# Towards Productive and Ergonomic Order Picking: Multi-Objective Modeling Approach
## 6 Discussion  
The developed general model supports manual man-to-goods order picking in arbitrarily long racks with several shelves and one entry and exit point. As this is manual order picking, the highest shelf should not be placed higher than 177 cm. As a novelty in the paper, we develop three single-objective functions and one multi-objective function to support decision-making on storage assignment in a described warehouse environment. They allow minimization of time, energy expenditure and health risk individually or in a weight-adjusted trinity. In addition to developing individual models, we are interested in comparisons between results from the use of functions from a decision-maker perspective. For them, the most important is time-saving and consequently lowering costs. The cost of order picking largely depends on the number of items collected per unit time. The introduction mentioned authors that have already experimented with time, energy expenditure, and health risk objective functions, but in different environments or by addressing less influential factors. This paper addresses order picking of smaller boxes in various dimensions and masses, which is a daily practice in packet distribution for the retail sector's needs. Including only time and energy expenditure would represent a sound basis for safer order picking. Still, it would not protect against MSDs due to the unanalyzed presence of situations with a high risk for injuries. Therefore, it makes sense to explore the possibility of a multi-objective assignment model and check if it is possible to achieve increased productivity while preserving the order pickers' physical health. Our experiment shows that less strenuous and safer work requires longer operating times and results in lower productivity. This result was logical and expected. The question is, what is the magnitude of this deviation. Focusing only on lower energy expenditure is reflected in a less than 3% increase in the order picking time, which is not much, and the benefits are questionable. A 3% time savings in one shift equals a 15-min break. Similar results were already observed by authors [9], who proposed a bi-objective model, including time and energy expenditure. They calculated that the maximum difference is about 2% in time and 2.5% in energy. In their case, the order fulfillment time in employing the energy-based storage assignment was 2% higher than the total time that results if the time-based assignment was used. Comparable results from the previous study contributed to the validation of our model. By extending the model to a multi-objective one, we find another concern about energy-based assignment usability, which was not previously understood from the mentioned bi-objective model. A comparison of average health risk values shows that the order picker is on average more often exposed to very-high health risks (unacceptable, immediate action required) in energy-based assignments than in time-based assignments. Scientific literature suggests a dynamic determination of breaks according to energy expenditure amounts. Still, even in this case, we cannot guarantee the absence of individual pick activities with a high risk of injury without additional modeling. Ergonomic assessment needs to be built into decision models, as it is not enough to protect workers from overwork; they also need to be protected from injury and MSDs. It has been revealed that with the health-risk optimization function, the order picker is never exposed to very-high risk (unacceptable, immediate action required). Only some boxes are positioned in storage spaces with some health risk, and the majority of them in spaces with negligible risk. On the other hand, this increases the average time by approximately 33% compared to single-objective time-objective optimization. A single-objective health risk-based assignment is suitable from a managerial perspective for a time frame when the number of customer orders is low.