  Legged locomotion has received increasing attention from the robotics
community. In this respect, contact stability plays a critical role in ensuring
that robots maintain balance, and it is a key element for balancing and walking
controllers. The Center of Pressure is a contact stability criterion that
defines a point that must be kept strictly inside the support polygon in order
to ensure postural stability. In this paper, we introduce the concept of the
sensitivity of the static center of pressure: roughly speaking, the rate of
change of the center of pressure with respect to the system equilibrium
configurations. This new concept can be used as an additional criterion to
assess the robustness of the contact stability. We show how the sensitivity of
the center of pressure can also be used as a metric to assess balancing
controllers by considering two state-of-the-art control strategies. The
analytical analysis is performed on a simplified model, and validated during
balancing tasks on the iCub humanoid robot.
