Human walking has been described by spring model recently. It has been verified that the stiffness of the spring model increases as the walking speed increases. However, there has been no study on the physiological meaning or the reason to the stiffness increase of the spring model as the waking speed increases. In this study, we will investigate how the stiffness increase of spring model represents the behavior of human leg and joint stiffness. We established that the increase in joint stiffness makes the leg stiffness to increase for leg segment model, especially knee joint stiffness. To verify the leg stiffness change, the joint stiffness change of human should be examined. If the result shows that the increase of the joint stiffness as walking speed to increase, this feature can be used to make the walking assistance system using the joint stiffness variation to the walking speed.