Progressive stiffness has been proposed as a solution to the common trade-off in the design of series elastic actuators: higher torque-tracking performance, achieved by stiffer springs, or safer physical interaction with the environment and enhanced torque transparency, offered by softer springs. In this work, we investigate the application of series elastic actuators with stiffening characteristic in the context of dynamic legged locomotion. A planar one-legged robot with an articulated leg is modeled and simulated using a multi-body dynamics framework, with a control strategy based on the Spring-Loaded Inverted Pendulum template. The actuators incorporate compact, monolithic compliant mechanisms based on flexure beams that exhibit a progressive torque deflection characteristic. The design of these mechanisms is parameterized in terms of length, thickness, and number of beams, and then optimized with the goal of reducing both the joints’ torque requirements and the mechanical cost of transport during a running task. Simulations results show that, compared to stiff actuation and linear series elastic actuation configurations, properly tailored nonlinear stiffness profiles can improve actuation efficiency and reduce energy requirements in dynamic legged locomotion.