J Manipulative Physiol Ther. 2013 (Jul); 36 (6): 333–341 ~ FULL TEXT
John Z. Srbely, DC PhD, Howard Vernon, DC, PhD,
David Lee, DC, Miranda Polgar, BSc
Department of Human Health and Nutritional Science,
University of Guelph, Guelph,
OBJECTIVE: The purpose of this study was to investigate if spinal manipulative therapy (SMT) can evoke immediate regional antinociceptive effects in myofascial tissues by increasing pressure pain thresholds (PPTs) over myofascial trigger points in healthy young adults.
METHODS: A total of 36 participants (19 men, 17 women; age, 28.0 [5.3] years; body mass index, 26.5 [5.7] kg/m(2)) with clinically identifiable myofascial trigger points in the infraspinatus and gluteus medius muscles were recruited from the University of Guelph, Ontario, Canada. Participants were randomly allocated to 2 groups. Participants in the test group received chiropractic SMT targeted to the C5-C6 spinal segment. Participants in the control group received sham SMT. The PPT was recorded from the right infraspinatus and gluteus medius muscles at baseline (preintervention) and 1, 5, 10, and 15 minutes postintervention.
RESULTS: Three participants were disqualified, resulting in a total of 33 participants analyzed. Significant increases in the PPT (decreased pain sensitivity) were observed in the test infraspinatus group when compared with test gluteus medius, control infraspinatus, and control gluteus medius groups (P < .05). No significant differences in PPT were observed at any time point when comparing test gluteus medius, control infraspinatus, and control gluteus medius groups (P > .05).
CONCLUSIONS: This study showed that SMT evokes short-term regional increases in PPT within myofascial tissues in healthy young adults.
Key Indexing Terms: Manipulation, Myofascial Pain Syndrome, Myofascial Trigger Point, Pain Threshold, Chiropractic
From the Full-Text Article:
Chronic myofascial pain syndrome (MPS) is one of the most commonly encountered pain conditions and represents a significant burden to our health care system. Population-based studies report that chronic regional pain, such as MPS, impacts up to 25% of the population at any given time.  The prevalence of MPS among the middle-aged population (30–60 years) is reported at 37% and 65% in men and women, respectively,  and increases to 85% in the elderly (>65 years).  Given that the elderly population is expected to double by the year 2040,  chronic MPS is poised to become one of the most significant health challenges in the future. Consequently, advancement of conservative and cost-effective therapeutic approaches is imperative to easing its impending burden on our health delivery system.
The application of spinal manipulative therapy (SMT) is a cost-effective and widely recognized manual intervention used by a variety of health care professionals in the management of musculoskeletal pain. [5, 6] A growing body of empirical evidence supports the use of SMT for the treatment of a broad scope of musculoskeletal disorders, [7, 8] citing short-term antinociceptive (pain-relieving) effects  and restoration of normal joint mechanics. [10, 11]
Although the practice of manipulative therapy dates back to 2700 bce,  its underlying physiological mechanisms of action are still poorly understood. One of the unresolved questions is whether SMT is predominantly governed by regional (segmental, heterosegmental) or general (nonsegmental) physiological mechanisms, or a complex interaction of both. The limited data addressing this question are equivocal, supporting either one or both of these mechanisms. [13, 14]
One objective of SMT research is to explore the neurophysiologic mechanisms and potential therapeutic applications of SMT in the treatment and management of MPS. Although the pathophysiology of MPS is still unclear, research suggests that myofascial trigger points (MTPs) play an integral role in the pathophysiology and clinical manifestation of MPS. [15, 16] It is currently unknown if the antinociceptive effects of SMT in myofascial tissues are manifest predominantly via regional or general mechanisms, or a combination of both. A study is needed to specifically investigate the hypothesis that SMT evokes robust antinociceptive effects in MTPs preferentially located within neurosegmentally linked myofascial tissues. To our knowledge, no previous study has explicitly examined the direct impact of regional antinociceptive mechanisms post-SMT in myofascial tissues using a single-session, validated, sham-controlled design. Therefore, the purpose of this study was to investigate if SMT can evoke immediate regional antinociceptive effects in myofascial tissues by increasing pressure pain thresholds (PPT) over MTPs in healthy young adults.
Our data suggest that SMT evokes statistically significant short-term increases in PPT in segmentally related myofascial tissues in young adults. Decreased pressure sensitivity (increased PPT score) was observed at all time intervals beyond baseline within neurologically linked infraspinatus muscle after real, but not sham, manipulation; no difference in pressure sensitivity from baseline was observed at either muscle in controls or from the gluteus medius muscle in test participants. Furthermore, no difference in pressure sensitivity (PPTdiff) was observed at the gluteus medius muscle between test and control conditions, suggesting that SMT did not evoke significant generalized antinociceptive responses. The peak antinociceptive effect was measured as a 36% decrease in pressure sensitivity from baseline values and was recorded at 5 minutes post-SMT. Significant antinociceptive effects in the test group were maintained throughout the 15–minute record period, whereas no significant antinociceptive effects were observed at any time point in controls. The findings of this preliminary single-session, manual sham-controlled study illustrate the robust regional mechanisms of SMT in myofascial tissues and may offer a basis for further research into potential applications of SMT in the conservative treatment and management of MPS.
Historically, SMT has been regarded and used as a technique to primarily restore joint mechanics. However, studies investigating changes in joint mobility post-SMT have challenged this paradigm.  Recent research suggests that the primary physiological effect of SMT may be neurophysiological; yet, the mechanisms are still poorly understood. In particular, it is still unclear whether the predominant mechanism of action of SMT in myofascial tissues is regional (segmental, heterosegmental), general (nonsegmental, supraspinal), or a combination of both. This is an important, yet unresolved, question that forms a foundational principle in understanding potential therapeutic applications for SMT in the management of MPS.
Our data support the existing literature showing that SMT elicits regional inhibitory mechanisms in myofascial tissues. Previous work has demonstrated that areas of secondary hyperalgesia surrounding painful lumbar spinal segments reduce in size to a greater extent after manipulation when compared with other interventions.  In addition, Terrett and Vernon  report significant increases (2.4–fold) in electrical cutaneous pain tolerance post-SMT in skin regions immediately surrounding painful spinal segments, whereas 2 other studies demonstrate reduced PPT in MTP in tissues surrounding painful spinal lesions by up to 45%, when compared with other interventions. [14, 32] Furthermore, regional decreases in thermal pain sensitivity,  increased electromyographic amplitude/fatigue resistance,  and bilateral increase in PPT in the wrist extensors of patients with lateral epicondylitis  have all been reported; however, none of these studies addressed the relative contributions of segmental vs nonsegmental mechanisms to their findings. Furthermore, there have been a considerable number of single-session studies investigating the effect of SMT; however, none of these have used a validated manual sham procedure.  Vernon and Humphreys  also note that “there is moderate-to-high quality evidence that immediate clinically important improvements (in pain scores) are obtained from a single session of (cervical) spinal manipulation.” However, most of these studies are so poorly controlled that the differential effect of the index therapy was subject to the confounding effects of unblinding of controls.
Despite the strong regional antinociceptive responses observed in our study, existing research suggests that SMT can potentially evoke both regional and supraspinal antinociceptive effects. [13, 14] Studies report increases of up to 25% in PPTs at remote anatomical sites post-SMT.  In addition, SMT applied to the upper cervical spine segments (atlantooccipital joint C0–C1) has been shown to reduce the PPT over the sphenoid bone in women with mechanical neck pain.  To distinguish between the relative impact of regional (segmental, heterosegmental) and supraspinal mechanisms, we compared the PPTdif) between the infraspinatus and gluteus medius muscles over time under test vs control conditions. Descending pain modulation is not somatotopically organized, but a generalized response mediated by serotonergic fibers within the dorsolateral funiculus.  The dorsolateral funiculus descends from the nucleus raphe magnus in the midbrain and terminates at all levels of the spinal cord. [40, 41] Numerous studies have shown that descending inhibitory mechanisms can block afferent pain transmission and nociresponsive mechanisms at all levels of the spinal cord and trigeminal pathway. [42, 43] Based on this rationale, we assumed that SMT-evoked supraspinal effects would impact pain sensitivity at both trigger point sites comparably; therefore, observed differences in the PPT profile between the infraspinatus and gluteus medius trigger points would be attributed predominantly to direct spinal segmental mechanisms acting at the infraspinatus trigger point site. A study investigating regional thermal pain sensitivity and using a similar design/rationale has been previously published. 
Our study contributes to the existing body of literature, as it is the first single-session, validated manual sham-controlled study directly investigating the impact of segmental mechanisms of SMT on pain sensitivity in MTPs. Although one other study similarly compares PPT changes in segmentally related tissues using a controlled (not sham-controlled) design,  it did not distinguish between the relative contributions from segmental vs central mechanisms.
We quantified the PPT using pressure algometry. Pressure algometry is a simple and reliable technique of quantifying trigger point sensitivity.  Previous research has shown a high inter- and intraexaminer reliability  in the PPT measure as well as a strong correlation to pain perception.  The PPT measure is consistent in normal individuals,  and measured changes in the PPT reflect changes in pain processing that could reflect changes in segmental and/or nonsegmental mechanisms. To compare segmental vs nonsegmental mechanisms, our study design used a regional control point within the gluteus medius muscle. This site was chosen because of its segmental distance from the manipulated C5–C6 segment, precluding it from direct segmental and/or heterosegmental effects. Based on this rationale, we assumed that the measureable differences in the PPT response between the 2 trigger point sites would be predominantly attributable to direct local spinal segmental and/or heterosegmental mechanisms acting on the infraspinatus trigger point. Further research along this line is needed to enhance our understanding of these mechanisms and how they may contribute to the conservative management of MPS.
Limitations and Future Studies
There are several limitations to consider when interpreting the results of this study. The primary limitation is the potential for participant group bias. Although we used a validated sham SMT procedure,  the reliability of the sham could potentially be affected by previous experience with SMT. We did not exclude participants with previous SMT experience, and participants taking part in this study may have been able to identify their assigned intervention. The potential impact of such bias on our outcome, however, is significantly mitigated by our study design. Participants were informed before the study that the aim of this study was to investigate the effect of a spinal manipulative intervention on muscle pain; they were not aware of the specific manual technique(s) used, nor were they preconditioned to expect either outcome (regional, general). Furthermore, our specific outcome measure was the difference in PPT (PPT diff) scores between the infraspinatus and gluteus medius trigger points at each time interval. The interpretation of our results relies on the assumption that the effects of participant bias would manifest as a generalized response and, therefore, be comparable at both trigger point sites. Consequently, participant bias would not have a meaningful impact on the primary outcome of this study.
Age and sex are important considerations in the interpretation of pain research. Given that age has a significant impact on PPT,  we chose to investigate these mechanisms in a young population; accordingly, the findings of this study cannot be extrapolated to clinical populations. Moreover, previous research investigating the effects of sex on pain is equivocal. Most studies in this field do not report a sex bias,  and those that do demonstrate small and inconsistent differences. 
Previous research suggests that the specificity of SMT can vary between practitioners, potentially impacting additional spinal segment(s) above and below the target segment.  Although, the treating clinician in our study was experienced in delivering the intervention, manipulation of additional segment(s) above and/or below the target C5–C6 segment would not significantly impact our specific objective of investigating regional effect(s) post-SMT in myofascial tissues given that regional antinociceptive effects evoked via heterosegmental effects are governed by similar neurophysiologic mechanisms. In addition, we only studied the short-term effects (15 minutes) of SMT. Although we observed significant regional antinociceptive effects throughout the 15–minute record period, these findings cannot be extrapolated long term. A final limitation is the possibility for modulating trigger point sensitivity via repetitive pressure testing; however, previous research has shown that repeated pressure algometry over a period of 1 hour does not significantly impact the PPT. 
Cost-effective manual therapies, such as SMT, are widely used in the management of a broad spectrum of musculoskeletal conditions, although their mechanisms and clinical applications are still poorly understood. An important outstanding question remains whether the physiologic mechanism governing SMT is predominantly regional (segmental, heterosegmental), generalized, or a combination of both. These data provide insight into the physiological effects of SMT and support the need for further research into potential clinical applications of SMT in the conservative treatment and management of MPS. Further research is needed in clinical populations to better understand the clinical relevance and application of these findings.
This study demonstrated that SMT evoked robust short-term segmental antinociceptive effects in myofascial tissues in a young population. The findings of this study contribute toward a better understanding of the basic physiological mechanisms of SMT and provide a foundation for further research investigating the potential therapeutic applications of SMT in the treatment and management of MPS.