# What is the dynamic model of a biped robot in double support phase?

I would like to know What is the dynamic model of a biped robot in double support phase? As drawn in Fig.b and Fig.c.

I have already known (partly) how to model the single support phase drawn in Fig.a. That is to treat it like a classical robotic arm. The support foot is firmly attached to the ground as long as the center of mass coincides with the ZMP. Then I can use Newton-Euler or Lagrangian approach to get this dynamic model: $$\tau = M(\theta)\ddot{\theta} + C(\theta,\dot{\theta})\dot{\theta} + G(\theta)$$ But continue using this model to control the biped robot to follow a certain trajectory when the CoM exceeds the supporting foot area might render the supporting foot unstable. (Though that's not mainly my question.) All I am saying is, I am more familiar with classical open-chain robotic arm modeling. How should I model the robot in a double supporting phase where it's actually a close-chain robot.

Recommending books or other materials are also appreciated.