Reliability of measurements of lumbar spine sagittal mobility obtained with the flexible curve

Presently, there is no available scientific information that examines the interchangeability of tangent and trigonometric methods used to calculate measurements of sagittal mobility of the lumbar spine obtained with a flexible curve. Repeated measurements of the lumbar curvature were made with a flexible curve by using a standardized protocol on 10 healthy volunteers under three conditions: 1) standing, 2) sitting with maximum trunk forward bending, and 3) lying prone with maximum backward bending. Measurements were made by a team of two physical therapists working together; one therapist instructed the subject, and the other therapist performed the measurement. Agreement between the tangent and trigonometric methods was assessed graphically by plotting the difference between methods against the mean value of each pair of readings for each of the three conditions. Measurements differed by 4 to 7° for each of the three positions of the lumbar spine. We believe such error is clinically acceptable and should not affect the clinical decision made on the basis of the measurement.