Mathematical modeling of muscle fatigue recovery time for drilling operations

Workers exposed to hand-transmitted vibration are at high risk of developing musculoskeletal disorders due to excessive muscle fatigue, and prolonged vibration exposure impairs the ability to recover effectively. Effective work design, including adequate rest intervals, is essential to minimize fatigue accumulation and its effects. This study evaluated the influence of four drilling parameters on muscle fatigue recovery time (MFRT) and developed a predictive mathematical model. The parameters investigated were drilling duration, feed force, material hardness, and drill bit diameter, each tested at two levels. A randomized complete block design was employed, with 16 treatment combinations replicated thrice. Participants were randomly assigned to treatment combinations. The MFRT was defined as the duration for muscle electrical activation to reach 80% of its initial maximum voluntary EMG Root Mean Square (RMS) value. The RMS value of the EMG was normalized using the amplitude analysis function and presented as a percentage of the maximum voluntary contraction. The results show significant impacts of feed force, drilling duration, and material hardness on MFRT, increasing it by 64.28%, 64.2%, and 22.78%, respectively. The two-factor interactions between feed force and material hardness, as well as duration and material hardness, were significant, while the three-factor interaction between feed force, duration, and material hardness also significantly impacted MFRT. The predictive model developed was validated by comparing experimental data with the model predictions. This study's findings can guide the optimization of drilling work design to ensure sufficient recovery time, thereby reducing the risk of musculoskeletal disorders among workers.