INTRODUCTION: There are many different tasks involved within a workplace, and assessing the required efforts for performing them depends on factors such as the human's body posture, task nature, and number of repetitions. This study aims to develop an equation for back-involved repetitive tasks that relates the maximum acceptable effort (MAE), the maximum acceptable efforts that an individual can sustain for a specific task and is expressed as a percentage of the maximum strength, to the duty cycle, the amount of time an individual is engaged in a task relative to the total time. The equation was derived based on psychophysical data collected from previous studies on lifting, lowering, and carrying tasks. The literature search identified studies reporting maximum acceptable loads (e.g., forces and toques) for back-involved tasks. METHOD: Data analysis was done by calculating duty cycles and for each task. Statistical tests were conducted to compare the results across different parameters, such as sex, task nature, lifting box size, box distance from the body, and population percentages. RESULTS: The results showed a strong negative relationship between duty cycle and MAE. This relationship shows that by increasing the duty cycle, MAE should be decreased to be acceptable and prevent worker's fatigue. The developed equation was compared to existing equations for upper-limb tasks and demonstrated a close resemblance. Additionally, statistical analysis indicated that the proposed equation eliminated the effects of various parameters. The proposed equation provides an individual-specific approach for estimating MAEs and can contribute to preventing workers' fatigue and injury and reducing their associated costs.