This study presents the theory of differential compliance and the clinical application of instrumentation designed for its detection in the clinical setting. The engineering principle of using a mechanical impulse to excite a structure coupled to sensors to measure the response, has been employed in the design of the Function Recording and Analysis System (FRAS). This system, linked to software which analyzes input data, is further designed to graphically display relative differences in vertebral compliance. The present study employed assessment of a region of the spine encompassing the occiput to the third thoracic vertebra as a model to demonstrate how compliance measurements can be obtained, graphically displayed, and interpreted. The reliability of the instrumentation to accurately and consistently measure different levels of compliance was tested on three artificial substrates of varying “stiffness” and three human tissue locations exhibiting different “stiffness” including the heel, palm, and finger tip. Moreover, intraand inter-examiner reliability in the use of the system has also been reported. Statistical evaluation of repeated measures for each of these categories included analysis by chi-square, Pearson product moment correlation, and intraclass correlation coefficients (ICC). Results indicated a high level of consistency of the instrument in measuring substrates of varying compliance. Moreover, strong intra-examiner reliability (ICC ≥ 0.90), and inter-examiner reliability (ICC = 0.65) suggest a consistent use of the instrument among and between practitioners. These findings indicate a useful role for the FRAS in the characterization of vertebral subluxation, as well as serving as one means of assessing changes in flexibility of the spine following the chiropractic adjustment.