Human–robot interaction is an important aspect of Industry 4.0, and the extended use of robotics in industrial environments will not be possible without enabling them to safely interact with humans. This imposes relevant constraints in the qualitative characterization of the motions of robots when sharing their workspace with humans. In this paper, we address the trade-off between two such constraints, namely the smoothness, which is related to the cognitive stress that a person undergoes when interacting with a robot, and the speed, which is related to normative safety requirements. Given an execution time, such an approach will allow us to plan safe trajectories without neglecting cognitive ergonomics and production efficiency aspects. We first present the methodology able to express the balance between these qualities in the form of a composite objective function. Thanks to the variational formalism, we identify the related set of optimal trajectories with respect to the given criterion and give a suitable parametrization to them. Then, we are able to formulate the safety requirements in terms of a reparametrization of the motion. Finally, numerical and experimental results are provided. This allows the identification of the preferable sets of the possible motions that satisfy the operator’s psychological well-being and the assembly process performance by complying with the safety requirements in terms of mechanical risk prevention.