The Influence of External Load Configuration on Trunk Musculature and Spinal Stability during Manual Material Handling

The performance of manual material handling (MMH) tasks is highly associated with lower back injuries due to the excessive acute and/or cumulative mechanical loading that spinal tissues experience. Therefore, it is critical to understand how different characteristics of MMH tasks could become potential risk factors that change back injury risks and to develop proper MMH strategies that could reduce their biomechanical impacts to the spine. In this study, we explored the effects of external load configuration on trunk musculature and spinal stability during static loading and sudden loading scenarios.;The main objective of the current research was to explore how the configuration of an external weight (e.g. weight distribution or arrangement of the parts of the weight) can influence trunk biomechanics and spinal stability during the performance of static loading and sudden loading. To this end, we have conducted two experiments each of which was designed to simulate the two scenarios mentioned above.;In the first experiment, we investigated the influence of the weight configuration of hand loads on trunk muscle activities and the associated spinal stability during static weight holding. Thirteen volunteers each performed static weight holding tasks using two different 9 kg weight bars (with medial and lateral weight configurations) at two levels of height (low and high) and one fixed horizontal distance (result in constant spinal joint moment across conditions). Results of this study demonstrated that holding the laterally distributed load significantly reduced activation levels of lumbar and abdominal muscles by 9 to 13% as compared with holding the medially distributed load.;In the second study, we examined the effects of different configurations of hand load on spine biomechanics and trunk stability during sudden loading events. Fifteen asymptomatic volunteers experienced sudden loadings using the same magnitude of weight (9 kg) with two different configurations (medially or laterally distributed) at three levels of height (low, middle, and high) and one fixed horizontal distance (constant spinal joint moment across conditions). Results of this study revealed that holding the medially distributed weight resulted in a significantly higher effective trunk stiffness (on average, lateral: 1785 Nm/rad and medial: 2413 Nm/rad) and peak L5/S1 joint compression force (on average, lateral: 2694 N and medial: 2861 N) compared with the laterally distributed weight.;We believe such effects are due to an elevated rotational moment of inertia when the weight of the load is laterally distributed. These findings suggest that during the design and assessment of manual material handling tasks such as lifting and carrying, the weight configuration of the hand load should be considered. According to the results, it was concluded that when confronted with static and sudden loading incidents, the load with larger moment of inertia (i.e. laterally distributed load) could help reduce the risk of low back injury compared to the load with a smaller moment of inertia (i.e. medially distributed load)