Predicting maximum acceptable efforts for repetitive tasks: an equation based on duty cycle.

OBJECTIVE: The objective was to develop an equation, for repetitive tasks, that uses frequency and/or duty cycle (DC) to predict maximum acceptable efforts (MAE) relative to maximum voluntary efforts (MVE).
BACKGROUND: Ergonomists must determine acceptable physical demands for a wide variety of tasks. Although a large database exists in the literature for maximum single-effort strength, far fewer repetitive tasks have psychophysical and/or physiological data available to guide the prediction of acceptable submaximal, repeated efforts.
METHOD: DC represents the total effort duration divided by the cycle time. MAEs were calculated by dividing average psychophysics-based acceptable loads by corresponding single-effort maximum strength using 69 values from studies of the upper extremities. The author developed an equation to characterize the relationship between MAE and DC.
RESULTS: The resulting equation had DC taken to the exponent 0.24, and it predicted MAE very well (r2 = 0.87%, root mean square [RMS] difference = 7.2% of the maximum strength). At higher DC values, the equation also demonstrated good agreement with the published physiological data.
CONCLUSION: The limited psychophysical database in the literature makes it difficult for ergonomists and engineers to recommend acceptable efforts for the large variety of repetitive tasks they evaluate. However, the proposed equation now allows for a correction of the large strength database to estimate acceptable force and torque limits for repetitive occupational tasks. APPLICATION: The proposed equation will have wide applications for ergonomic practitioners performing evaluations of repetitive tasks.