PURPOSE:Determine the reliability of and relationship between 3-point bending and displacement methods of lumbar stiffness measurement. RELEVANCE:Three-point bending and displacement methods of measurement of spinal stiffness are present in the literature without discussion of their relative merits. The 3-point bending method measures the change in angle of sensors placed on L1 and the sacrum, whereas the displacement method measures vertical displacement at the point of force application. This is the first study to compare measurements collected simultaneously using both methods. METHODS:Twenty-nine subjects (15 asymptomatic and 14 with low back pain) participated. Stiffness was measured simultaneously via 3-point bending (force/angle) and displacement (force/displacement) methods. To compare reliability of the methods, measurements were taken 3 times on 2 occasions, in asymptomatic participants. For symptomatic participants, responsiveness to treatment was assessed by correlating percentage change after both 3 and 6 minutes of treatment. RESULTS:Within-day ICCs for stiffness were 0.56 to 0.77 (SEM, 0.46%- 1.46%), between-day ICCs for stiffness were 0.47 to 0.61 (SEM, 0.72%- 1.36%). Correlation analysis on asymptomatic participants demonstrated that there was a significant (P = .01) but moderate (r = 0.64; 95% CI: 0.20, 0.87) correlation between the 2 methods. On symptomatic participants, the correlation using baseline data revealed dissociation between the 2 methods (P = .167, r = 0.376). There was dissociation between percentage changes in the 2 stiffness measurement methods after 3 minutes of treatment (P = .751, r = 0.090); however, there was a significant correlation between the percentage changes after 6 minutes of treatment (P = .004, r = 0.693). CONCLUSIONS:Reliability statistics suggest that stiffness measurements are not sufficiently reliable to monitor the changes observed with treatment. The varying relationship between the 2 methods suggests they are not measuring the same behavior, highlighting the need for further investigation into their validity. IMPLICATIONS:The usefulness of stiffness measurement in assessing response to mobilization treatment warrants further investigation.