Which criteria determine the formation of rest-to-rest arm movements when interacting with virtual mass-damper dynamics? A novel bilevel optimization approach is used to find the optimal linear combination of common optimization criteria for human trajectory formation such that the resulting trajectory comes closest to the humanperformed one. The goal is to utilize this optimal combination to predict human motions in robot control. Experimental results show that a subject-dependent criteria combination can be found for different massdamper dynamics. Key words: Human trajectory formation, Bilevel optimization