It turns out that we can model a biped robot quite simply as a single point mass with two force vectors representing the feet. Modeling the robot this way allows for easy mathematical modeling as well as being easier to visualize than a multi-mass model with many joints.
One further simplification which is very popular at the moment is to restrict the robots motion to the X-Y plane and prevent motion in the up down direction. This is called the Linear Inverted pendulum model (LIPM)  and is used extensively in current biped robotics research.
The equation of motion for the LIPM is described as follows
and is the height of the COM which should be a constant.
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. “The Linear Inverted Pendulum Model: A simple modeling for a biped walking pattern generation”, Shuuji Kajita, Fumio Kanehiro, Kenji Kaneko, Kazuhito Yokoi and Hirohisa Hirukawa. Proceedings of the 2001 IEEE/RSJ International Conference on Intelligent Robots and Systems Maui, Hawaii, USA, Oct. 29 – Nov. 03, 2001