Objective-A system for measuring posterior-to-anterior spinal stiffness (PAS) was developed for use in clinical trials of manipulation for low back pain (LBP). The current report is an analysis of the baseline PAS data, with particular emphasis on relationships between PAS and clinical and demographic characteristics.Methods-Posterior-to-anterior spinal stiffness measurements were recorded over the spinous processes of the lumbar spines from patients who had LBP. The system uses electronic sensors to record displacement and force, whereas a human operator provides the force of indentation. Clinical and outcome measures were compared with spinal stiffness.Results-We recruited 192 patients (89 female and 103 male; average age, 40.0 years; SD, 9.4 years). The average Roland-Morris score was 9.7 (SD, 3.2) on a 24-point scale. The Visual Analog Scale pain scores were 55.7 (SD, 20.9) on a 100-mm scale. Stiffness values ranged from 4. 16 to 39.68 N/mm (mean, 10.80 N/mm; SD, 3.72 N/mm). Females' lumbar spines were, on the average, 2 N/mm more compliant than males (P < .001).Conclusions-The PAS system of computer-monitored equipment with human operation performed well in this clinical study of LBP. Spinal stiffness was found to be different between males and females, and age and body mass index were related to PAS. We found no significant relationship between the severity or chronicity of the LBP complaint and spinal stiffness. There was little agreement between the stiff or tender segments identified by the clinicians using palpation and the segment that measured most stiff using the PAS device. Spinal stiffness assessments are frequently performed as part of patient evaluation in chiropractic, osteopathy, and physical therapy. Often referred to as posterior-to-anterior spinal stiffness (PAS), in chiropractic, the method might be called motion palpation or joint end-play assessment. The clinician typically will use the palm of his or her hand to press on the spine and feel for restricted movement.Studies of spinal stiffness or motion palpation in chiropractic and physical therapy most often find limited interexaminer reliability of the assessment.1,2 Because these are commonly used clinical tests, researchers have attempted to develop instruments to help improve the objectivity of the measures. Over the past 20 years, several devices have been reported in the literature.3-11 Such devices typically use a computer-controlled motor to indent the spine or adjacent tissues while monitoring the amount of displacement of the skin surface and the amount of force developed. These devices report very good reliability and accuracy.6Posterior-to-anterior spinal stiffness measurement is a more complex task than it might seem at first. Investigators have identified a host of variables that can influence the measurement. Relative to the patient, variables include patient positioning and the support surface,12,13 respiration and abdominal pressure,14,15 paraspinal muscle activity,16,17 and the thickness of soft tissue between t...