Trunk flexion–extension is accomplished with pelvic and thoracic rotation in the sagittal plane and a smooth coordination of lumbar vertebral translation and rotation. Several systems exist that measure gross trunk movements, but they typically do not track changes in lumbar arc length or curvature. A method for dynamic measurement of changes in lumbar arc length that uses a lordosimeter is presented. A static validation study of postures showed that the measurements are repeatable and compare favorably with a standard measure of lumbar curvature. In a dynamic validation study of a lifting task with a 23-kg load, the measurement method was highly correlated with thoracic and pelvic angular displacement. A discussion of the utility of this measure in biomechanical modeling is presented. A clinical or field version of the lordosimeter is also described.