Biomechanics, in general, is the science of internal and external forcesand their impact on biological systems. As all readers know, but may not often think about, spinal manipulation is a mechanical event. Clinicians exert a force of a certain magnitude, in a specific direction, at a specific target site (or area) to affect changes in a joint.
Study title: The biomechanics of spinal manipulation
Authors: Herzog W, Faculty of Kinesiology, University of Calgary, Canada
Publication information: Journal of Bodywork and Movement Therapies 2010; 14: 280-286.
Biomechanics, in general, is the science ofinternal and external forces and their impact on biological systems. As all readers know, but may not often thinkabout, spinal manipulation is a mechanical event. Clinicians exert a force of a certainmagnitude, in a specific direction, at a specific target site (or area) toaffect changes in a joint. This causes adeformation of the spine and surrounding soft tissues. In high velocity,low-amplitude (HVLA) manipulations commonly employed by chiropractors, forcemagnitudes and rate of force development are high. Each patient’s spine differs in terms oftissue stiffness, joint mobility, and so on – presenting significant challengesto those who study the biomechanics of this treatment intervention. Despite the emergence of advanced researchtechniques and ideas, we still know relatively little about the biomechanics ofspinal manipulation. In this paper, Dr.Herzog describes the current state of knowledge on selected topics in thisarea, discussing the forces delivered to patients, the effects of those forces,and the stresses placed upon the vertebral artery (this aspect will not bereviewed in detail here).
ExternalForces Applied by Chiropractors During HVLA Manipulations
- Early works investigated forces applied when manipulating dummies – the obvious shortfall being that human subjects were not used. Subsequent studies utilized human subjects and measured forces using thin pressure pads and other similar devices. When the results of these studies are combined, the following patterns emerged:
- Peak and preload forces vary dramatically depending on the location of treatment application. Peak forces for neck manipulations (~100N) are substantially smaller than peak forces during thoracic, lumbar, or sacroiliac manipulations (all ~400N).
- Treatment forces vary highly between clinicians, and have been measured from 200N-1600N in Herzog’s lab! (1)
- Interestingly, the average forces between males and females are about the same (2), as are forces between novice and experienced practitioners.
- Clinicians who adjust with greater force tend to do so consistently, as do those who use less force (in fact, some “soft” adjusting clinicians do not even reach preload levels of their “harder” adjusting colleagues!) – this raises questions about the importance of force in determining treatment outcome.
- Thrust times typically range from 100ms in the cervical spine to 150ms in other regions. Since peak force varies significantly, it follows that rate of force development varies as well, also calling into question the importance of this variable.
SelectedEffects of HVLA Spinal Manipulation
Most clinicians apply manipulation with the goal ofmoving a particular joint or spinal level, and can appreciate that joint motiondoes occur. However, can one be certainthat the target joint is the one that receives the intended force, and does itmatter? Or, are neighboring joints alsomoving/deforming? Studies have revealedthat target segments and adjacentsegments experience substantial relative movement in the preload phase, aswell as additional movement during the thrust phase (3-5). The relevance of this to treatment outcomehas not been extensively studied.
Although the primary goal of SMT is to restoreproper joint motion, it has been proposed that reflex responses in adjacentmuscles can also produce a treatment effect. EMG activity has been measured in paraspinal musculature during manualSMT, revealing:
- No changes were noted during the preload phase
- Increased activity noted 200-400ms after the thrust (suggesting a reflex mechanism), and then it dissipated
- This effect was not necessarily localized to the area of thrust application
This suggests that the reflex response isassociated with the speed of force application, but does not speak to theoverall clinical relevance of this spike in muscle activity (6). However, other data revealed that in thosepatients with spastic muscle activity, SMT abolished EMG activity (hencerelaxing the muscles) in some, but not all patients – this requires furtherstudy.
Role ofthe Cavitation:
The cavitation, or “pop”, is an indicator of asuccessful manipulation to many clinicians. In fact, if a cavitation is not heard, many clinicians will repeat theprocedure. This is a controversialsubject to say the least and the jury is still “out”, but the literature todate suggests that cavitation is (at least) not associated with thereflex-mediated responses discussed above. Further research is required in this area.
CONCLUSIONS AND PRACTICAL APPLICATIONS
This paper provided a brief snapshot on theliterature dealing with some selected issues surrounding the biomechanics ofSMT, most of which have been discussed here. This review did not include a summary of the section on internal forcesduring SMT (specifically to the vertebral artery) as there are more detailedreviews elsewhere.
As mentioned, the literature in this area is stilldeveloping. As new research emerges,prudent clinicians will adapt their techniques, views and patient educationlanguage/materials appropriately.
This was a narrative literature review by one ofthe leading authorities in this topic.
STUDY STRENGTHS AND WEAKNESSES
Thiswas not an exhaustive review and the reader should keep this in mind.
1) Herzog W et al. Forces exerted during spinalmanipulative therapy. Spine 1993; 18: 1206-1212.
2) Forland D et al. The forces applied by female andmale chiropractors during thoracic spinal manipulation. Journal of Manipulative& Physiological Therapeutics 2004; 27: 49-56.
3) Ga´l J et al. Movements of Vertebrae during manipulativethrusts to unembalmed human cadavers. Journal of Manipulative &Physiological Therapeutics 1997; 20: 30-40.
4) Ga´l J et al. Biomechanical studies of spinal manipulativetherapy (SMT): Quantifying the movements of vertebral bodies during SMT.The Journalof the CCA 1994; 38: 11-24.
5) Ga´l J et al. Measurements of vertebral translations usingbone pins, surface markers and accelerometers. Clinical Biomechanics 1997;12(5): 337-340.
6) Herzog W et al. Reflex responses associated withmanipulative treatments on the thoracic spine. Journal of Manipulative &Physiological Therapeutics 1995; 18: 233-236.
This brief review is a sample of the content posted weekly on Research Review Service and is one of over 60 reviews available per year of the latest scientific literature in chiropractic. RRS is approved for 4.0 CEUs/year by CMCC.
Dr. Shawn Thistle is the founder and president of Research Review Service Inc., an online, subscription-based service designed to help busy practitioners to integrate current, relevant scientific evidence into their practice. Shawn graduated from CMCC and holds an Honours Degree in Kinesiology from McMaster University. He also holds a certificate in Contemporary Medical Acupuncture from McMaster University, and is a Certified Active Release Techniques (ART®) Provider. For more information about the Research Review Service, visit www.researchreviewservice.com .
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