Tolerance of the lumbar spine to shear: a review and recommended exposure limits

The lumbar spine may experience significant shear forces during occupational tasks due to the force of gravity acting on the upper body when bending the trunk forward, or when performing tasks involving pushing or pulling. Shear force limits of 1000 N and 500 N have been recommended by previous authors for maximum permissible limit and action limit, respectively.

The present paper reviews literature in terms of shear tolerance (ultimate shear stress and fatigue life in shear stress) of the lumbar spine and develops recommended limits based on results of studies examining shear loading of human motion segments. Weibull analysis was used to assess fatigue failure data to estimate distributions of failure at different percentages of ultimate shear stress.

Based on Weibull analysis of fatigue failure data from the best available data, a 1000 N shear limit would appear acceptable for occasional exposure to shear loading (≤ 100 loadings/day); however, a 700 N limit would appear appropriate for repetitive shear loading (100-1000 loadings/day) for most workers.

Results of the current analysis support the 1000 N limit for shear stress, but for a rather limited number of cycles (<100 per day). Due to the logarithmic nature of the fatigue failure curve, a 700 N shear limit would appear to be acceptable for frequent shear loadings (100-1000 per day). This value is slightly higher than the action limit of 500 N previously recommended.