Lifting Unexpected LoadsMany studies over the years suggest that low back pain can be caused by lifting an unexpected weight or a load that shifts suddenly. Now the question has been raised: if the load is uneven, is the risk of injury even greater?
The authors of this study say, âNo and yes.â No, unexpected mass to one side doesnât increase the compressive force on the spine. In other words, the bones in the spine donât press down on each other with the new load. But, yes, there may still be an increased risk of injury from another source.
The muscles of the trunk pull to one side in response to an unexpected load placed on one side. Lifting objects while bending to the side or twisting increases the risk of injury. The muscles generate a high force to offset the load. The resulting forces may be damaging to the spine.
In this study, a video-based recording system was used to show the effects of expected versus unexpected loads. Tiny lights placed on various joints marked the starting posture. As the person lifted the weight, the lights moved and any change in the posture was recorded.
At the same time, electrical activity in the muscle was recorded for the back and abdominal muscles. Muscle force and spinal compression were measured. The results show that total muscle force was decreased when lifting uneven weight loads.
While the load on the spine wasn't increased, delayed muscle contraction or too little muscle contraction may still lead to spinal rotation. The rotation combined with pressure through the spine may be enough to cause injury to the ligaments or disc.
According to these authors, the bottom line is this: unexpected weight shifts when lifting loads can increase the risk for injury-- just not for the reasons we might expect!
J. (Petra) C. E. van der Burg, MSc, et al. Effects of Unexpected Lateral Mass Placement on Trunk Loading in Lifting. In Spine. April 15, 2003. Vol. 28. No. 8. Pp. 764-770.
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