OBJECTIVE: To determine the general nature of the biomechanical response of the vertebrae to small forces, such as spinal manipulative therapy (SMT).

DESIGN: Perturbation theoretical methods of physics and mechanical energy considerations are used to derive the equations of motion of the vertebral bodies moving under the combined influences of ligamentous and discogenic forces, applied forces and dissipative forces attributable to surrounding tissues.

RESULTS: The allowable solutions to the equations of motion determine that the mechanical response of any vertebra to SMT should consist of a superposition of damped oscillations. This is based on the most general assumptions about the spine that are consistent with clinical observations, namely, that patients can lie stably motionless, and is independent of the specifics of any spinal model.

DISCUSSION: The extant data are shown to be consistent with this theory. The implications for future research and clinical practice are explored.

CONCLUSIONS: Vertebral motion in response to SMT seems to occur in two distinct phases: an initial, (passive) oscillatory response to the SMT thrust, governed by ligamentous and discogenic forces, and a later, less regular motion, probably caused by muscular reflex contractions. Evidence of this includes direct measurement of oscillations, surface electromyogram measurements of muscle responses and detection of multiple spinal resonances. Further research on the muscular reflex responses to SMT is necessary. Most SMT should initiate some of the normal-mode oscillations of the vertebrae. There may be up to 144 different frequencies of vertebral oscillatory motion in each individual in any posture; those frequencies detected thus far are consistent with the predicted relationship between frequencies, vertebral body masses and coefficients of stiffness. Further data are needed to confirm the detailed validity of this theory.

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