Human workers are confronted to an increased level of interaction with machines on work settings and manufacturing environments. The design of adaptative production systems that further enhance human's physical, sensorial and cognitive capabilities requires a thorough understanding of their capacities and a deep analysis of their interactions. This highlights the need for tools that facilitate the study of human-machine interactions on the early design phases. In this paper, we present the development of a tool for prospective ergonomic assessment in virtual reality environments. The proposed tool is based on inertial motion units for motion capture and builds upon an ergonomic analysis software for activity analysis. A warehouse item storage task analysis is presented to illustrate the use of the tool.