A simple, inexpensive method has been developed for measuring the position of the back, projected onto the sagittal plane, during flexion. Light-emitting diodes (LEDs) were attached to the skin overlying the T6, L1, L3 and L5 vertebrae. They started to flash, at a controlled frequency, at the beginning of movement from an upright to a flexed position. The positions of the markers together with the initial (upright) and final (flexed) positions of the back were recorded on a single photograph. A plumb line provided a vertical reference. Times were determined from the flashing frequency. Vertical and horizontal components of the position were measured from the photograph; their resultant represented total displacement at a given instant in time. The results were scaled so that the displacements were expressed as a fraction of their maximum value, and time was expressed as a fraction of the total movement time. The reproducibility of the technique was tested by making repeated measurements from a single healthy volunteer. At any instant, the horizontal, vertical and resultant displacements of the T6 marker could be determined to within 7% of their maximum values; the resultant displacement of the L5 marker could be determined to within 10%. The results from the other markers were less reliable.