The important message to take from this article is that a robot is a unified dynamic system comprising electronics, software, and mechanical components, and for certain tasks such as running, a significant portion of the behavior is best exhibited through natural dynamics of the mechanism. Therefore, the mechanical system must be specialized for the task and designed with the same care for dynamic control as the software control system. In constructing the ECD leg, we have attempted to follow this philosophy and design the mechanical system for the specific tasks of walking and running. The prototype actuator, with dynamics verified by testing, exhibited behavior that enabled running in simulation. The ECD leg builds on design revisions from the BiMASC prototype, and the successful experiments with Thumper hopping around the laboratory have proven the ideas and engineering behind the design. Prof. Grizzle's group at the University of Michigan has already demonstrated tentative walking with MABEL, and we expect to demonstrate robust and efficient walking and running gaits in the near future.