One of the biggest challenges of humanoid robots is to keep the balance at any moment, as they can be subject to different types of external perturbations. Therefore, push recovery is a relevant issue in humanoid robotics, which still represents an open challenge. In literature, the most used method is based on the capture point, where a reduced model of the robot is used to compute recovery motions. In this paper we want to explore the problem with a different approach, by using whole-body models combined with optimal control. The robot is subject to an external perturbation, which is taken into account in the system dynamics as continuous external force. Optimal control is used to generate whole-body recovery motions by using the whole-body dynamic model of the robot. Instead of the capture point criterion, we optimize for a stable motion that allows to perform recovery within one step, at the end of which the robot is in a state of zero velocity. We apply the method to the model of the humanoid robot HeiCub, where serial springs can be mounted in the knee and ankle pitch actuators to change them into Series Elastic Actuators (SEA). The recovery motion is computed for both the model with and without SEA.