Functional anatomy of trunk flexion-extension in isokinetic exercise: muscle activity in standing and seated positions.

This study examined the functional differences existing in the trunk flexion-extension movement in standing and seated positions during isokinetic exercises, as well as the influence of position on overload of the lumbar column.Nine females underwent an isokinetic test at 60 and 180 degrees /s in standing and seated positions. Muscle electrical activity was assessed by means of surface electromyography (EMG). Motion angles related to the different body segments were extrapolated from video images.The range of motion for hip movements was statistically significantly higher in the standing position, whereas it was similar for all the other segments studied in both positions. During the first phases of the movement, the lumbar tract showed a more marked lordosis in standing position. Peak torque values were significantly higher for flexion in standing position. Surface EMG showed significant differences in both positions only for the gluteal and biceps femoris muscles.The seated position allowed made it possible to limit the involvement of the hip muscles, particularly the iliopsoas during flexion, whereas there was little contribution to the trunk extension from hip extensor muscles. In addition, trunk extensors/flexors ratio showed values variable with velocity in standing position. Therefore, if the participation of accessory muscles is avoided, the seated position allows us to more accurately assess these two groups of antagonist muscles, whose balanced ratio is essential in the prevention of spine pathologies. The seated position has also been found to be more suitable in order to limit functional overload of the lumbar column.