Brian C. Clark, PhD

JAOA Full Text Article:  http://www.jaoa.org/content/112/9/617.full

JAOA Full Text PDF:  http://www.jaoa.org/content/112/9/617.full.pdf+html

Abstract

Each year, more than 18 million adults in the United States receive manual therapies, at a total annual out-of-pocket cost of $3.9 billion. Although there is growing evidence supporting the efficacy of manual therapies, little is known about the mechanisms underlying these treatments. This lack of basic knowledge significantly limits the development of rational strategies for the use of these treatments and potentially hinders their acceptance by the wider scientific and health care communities. Many authors have hypothesized that manual therapies act by disrupting the pain-spasm-pain cycle, but relatively little experimental evidence has supported this hypothesis. The authors have tested this hypothesis and summarize their work on the biology of manual therapies.

Many scientists and clinicians have postulated that manual therapies exert biologic effects on the nervous system.1-16 It has been hypothesized that manual therapies act mechanistically to disrupt the pain-spasm-pain cycle.9,17 In brief, the conceptual basis for this cycle is that pain leads to muscular hyperactivity (spasm), which in turn causes or exacerbates pain (see Pain-Spasm-Pain Cycle).18 Although 2 distinct neural pathways have been proposed to support this model,19 each pathway operates on the basis of the same concept, namely, that increased excitatory input to the α-motoneuron pool leads to more sustained and intense muscle activity. More than 3 decades ago, Korr7 hypothesized that manual therapies act to disrupt the pain-spasm-pain cycle by reducing the excitability of the monosynaptic stretch reflex (also known as myotatic reflex, deep tendon reflex, or muscle spindle reflex). However, relatively few studies have quantified the effects of manual therapies on muscle activity20-24 or stretch reflex excitability in humans,4,25-28 and thus, there are still limited empirical data describing the effects and consequences of manual therapies.

Our work has focused on the mechanistic effects of manual therapies. The studies profiled here tested the hypothesis that manual therapies act to disrupt the pain-spasm-pain cycle. In this article, we review our latest work on the biology of manual therapies. We will briefly review the anatomy and physiology of muscle spindles, then discuss the pain-spasm-pain cycle, and finally present the findings from these studies, providing our perspectives on key questions to be addressed in future research.