The effects of force and exertion duration on duty cycle time: Implications for productivity

Ergonomics has positive effects on both physical health and productivity, but estimating productivity benefits is difficult at the task design/redesign stage. Rest-allowance prediction models are not suitable for repetitive, short-cycle dynamic tasks, and methods–time measurement (MTM) techniques are limited in their suitability for considering ergonomics risk factors such as posture and force. The purpose of this study was to investigate the relationship between force and exertion duration on self-selected duty cycle time and discomfort. Twenty-one participants completed repetitive, upper-limb exertion treatments, each of a 10-minute duration. Five levels of force (10, 20, 40, 65, and 80% maximum voluntary contraction [MVC]) and exertion period (1, 2, 4, 6.5, and 8 seconds) were investigated. The psychophysical adjustment method was used whereby participants self-selected a work pace for the second half of each treatment. Duty cycle, derived from the self-paced cycle time, was the measure of productivity effects in the experiment. Analysis of variance revealed a significant effect on duty cycle time for force, exertion period, and their interaction (each p < 0.0001). Friedman's test indicated a significant effect of force (p < 0.0001) and exertion period (p < 0.0001) on discomfort. Spearman's correlation analysis showed a strong correlation between discomfort and duty cycle time (p < 0.05). Multiple regression analysis was used to develop a predictive model for duty cycle time based on force and exertion period, and this was a good fit to the data (R2 = 0.98, p < 0.05). Profiles were generated presenting zones of acceptable self-selected duty cycle times based on force and exertion duration.