Intraabdominal pressure (IAP), movements of the body in the sagittal plane, and the forces applied to the load were recorded while 10 male subjects lifted or lowered a 15-kg box using six different lifting techniques and two lowering techniques. IAP data were compared with calculated peak values of lifting velocity, lumbosacral compression and hip torque, and with the integral of lumbosacral compression over time. No consistent relationship between IAP increases and any one of these values emerged. The variation in peak IAPs was considerable. Nonetheless, there were significant differences in IAP between different lifting and lowering techniques. IAP was, in general, less when the trunk was flexed for lifting and lowering than when it was used in a posture nearer to the vertical.
Biomechanical models used for the evaluation of spinal stress have mostly been static: they reveal the postural stress caused by gravity but ignore the inertial forces and' torques induced by the accelerations of body segments and the load handled. In this study both static and dynamic models for determining lumbosacral compression were used, and the results from different models were compared.Twenty subjects lifted a 15 kg box from a 10 cm high shelf to knuckle height with four lifting techniques: the conventional back and leg lifts and two 'kinetic' lifts with which the subjects gave kinetic energy either to the horizontally moving load or to the vertically moving body before lifting the load itself vertically.The results showed that the inertial factors increase the spinal load considerably. Comparison between the lifting techniques showed that the dynamic peak stress was clearly smaller in the leg lift than in the back lift, although the static peak stress was smaller in the back lift The L5/Sl-compression x time integral, describing the total stress of a lift. was smallest in the back lift with both models.
The purpose of this study was to measure dose of spinal load when different pacing methods were applied to lifting work and to develop methodology for such measurements. The compressive load on the spine computed by a dynamic biomechanical model and the electromyographic activity of back muscles were used for describing the spinal load. Five men and five women worked in a laboratory on two days lifting a box up and down for 30 min on both days, on one day force-paced (4 lifts/min), and on the other self-paced in random order. The weight of the box was rated by the subjects to be acceptable for the work done. The lift rate of our female subjects was higher and that of the male subjects lower in self-paced than in force-paced work. There were no significant differences in peak lumbosacral compressions nor in the amplitude distributions of electromyography between the two pacing methods. The biomechanically-calculated compressive forces on the spine were lower (about 2.7 kN for the men and 2.3 kN for women) than the biomechanical recommendations for safe lifting, but the EMG activity showed quite high peaks so that for 1% of work time the activity was on women above 60% and on men above 40% of the activity during maximum isometric voluntary test contraction.
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