Quantifying the Precision of Biomechanical Calculations using Experimental Data

Biomechanical calculations are a useful tool to evaluate the severity of manual materials handling tasks. The exactness of the calculated forces depends on a number of factors. On one level is the precision of the inputs such as postural data and the force exerted by the operator. At another level is the exactitude of the biomechanical model itself. The effect of the imprecision of each factor upon the final result can be calculated so that on one hand, the range of values within which the final result is likely to fall is known, and on the other hand, the importance of each factor can be assessed, by comparing the standard deviation of one or more factors with the standard deviation of the result. Calculations of forces on the low back have been carried out in the laboratory using an optical scanner (CODA-3) to record posture on-line to a computer, as well as a handle equipped with strain gauges to record the force exerted. The program automatically carries out biomechanical calculations from these inputs. It takes into account the uncertainties on muscle lever arms and intra-abdominal pressure, using mean values for these and estimating the confidence limits within which the calculated low-back forces will lie, given the variance of one or more of the inputs.