OBJECTIVE: To evaluate the relationship of muscle strength response to a provocative vertebral challenge and to spinal manipulation.

DESIGN: Prospective double-blind randomized controlled trial: crossover and between subjects designs.

SETTING: Laboratory: Center for Technique Research.

PARTICIPANTS: Sixty-eight naive volunteers from the student body, staff and faculty of the college.

INTERVENTIONS: Provocative vertebral challenge: standardized 4-5 kg force applied with a pressure algometer to the lateral aspects of the T3-12 spinous processes.

INTERVENTION: manual high velocity low amplitude adjustment or switched-off activator sham.

MAIN OUTCOME MEASURES: Piriformis muscle response was defined in two ways: reactivity (a decrease in muscle resistance, yes or nor, following a vertebral challenge); responsiveness (the cessation of reactivity following spinal manipulation). Relative response attributable to the maneuver (RRAM): the percent of an outcome attributable to the challenge or adjustment itself.

RESULTS: Average RRAM = 16% reactivity to vertebral challenge; average RRAM = 0% responsiveness to spinal manipulation. Six to 10% of muscle tests were positive regardless of examiner, previous finding or intervention.

CONCLUSIONS: For the population under investigation, muscle response appeared to be a random phenomenon unrelated to manipulable subluxation. In and of itself, muscle testing appears to be of questionable use for spinal screening and post-adjustive evaluation. Further research is indicated in more symptomatic populations, different regions of the spine, and using different indicator muscles.

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