Rest Interval Modeling for Repetitive Lifting Using Task Characteristics

Overexertion, usually caused by manual material handling (MMH), often leads to work-related musculoskeletal disorders (WMSDs). These can be prevented during MMH tasks by redesigning them to include sufficient rest periods. Task characteristics play an important role in the redesign of the job. This study evaluated the effect of five task characteristics—weight, duration, frequency, distance, and angle of symmetry—at two levels on the maximum voluntary contraction recovery rate (Muscle Voluntary Contraction Recovery Time (MVCRT), defined as the time required for the muscle to recover to 80% of the initial Maximum Voluntary Contraction (MVC) value) of the muscles and developed a work-rest interval model based on these task characteristics. The results show that the weight and duration had a significant effect on MVCRT, increasing it by 32% and 21%, respectively. The interaction between frequency and distance also significantly impacted MVCRT. A simplified mathematical model was developed and validated by comparing a set of the model-generated data with experimental data. The simplified regression model demonstrated moderate predictive capability with an R2 value of 0.52. The ANOVA shows no significant difference between the data. The study’s results and mathematical models can be used during task design to reduce WMSDs. Because approximately 48% of the variability in MVCRT remained unexplained, direct application of the model to establish worker-specific work-rest intervals may result in underestimation or overestimation of recovery requirements. Therefore, the model should be interpreted as a preliminary estimation framework.