Work-related musculoskeletal disorders are a major health issue often caused by awkward postures. Identifying and recommending more ergonomic body postures requires optimizing the worker's motion with respect to ergonomics criteria based on the human kinematic/kinetic state. However, many ergonomics scores assess different risks at different places of the human body, and therefore, optimizing for only one score might lead to postures that are either inefficient or that transfer the risk to a different location. We verified, in two work activities, that optimizing for a single ergonomics score may lead to motions that degrade scores other than the optimized one. To address this problem, we propose a multi-objective optimization approach that can find better Pareto-optimal trade-off motions that simultaneously optimize multiple scores. Our simulation-based approach is also user-specific and can be used to recommend ergonomic postures to workers with different body morphologies. Additionally, it can be used to generate ergonomic reference trajectories for robot controllers in human-robot collaboration.