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.