A bipedal spring-mass model with compliant legs explains the basic dynamics of walking and running. The model, which includes two massless springs representing legs, shows that compliant legs are essential for reproducing the characteristic stance dynamics of walking, including small vertical oscillations and out-of-phase changes in kinetic and potential energies. The model incorporates double support, a key feature of walking, and reproduces the observed patterns of ground reaction forces (GRF) and COM motion. It also reveals that walking and running are two of many possible solutions to legged locomotion, depending on energy or speed. The model shows that walking occurs at lower energies and slower speeds, while running requires higher energy and speed. The model's parameter space includes three sub-domains corresponding to different walking patterns, with variations in symmetry and amplitude reflecting differences in speed. The model also demonstrates that walking and running can be unified in one mechanical framework, with running occurring at higher energies and speeds. The study challenges the notion that walking efficiency depends on COM motion resembling an inverted pendulum, showing that energy storage in compliant legs during double support is crucial for efficient walking. The model provides a simple yet effective framework for understanding legged locomotion, combining bipedalism and leg compliance to explain both walking and running mechanics.A bipedal spring-mass model with compliant legs explains the basic dynamics of walking and running. The model, which includes two massless springs representing legs, shows that compliant legs are essential for reproducing the characteristic stance dynamics of walking, including small vertical oscillations and out-of-phase changes in kinetic and potential energies. The model incorporates double support, a key feature of walking, and reproduces the observed patterns of ground reaction forces (GRF) and COM motion. It also reveals that walking and running are two of many possible solutions to legged locomotion, depending on energy or speed. The model shows that walking occurs at lower energies and slower speeds, while running requires higher energy and speed. The model's parameter space includes three sub-domains corresponding to different walking patterns, with variations in symmetry and amplitude reflecting differences in speed. The model also demonstrates that walking and running can be unified in one mechanical framework, with running occurring at higher energies and speeds. The study challenges the notion that walking efficiency depends on COM motion resembling an inverted pendulum, showing that energy storage in compliant legs during double support is crucial for efficient walking. The model provides a simple yet effective framework for understanding legged locomotion, combining bipedalism and leg compliance to explain both walking and running mechanics.