OBJECTIVE: The purpose of this study was to determine the amplitudes and variations of preload positioning and displacements of the head and thorax during spinal manipulative therapy (SMT) applied to C2.

DESIGN: This experimental study measured the biomechanics of SMT applied to C2.

SETTING: Biomechanics Laboratory, Mechanical Engineering and Applied Mechanics, University of Michigan and Spinal Ergonomics and Joint Research Laboratory, National College of Chiropractic.

PARTICIPANTS: Eighteen healthy volunteers from the University of Michigan and the National College of Chiropractic were manipulated by a total of eight experienced chiropractic physicians licensed in the state in which they participated.

INTERVENTION: Sixty-six Direct Break (DB) and twenty-three Rotary Break (RB) diversified procedures were administered to C2. Randomization of the DB was made on two variables, direction and intended load intensities. The RB was administered only from the right by each physician.

MAIN OUTCOME MEASURES: Linear and angular displacement time histories for the head and thorax mass centers were monitored during the preload setup of each procedure and the dynamic delivery of the treatment procedures.

CONCLUSIONS: DB procedures were found to have sagittally symmetric positioning and displacements as a function of intended direction of the procedure. Both DB and RB methods consisted of preload positioning with head flexion. RB rotation and lateral bending preload positions approached the maximal voluntary ranges of motion for the upper cervical spine. Variations among procedures by one manipulator were approximately the same as for variations among manipulators. Data indicates that SMT procedures can be successfully modified to control amplitude and direction of body segment displacements that arise.

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