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Purpose: A strategic change from parallel-cell based assembly (OLD) to serial-line assembly (NEW) was investigated in a Swedish company with special reference to how production system design elements affect both productivity and ergonomics. Methods: Multiple methods, including records and video analysis, questionnaires, interviews, biomechanical modelling, and flow simulation were applied. Findings: The NEW system, unlike the OLD, showed the emergence of system and balance losses as well as vulnerability to disturbances and difficulty handling all product variants. Nevertheless the NEW system as realised partially overcame productivity barriers in the operation and management of the OLD system. The NEW system had impaired ergonomics due to decreased physical variation and increased repetitiveness with cycle times that were 6% of previous thus increasing repetitiveness, and significantly reduced perceived influence over work. Workstations’ uneven exposure to physical tasks such as nut running created a potential problem for workload management. The adoption of teamwork in the NEW system contributed to significantly increased co-worker support – an ergonomic benefit. Implications: Design decisions made early in the development process affect both ergonomics and productivity in the resulting system. While the time pattern of 3 physical loading appeared to be controlled by flow and work organisation elements, the amplitude of loading was determined more by workstation layout. Psychosocial conditions appear to be affected by a combination of system elements including layout, flow, and work organisation elements. Practical Implications: Strategic use of parallelisation elements in assembly, perhaps in hybrid forms from configurations observed here, appears to be a viable design option for improved performance by reducing the fragility and ergonomic problems of assembly lines. Value: The interacting design elements examined here pose potential ‘levers’ of control by which