Soft robots join body and actuation, forming their structure from the same elements that induce motion. Soft actuators are commonly modeled or characterized as primary movers, but their second role as support structure introduces strain-pressure combinations outside of normal actuation. This manuscript examines a more complete set of possible strain-pressure combinations for McKibben actuators, including passive, or unpressurized, deformation, pressurized extension and compression of a pressurized actuator beyond the maximum actuation strain. Each region is investigated experimentally, and empirical force-displacement-pressure relationships are identified. Particular focus is placed on ensuring empirical relationships are consistent at boundaries between an actuator's strain-pressure regions. The presented methodology is applied to seven McKibben actuator designs, which span variations in wall thickness, enclosure material and actuator diameter. Empirical results demonstrate a trade-off between maximum contraction strain and force required to passively extend. The results also show that stiffer elastomers require an extreme increase in pressure to contract without a compensatory increase in maximum achieved force. Empirical force-displacement-pressure models were developed for each variant across all the studied strain-pressure regions, enabling future design variation studies for soft robots that use actuators as structures.