Work-related musculoskeletal disorders in industry represent a major health problem in many developed countries. Collaborative robotics, which allows the joint manipulation of objects by both a robot and a person, is a possible solution. But efficiently designing such assistive robots requires to assess the ergonomic benefit they offer. Similarly to other domains such as automotive or workstation design, the use of a digital human simulation (DHS) can cut down the development cost and time by replacing the physical mock-up with a virtual one easier to modify. However, simulating human-robot collaborative tasks poses specific challenges because the human and the robot form a highly coupled dynamic system in which the motion of each partner depends on the forces exchanged. Therefore a dynamic simulation is required to obtain reliable measurements for ergonomic assessments. The first part of this chapter details the challenges of DHS for collaborative robotics. State-of-the-art work on DHS with collaborative robots is reviewed to identify which questions currently remain open. An optimization-based controller is then proposed to animate a digital human model (DHM) in the context of human-robot collaboration. The second part of this chapter presents an application of the DHM controller. A human-robot collaborative task is successfully simulated and allows to quantify the effect of kinematic, dynamic and control parameters of the robot on the DHM posture and effort.