By interactionofbath, 24-Feb-2012 12:52:00
Ergonomists must often make recommendations for acceptable physcial demands for a wide variety of tasks. Although there is a large database in the literature for single-effort strength, most occupational tasks require repetitive efforts. In such cases the recommended demands should be lower than the maximum strength. But by how much? As yet, there is not a database of data available to guide the prediction of acceptable submaximal, repeated efforts.
In an article from Human Factors, December 2011, Jim Potvin of McMaster University, Canada describes an equation for "Predicting Maximum Acceptable Efforts [MAE] for Repetitive Tasks" based on Duty Cycle [DC], where DC represents the total effort duration divided by the cycle time.
According to Jim Potvin, this equation, which takes DC to the exponent 0.24, predicted MAE very well
MAE = (1 – DC ^0.24) x maximum voluntary effort
On an assembly line where women are required to make ten power grip efforts during a 65s cycle, with each grip of 0.55s duration:
DC = (10 efforts/cycle x 0.55s/effort) / 65s = 0.085 (8.5% of each cycle)
MAE = (1 - 0.553427) x MVE = 44% of maximum grip strength
This equation allows practitioners to make corrections of the large strength database to estimate acceptable force and torque limits for repetitive occasional tasks.
As ever, there are a few caveats: the equation applies only when the demands of work do not vary substantially over 8-hour day and when the work is not complex, with multiple task elements; at this stage, too, the equation should only be used to determine female MAE values.
We accept that the equation is not perfect and that it doesn't provide all the answers, but we also believe that, as part of the ergonomist's toolkit, it could help many practitioners in a range of settings.
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