# Towards Productive and Ergonomic Order Picking: Multi-Objective Modeling Approach
## 1 Introduction
### Lean and Sustainability as Drivers for Multi-Objective Optimization Development  
A lean approach is being introduced to production environments to decrease costs and increase productivity, which creates an intensified work pace and demands [4]. However, this does not increase the need for workers' decision-making and self-organization in the workplace. Work becomes standardized, movements prescribed. Such work can become more or less burdensome. In a society where more and more companies are introducing lean, intensive work, this can be reflected in increased MSDs [5; 6; 7]. In Germany, for example, musculoskeletal and connective tissue disorders accounted for EUR 17.2 billion (EUR 17,200 million) of production loss (production loss costs based on labor costs) in 2016 and EUR 30.4 billion in loss of gross value added (loss of labor productivity), representing 0.5% and 1.0% of Germany's gross domestic product, respectively [8]. Despite that, the main objective of decision support models and Warehouse Management Systems remains the reduction of the average distance the order picker needs to travel, thus lowering the order picking time [9] and increasing the number of picks per order picker. We can achieve nearly the same order picking time with different proportions of overall traveling, searching and picking times for identical customer orders from different item layouts at storage locations. We assume that viable alternatives differ in contribution to order picker's fatigue, workload, and proportions of postures with a negative impact on the worker's physical health. Manual \"man-to-goods\" order picking requires manual material handling tasks, such as stretching, turning, lifting, lowering, squatting and carrying, and is considered highly repetitive because the cycle times are usually measured in seconds. Manual lifting and exposure to other ergonomic stressors are proven to be linked to low back pain [10; 11; 12; 13]. Up to 80% of the population experiences low back pain approximately at least once in their lifetime [14; 15; 16]. The cost of temporary and permanent incapacity for work associated with manual material handling is an important, influential element for the industry [17]. Although this is often overlooked, the lean concept is not just about increasing productivity but also about improving safety. In some areas, like order picking, it is challenging to achieve productivity and safety simultaneously. Today, when the interaction of productivity and ergonomic suitability is still relatively unexplored, managers prefer not to talk about the potential detrimental impact of order picking on people's wellbeing and even encourage overloading through the variable part of pay [18]. However, this problem cannot be omitted because the implemented optimization strategy within a logistic system must be consistent with workers' consent and commitment for whole system efficiency [19]. Order picking activities can be seen in optimization aspects as one of the steps towards a sustainable organization without a waste of time [20] but still friendly to workers' wellbeing. It is difficult to achieve consistent and conscious efforts toward successful lean implementation in a warehouse environment due to the diversity and high frequency of tasks with the differing workload, but not impossible when considering industry 4.0 based on digitalization. Research has already proven a positive correlation between lean manufacturing and industry 4.0 [21; 22] with its essential information technology (IT) element. A warehouse management system could then be seen as a lean and sustainable solution all at once. It could inform order pickers about the achieved level of safe daily workload, load them with ergonomically acceptable sequences of tasks, and among others, propose a more ergonomically favorable picking sequence outside the rush periods.