Effects of body posture, weight and frequency on time-dependent muscle strengths during dynamic materials handling tasks

We examined the effects of body gesture, weight and frequency on time-dependent muscle strengths during dynamic materials handling tasks. Maximum voluntary isometric strengths (MVIS) of nine healthy males were evaluated for symmetric and asymmetric postures, respectively, and their time-dependent muscle strengths were measured after performing 2 hours of lifting and lowering tasks with varying conditions. The percentage decrease in normalized MVIS after performing each task was then reported. The mean MVIS across the subjects for the asymmetric posture of 90° lateral plane to the right was significantly lower than that for the symmetric posture. The percentage decrease in MVIS after performing the asymmetric tasks was found to be significantly greater than that for the symmetric tasks. A correction factor of 34.1% is recommended to modify the sagittally symmetric MVIS data when they are utilized in the asymmetric posture of 90°. Furthermore, the MVIS data should be adjusted by 13.4% and 24.2% for durations of 2 hours for symmetric and asymmetric tasks, respectively. Relevance to industry. Muscle strength is a basis for MMH job design and worker screening. Correction factors to compensate for the effects of task variables on muscle strength and variations in worker endurance on physiological fatigue were established for use in designing ergonomically sound MMH tasks.