Sources of variation in spinal indentation testing: Indentation site relocation, intraabdominal pressure, subject movement, muscular response, and stiffness estimation☆☆☆★

Background: Force-displacement properties of spinal tissues assessed by blunt indentation are thought to have clinical relevance; however, numerous variables with respect to spinal indentation have yet to be identified or characterized completely. Objective: To identify and quantify, where possible, previously unidentified or incompletely characterized variables with respect to spinal indentation. Design: Multiprotocol design. Methods: Four experiments were performed: (1) Twelve asymptomatic subjects were indented with concurrent electromyography during conditions of rest, held inspiration, increased intraabdominal pressure and lumbar extension. (2) Changes in the recumbent position of 12 subjects were measured while a series of movements was performed in restrained and unrestrained conditions. (3) Ten clinicians attempted to locate, and to relocate, a subcutaneous anatomical landmark through visualization/palpation and ultrasonic imaging. (4) Performances of 3 methods of force-displacement curve modeling were compared with respect to stiffness estimation. Results: (1) Spinal stiffness increased significantly in a minority of subjects awaiting indentation and in a majority of subjects during increases in intraabdominal pressure. (2) Changes in subject position were significantly reduced by a restraint system. (3) With respect to interclinician error in locating and relocating an indentation site, there was significant improvement with the use of ultrasonic visualization. (4) The error associated with linear techniques used to model curvilinear force-displacement data plots increased with increasing linear intervals. Conclusion: Several sources of variation in spinal indentation were identified: indentation site relocation, intraabdominal pressure, subject movement, muscular response, and stiffness estimation. These variables, which have been unaccounted for in previous indentation studies, might be responsible for the change or lack of change in force-displacement properties between preintervention and postintervention indentation trials. (J Manipulative Physiol Ther 2001;24:84-91)