The study explores the fundamental mechanics of human locomotion, focusing on the differences between walking and running. While running is well explained by a spring-mass model with compliant legs, walking is more complex and cannot be fully explained by a stiff-legged model. Using a simple bipedal spring-mass model, the authors demonstrate 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 also reveals that walking and running are two distinct solutions within a broader parameter space of compliant leg behavior, which can be accessed by energy or speed. This finding suggests that walking and running are not fundamentally different gaits but rather different manifestations of the same mechanical principle, emphasizing the importance of compliant leg behavior in both activities.The study explores the fundamental mechanics of human locomotion, focusing on the differences between walking and running. While running is well explained by a spring-mass model with compliant legs, walking is more complex and cannot be fully explained by a stiff-legged model. Using a simple bipedal spring-mass model, the authors demonstrate 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 also reveals that walking and running are two distinct solutions within a broader parameter space of compliant leg behavior, which can be accessed by energy or speed. This finding suggests that walking and running are not fundamentally different gaits but rather different manifestations of the same mechanical principle, emphasizing the importance of compliant leg behavior in both activities.