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The primary purpose of this study was to determine, at liftoff of the trailing foot under both slip and nonslip conditions in bipedal walking, the threshold values of the center of mass (COM) velocity relative to the base of support, required at a given COM position, in order to prevent backward balance loss. A 7-link bipedal model and forward dynamics simulation integrated with a dynamic optimization routine were employed to derive the threshold values. We found that the minimum COM velocity ranged from 0 to 0.29 and from 0 to 0.52, corresponding to the changes in the initial COM positions from the heel of the right foot, to 1.25-time the foot length posterior to the leading (slipping) heel required for walking respectively under nonslip and slip conditions. This required COM velocity was consistently higher under slip conditions than that under nonslip conditions. These findings were verified by extensive experimental data collected during human walking experiments. They may not only provide scientific basis for fall prevention among older adults, but also provide essential guidance in bipedal robotic design.