Lean on Me To Reduce Low Back Injury Risk

Changes to the design of work that are intended to reduce injury and severity rates should be directed to reducing or removing known risk factors.  Assessments of risk of low back injury in field settings are typically targeted towards single peak spinal loading exposures.  These are most often associated with heavy lifting.  However, low back pain (LBP) is often reported by workers in jobs that are not heavy, in the sense of single high forces on spinal tissues.  Often these jobs are characterized by trunk postures that present only low or moderate peak force demands on the spine but become problematic if they require adverse prolonged or repetitive postures.  In these situations it may be the effects of cumulative loading which produce the LBP.  Indeed, increased levels of cumulative spinal loading have been identified as a significant risk factor for reporting LBP in nursing aides (Kumar, 1990) and subsequently in automotive assembly and assembly support workers (Norman et al, 1998, Kerr et al., 2001).  

Figure 1 is the time history of a job that comprises several tasks of varying duration and varying peak spinal loading demands, in this example spinal compression is estimated at the level of the L4/L5 spinal motion unit. Two of the tasks show peak compression values that would be flagged, if one were to use the NIOSH Action Limit (AL) for disc compression (3400 N), as being a risk to some people. None of the other tasks are problematic on the NIOSH criterion.  The cumulative spinal loading is represented by the area underneath the L4/L5 compression force - time history.  While high peak forces contribute to high cumulative loading the tasks that have peaks well below the NIOSH AL contribute much more to the cumulative loading because collectively they represent substantially more of the total duration of the job. The effect of these smaller peaks, which may be held for long durations, and/or performed many times in a shift, also lead to higher risk because of their contribution to the cumulative loading. 

Figure 1: A schematic of the L4/L5 spinal compression time history, as produced by a biomechanical model, for a series of activities associated with a hypothetical production job.  The instances when the 3400 N NIOSH AL have been exceeded are clearly visible.  The cumulative compressive loading is determined by measuring the shaded area underneath the compression-time history.

High spinal compressive loads are usually caused by a combination of poor trunk posture and high forces in the hands. However, even in the absence of high hand forces, the weight of the upper body (about one half of body weight) continually produces measurable spinal compression. If the worker is required to lean forward the spinal compression becomes moderately high although it does not exceed the NIOSH AL for compression. But the cumulative loading caused by adverse trunk postures can, over time, become excessive.

So, how do you reduce these peak and cumulative loading risk factors?  One approach is to support yourself, by leaning, whenever possible.  Lardi and Frazer (2003) examined 10 subjects performing a single handed assembly task in 12 different static postures. Four trunk flexion angles (30°, 60°, 75° and 90°) were each evaluated with three different supporting/leaning non-dominant arm angles (25°, 35° and 50° from the vertical). The results indicated that subjects supported themselves by placing 10-15% of their body weight on the support arm.  This support produced a decrease in the peak spinal compression by 1,000 N!  Also, as the support was present while performing the entire task, leaning was identified as an effective method for reducing cumulative spinal loading as well.

Lardi and Frazer (2003) identified some cautions when using leaning as a work method for reducing peak and cumulative loading on the lumbar spine.  The first was that leaning has the potential to produce injuries at other body regions.  In their study they found the peak moments at the elbow and shoulder to be less than 10 N.m.  Secondly, they pointed out that this was not an appropriate administrative control measure for reducing spinal compression for work tasks or activities identified as exceeding threshold limit values.  Rather, they stressed the importance of implementing engineering controls when feasible as these diminish the exposure.

References

Kerr, M.S., Frank, J.W., Shannon, H.S., Norman, R.W.K., Wells, R.P., Neumann, W.P., Bombardier, C., the Ontario Universities Back Pain Study Group, 2001. Biomechanical and psychosocial risk factors for low back pain at work. American Journal of Public Health 91, 1069-1075.

Kumar, S., 1990. Cumulative load as a risk factor for back pain. Spine 15, 311-316.

Lardi, F., Frazer, M. How much assistance does single arm support provide when you lean forward? in: Proceedings of the ACE 2003 Conference- Fit, Form, Function, CD ROM, London, Ontario, October 15-18, 2003.

Norman, R., Wells, R., Neumann, P., Frank, J., Shannon, H., Kerr, M. and the Ontario Universities Back Pain Study (OUBPS) Group 1998, A comparison of peak versus cumulative physical work exposure risk factors for the reporting of low back pain in the automotive industry, Clinical Biomechanics, 13, 561‑573.