The main balance mechanism in legged systems is the control of the Center of Pressure (COP) over the Base of Support (BOS). The BOS dimensions are therefore strictly related to the control authority of the COP and are typically associated to foot dimensions. This geometrical BOS dimension is valid for the case of ideal rigid flat foot but does not represent the truly available range for COP control, hence the effective BOS, when a compliant foot-ground contact is modeled. This work presents experimental observations on the maximum COP range (i.e., effective BOS) for a subject perturbing its standing balance on rigid and soft grounds. A legged system with a viscoelastic foot contact is modeled to illustrate how a given foot model impacts the effective BOS and can lead to different prediction on the system's stability, quantified by existing margin of balance. This work motivates the need for novel methodologies for balance and falling prediction, currently focused on ideal rigid feet, in which the effects of a viscoelastic contact on the control of the COP are considered.