Methodological considerations for the calculation of cumulative compression exposure of the lumbar spine: a sensitivity analysis on joint model and time standardization approaches.

Cumulative lumbar spine loading has attracted much attention as a factor associated with the development of low back pain. While evidence supports cumulative loading to be a plausible mechanism in explaining several workplace injuries, research establishing a threshold limit value (TLV) for cumulative spine loading has been challenging. The lack of a TLV or even a trend towards harmful cumulative load values may suggest that methodological considerations are greatly influencing the results. This paper examines the impact of different joint models (single muscle equivalent, an electromyography-based third order polynomial, a modified version of the polynomial and a hybrid approach) to determine cumulative spine compression, as well as the importance of time standardization in the calculation of a daily cumulative loading dose. Findings demonstrated that the polynomial predicted cumulative compression values were 43-53% higher than those with all other models tested and the single muscle equivalent predicted loads 18% higher than loads predicted using a modified polynomial. Profound differences between modelling approaches suggest that caution should be taken when selecting a muscle model to determine cumulative spine compressive loading. Time standardized cumulative compression values were found to be 28.3% greater than non-standardized estimates, illustrating the importance of selecting a standard time frame in the calculation of cumulative spine compression.