Variability of Spinal Loads in Repetitive Lifting: Median vs. Peak Compression

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Variability of Spinal Loads in Repetitive Lifting: Median vs. Peak Compression

It is known that great variations in mechanical loads on the body exist when a person performs a lifting task repeatedly. This variance may be important in understanding back injuries because biomechanical and epidemiological studies suggest that peak loads predict injury more accurately than average loads. However, biomechanical studies are often based on average loads.

This study focused on within-subject variability of spinal loading during repetitive lifting. To assess the distribution of maximum compression on the spine from repetitive lifting, 10 male subjects performed 360 repetitive lifting tasks of a 45-liter crate for one hour in two separate sessions. In one session, the crate was stabilized with a 10-kilogram weight attached to the bottom. In the other session, the crate was unstable because it was filled with 10 kilograms of water. Participants used a "back technique," keeping the legs as straight as possible when lifting, and were paced with a metronome.
In each subject, peak compression varied substantially through trials and the peak force was substantially greater than the average, particularly with the unstable load. Peak values were significantly different between conditions, while median values were similar.
Conclusion: In repetitive lifting tasks, peak spinal compression may exceed median compression, and may be influenced by conditions independent of the median. The authors conclude that the inconsistency of spinal loads should be considered when devising lifts that can result in spinal injuries.
Note: This study is interesting as it investigates the peak load of force on the spine as being the probable factor in back injury, and not the accumulation or average force sustained by the back. In essence, it looks at the proverbial straw that breaks the camel's back.
Van Dieën JH, Dekkers JJM, Groen V, et al. Within-subject variability in low back load in a repetitively performed, mildly constrained lifting task. Spine, August 15, 2001:26(16), pp. 1799-1804