Axis for dynamic measurement of flexion and extension torques about the lumbar spine. A computer simulation.

Most reports of trunk testing with an isokinetic dynamometer describe the axis of the machine aligned with the lumbosacral junction. A computerized mathematical simulation of motion of the lumbar spine under three conditions was performed, representing variations on the physiologic motion of the lumbar spine. The instantaneous center of rotation of the T12 vertebral body moving about a fixed pelvis was calculated and displayed. Under normal conditions, the center of rotation started close to the center of the L3 vertebral body and moved in an arc similar to that of L3 during flexion and extension. Dynamic testing of the trunk musculature in the lumbar region, therefore, should be performed with the axis of the isokinetic dynamometer aligned with the L3 vertebral body, and motion of the pelvis and thoracic spine should be minimized. This position of the axis provides the best approximation of the fixed axis of the testing machine to the physiologic axis. Failure to observe these precautions would tend to make the motion of the lumbar spine nonisokinetic and, thus, interfere with angular measurement of the spinal motion.