A comparison of two motion analysis devices used in the measurement of lumbar spinal mobility

Objective. The aim of the study was to compare lumbar range of motion determined using two computerised dynamic motion analysis devices. Background. Measures of spinal motion are currently used in biomechanical, epidemiological and clinical studies of the low back. It is essential that the various devices used to measure mobility yield similar results, particularly when the absolute values are to be used to assess job suitability, the extent of injury or the need for rehabilitation. Methods. Eleven volunteers took part. The ranges of lumbar flexion, extension, lateral bending and axial rotation were measured using the CA6000 Spine Motion Analyser and the Polhemus Fastrak system, using standardised protocols. Results. Each device showed good test-retest reliability in itself (R > 0.82). The absolute values for range of flexion in a standing posture were significantly higher with the CA6000 than with the Fastrak (though well correlated); those recorded in sitting were comparable for the two devices. Values for lateral bending using the two devices were well correlated, although small (but significant) differences in the absolute values were found. For extension and axial rotation, the devices gave significantly different values that were also poorly correlated. The ‘limits of agreement’ for the two devices (calculated to examine whether they could be used interchangeably) were rather wide, especially for extension and axial rotation. Conclusion. The two devices do not always yield comparable measures for spinal mobility. The accuracy of each, in relation to true angular movements of the vertebrae, remains unknown.