying proprioceptive group 1 and 2 afferents.  Because the upper trapezius muscle receives nerve innervation from the C2 through C4 level, it may be possible that the stimulus induced by the manipulative procedure of this segment could have some effect on upper trapezius MTrP sensitivity. Furthermore, it seems that the spinal manual procedures can activate the periaqueductal gray substance resulting in central hypoalgesic effects. [28, 29] Probably, more than one mechanism explains the effects of spinal manipulation on mechanical pain sensitivity. 
It has been suggested that cervical joint dysfunctions could be a perpetuating factor for MTrPs located in those muscles innervated by this spinal segment.  It has been found that intervertebral joint dysfunction could increase the responsiveness of motor neurons of adjacent MTrPs.  It seems that there is a remarkable neurophysiologic analogy between the nervous system effects produced by intervertebral joint dysfunctions and MTrPs.  Our findings are clinically relevant because they suggest that it would be reasonable to suspect at least a self-perpetuating relationship between MTrPs and cervical joint dysfunctions  because a manipulative procedure evoked changes on pressure pain sensitivity in MTrPs that receive innervation from the manipulated segment. However, the ideal order in which these musculoskeletal disorders should be best treated is not known, and it will require randomized controlled trials assessing the effectiveness of the different manual therapy procedures.
Our study has several limitations. First, this study only examined the immediate effect of the cervical spine manipulation. A trend toward an increase in PPT occurred after spinal manipulation provides impetus for future research in this area. Further studies are needed to examine long-term effects of cervical manipulation on MTrPs.
Second, the participants in this study were pain-free and may not be typical of the population presenting to manual therapists for treatment. It is probable that the clinical effect of cervical manipulation would be greater in active MTrPs, for example, neck pain  or tension-type headache. 
Finally, joint cavitation bias can also have occurred because subjects in the experimental group were not “blinded” to the existence of joint cavitation. It is possible that joint cavitation can create an expectative or a placebo effect on patients. However, this situation is so difficult, if not impossible, to control in a spinal manipulation study.
Finally, Ylinen et al  have recently suggested that although the repeatability of PPT assessment allows algometry to be used for research purposes, because considerable variations exist on an individual level, caution is advised when interpreting the results in clinical practice. 
The application of a cervical spine manipulation directed at the C3 through C4 segment induces changes in pressure pain sensitivity in latent MTrPs in the upper trapezius muscle. Subjects who received the manipulative intervention showed a trend toward an increased in PPT vs those subjects receiving a sham manual procedure who showed a trend toward a decrease in PPT. Different therapeutic mechanisms, either segmental or central, may be involved at the same time.
The application of a cervical spine manipulation directed at the C3 through C4
segment evokes changes in pressure pain sensitivity in latent MTrPs in
the upper trapezius muscle in the pain-free subjects in this study.
Different therapeutic mechanisms, either segmental or central, may be involved
at the same time.
Cervical joint dysfunctions could be a perpetuating factor for MTrPs located
in those muscles innervated by this spinal segment.