During gait neurorehabilitation, many factors influence the quality of gait patterns, particularly the chosen body-weight support (BWS) device. Consequently, robotic BWS devices play a key role in gait rehabilitation of people with neurological disorders. The device transparency, support force vector direction and attachment to the harness vary widely across existing robotic BWS devices, but the influence of these factors on the production of gait remains unknown. As this information is critical to design an optimal BWS, we systematically studied these determinants in this work. With a highly transparent device and a conventional harness, we found that healthy subjects select a small backward force when asked for optimal BWS conditions. This unexpected finding challenges the view that during human-robot interactions, humans predominantly optimize energy efficiency. Instead, they might seek to increase their feeling of stability and safety. We also demonstrate that the location of the attachment points on the harness strongly impacts gait patterns, while harness attachment is hardly reported in literature. Our results establish new principles for the design of BWS devices and personalization of BWS settings for gait neurorehabilitation.