The Influence of Expectation on Spinal Manipulation Induced Hypoalgesia: An Experimental Study in Normal Subjects
Joel E Bialosky, Mark D Bishop,
Michael E Robinson, Josh A Barabas,
and Steven Z George
University of Florida Department of Physical Therapy,
Gainesville, Florida, USA.
BACKGROUND: The mechanisms thorough which spinal manipulative therapy (SMT) exerts clinical effects are not established. A prior study has suggested a dorsal horn modulated effect; however, the role of subject expectation was not considered. The purpose of the current study was to determine the effect of subject expectation on hypoalgesia associated with SMT.
METHODS: Sixty healthy subjects agreed to participate and underwent quantitative sensory testing (QST) to their leg and low back. Next, participants were randomly assigned to receive a positive, negative, or neutral expectation instructional set regarding the effects of a specific SMT technique on pain perception. Following the instructional set, all subjects received SMT and underwent repeat QST.
RESULTS: No interaction (p = 0.38) between group assignment and pain response was present in the lower extremity following SMT; however, a main effect (p < 0.01) for hypoalgesia was present. A significant interaction was present between change in pain perception and group assignment in the low back (p = 0.01) with participants receiving a negative expectation instructional set demonstrating significant hyperalgesia (p < 0.01).
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CONCLUSION: The current study replicates prior findings of c- fiber mediated hypoalgesia in the lower extremity following SMT and this occurred regardless of expectation. A significant increase in pain perception occurred following SMT in the low back of participants receiving negative expectation suggesting a potential influence of expectation on SMT induced hypoalgesia in the body area to which the expectation is directed.
From the Full-Text Article:
A growing body of evidence supports spinal manipulative therapy (SMT) as an effective treatment for low back pain. [1–6] Furthermore, the evidence is particularly strong when patients are classified into subgroups by patterns suggesting the likelihood of a favorable response. [2, 3, 6] Despite the positive findings of clinical trials, the mechanisms through which SMT acts are not established.
Hypoalgesia has been associated with SMT and has a postulated involvement in the clinical effectiveness. [7–16] For example, Vicenzino et al  observed greater pain free grip and pain pressure threshold in the forearm following SMT to the cervical spine. A prior study by our group found hypoalgesia of c- fiber mediated pain as measured by lessening of temporal summation in the lower extremity following SMT to the lumbar spine.  Temporal summation results from multiple painful stimuli of the same intensity applied at a frequency of less than 3 seconds and has been observed in both healthy subjects [17–19] and those experiencing chronic pain. [20, 21] Activation of the dorsal horn of the spinal cord has been directly observed with temporal summation in animal studies. [22–25] Subsequently, we interpreted our prior findings of hypoalgesia of temporal summation following SMT in healthy participants as indicative of a pain inhibiting effect occurring at the dorsal horn.