In this paper, the standard definition of the zero moment point as a location on the ground is extended to its angular definition around the center of mass of a humanoid robot. This new definition provides a more general way of expressing this stability parameter and enables its wider use, as this way it can be located anywhere on a specific line passing through the system’s center of mass.
https://link.springer.com/chapter/10.1007%2F978-3-030-75259-0_15