# Towards Productive and Ergonomic Order Picking: Multi-Objective Modeling Approach
## 1 Introduction
### Decision Support Models for Storage Assignment  
A recent literature review on picker routing revealed that in 83% of considered papers, straight or parallel shelves, which may be intersected by one or multiple cross aisles, were the subject of research [27; 30]. This indicates the popularity and prevalence of such  
Figure 1: Factors influencing order picking time, energy expenditure and health risk. layouts in practice. The number of items collected per unit time, fatigue, and physical health of order pickers are greatly influenced by determining storage positions for individual types of goods, determining the sequence of picking items from storage racks and layout. In the literature on order picking, there is a noticeable trend towards developing decision support models focusing on the assignment of items to storage positions [1; 30]. The assignment can be either random or based on specific criteria but always chosen to support an increase in the number of items picked per particular time unit or minimization of travel distance. Researchers have developed several storage assignment methods to shorten total fulfillment time. Three frequently in practice implemented examples are:  
* An ergonomic storage assignment method by classifying items at different heights in storage locations using flow racks [31];
* A concept of golden zone storage, where high-demand items are stored in the area between a picker's waist and shoulders [23; 32];
* Modeling order picker's fatigue/recovery level based on predicted energy expenditure rate calculated from results of monitoring heart rate [33]. The literature on storage assignment is focused on developing methods that reduce travel distance and order picking time which put warehouse managers in a precarious position because the results of the methods used do not inform them about health risks for operators [31]. Other objectives, such as minimizing order picker's discomfort or the prevention of MSDs, have been mainly overlooked in managerial decision support models for the storage assignment problem [23] or have only very infrequently been discussed [34]. In contrast to the management-oriented literature, order pickers' wellbeing has been studied in the ergonomics literature [9]. Several scientific works describe the manual order picking process as work in an environment that puts workers at risk of developing MSDs (e.g., [35; 36]). Order picking activities require the use of all order picker's body parts and thus creating a risk of various MSDs. The recognized need to incorporate ergonomics or human factors issues into planning models for order picking in the scientific literature in 2015 [23] was followed by a growing number of published works in this area in recent years. Works on ergonomics can be divided into three research streams. First is oriented on physical ergonomics, second on cognitive ergonomics and third on organizational ergonomics. Because of the topic discussed in this paper, our research is limited to physical ergonomics. It studies human anatomical, anthropometric, physiological and biomechanical characteristics and their links to physical activity. Research works on order picking, including a focus on physical ergonomics, observe the handling of loads and postures. Some multi-objective models, including time and health risk dimensions, are already proposed. A few of them are discussed in more detail in the following [9; 27; 28; 29; 37]. In [9], the authors analyzed the optimal allocation of items to storage locations to minimize total picking times and ergonomic strains, the latter based on the energy expenditure prediction model developed by [38]. In the next step, the rest allowance formulation of [38] is used to develop an integrated approach that considers both order picking time and energy expenditure.