Current design methods for vibration serviceability assessment of pedestrian structures are generally based on footfall loading models representing walking over rigid level surfaces rather than vibrating surfaces that might modify the loading due to human-structure interaction. A two-year project was carried out to identify vertical vibration conditions under which the human-structure interaction occurs. As part of the testing program, participants were asked to walk over either an outdoor footbridge or a rigid ground surface at various step frequencies. Ground reaction forces were measured by using insole sensors, and the effects of vibration amplitudes and walking to vibration frequency ratios on these forces were examined. It was found that the vibrating surface has no discernible effect on the step-to-step variability. Comparison of dynamic load factors generated on the vibrating footbridge with those on the rigid-level ground revealed that while the vibrating surface increased the first dynamic load factor, it actually reduced it when the step frequency approached the fundamental frequency of the structure.