Spring-like leg behavior was found in the global dynamics of human and animal running in sagittal plane. The corresponding template model, the conservative spring-loaded inverted pendulum (SLIP), shows stability for a large range of speeds and is, therefore, a promising concept for the design of legged robots. However, an anchoring of this template is needed in order to provide functions of biological structures (e.g., mass distribution, leg design) and engineers’ details for construction. We extend the SLIP template model towards two new models that we call M-SLIP and BM-SLIP by adding considerable leg masses to investigate the influence of leg rotation on running stability. Our study clearly reveals that the spring-loaded inverted pendulum can be anchored in a leg mass model. This supports model- and simulation-driven engineering towards robotic behavior inspired from biological systems.