Validation of the force and frequency characteristics of the activator adjusting instrument: Effectiveness as a mechanical impedance measurement tool

Objective: To determine the dynamic force-time and force-frequency characte ristics of the Activator Adjusting Instrument and to validate its effective ness as a mechanical impedance measurement device; in addition, to refine o r optimize the force-frequency characteristics of the Activator Adjusting I nstrument to provide enhanced dynamic structural measurement reliability an d accuracy. Methods: An idealized test structure consisting of a rectangular steel beam with a static stiffness similar to that of the human thoracolumbar spine w as used for validation of a method to determine the dynamic mechanical resp onse of the spine. The Activator Adjusting Instrument equipped with a load cell and accelerometer was used to measure forces and accelerations during mechanical excitation of the steel beam. Driving point and transfer mechani cal impedance and resonant frequency of the beam were determined by use of a frequency spectrum analysis for different force settings, stylus masses, and stylus tips. Results were compared with beam theory and transfer impeda nce measurements obtained by use of a commercial electronic PCB impact hamm er. Results: The Activator Adjusting Instrument imparted a very complex dynamic impact comprising an initial high force (116 to 140 N), short duration pul se (<0.1 ms) followed by several lower force (30 to 100 N), longer duration impulses (1 to 5 ms). The force profile was highly reproducible in terms o f the peak impulse forces delivered to the beam structure (<8% variance). S pectrum analysis of the Activator Adjusting Instrument impulse indicated th at the Activator Adjusting instrument has a variable force spectrum and del ivers its peak energy at a frequency of 20 Hz. Added masses and different d urometer stylus tips had very little influence on the Activator Adjusting I nstrument force spectrum. The resonant frequency of the beam was accurately predicted by both the Activator Adjusting Instrument and electronic PCB im pact hammer, but variations in the magnitude of the driving point impedance at the resonant frequency were high (67%) compared with the transfer imped ance measurements obtained with the electronic PCB impact hammer, which had a more uniform force spectrum and was more repeatable (<10% variation). Th e addition of a preload-control frame to the Activator Adjusting Instrument improved the characteristics of the force frequency spectrum and repeatabi lity of the driving point impedance measurements. Conclusion: These findings indicate that the Activator Adjusting Instrument combined with an integral load cell and accelerometer was able to obtain a n accurate description of a steel beam with readily identifiable geometric and dynamic mechanical properties. These findings support the rationale for using the device to assess the dynamic mechanical behavior of the vertebra l biomechanical effectiveness of various manipulative, surgical, and rehabi litative spinal procedures.