Surface stiffness plays an important role in human locomotion mechanics. This would affect both the energy expenditure and gait of the human. These effects have been numerically investigated for runners. Such effects, however, have not been fundamentally addressed for other locomotion scenarios such as walking or running. Towards this goal, we designed and developed a novel Treadmill with Adjustable Stiffness (TwAS) with the Ability to Regulate the Vertical Stiffness of the Ground. The novelty of the system is on its stiffness adjustment mechanism which allows the “vertical” stiffness of the surface to change quickly (less than 0.5 second) from completely passive (i.e. theoretically zero stiffness) to extremely rigid (i.e. theoretically infinite stiffness) with minimum energy consumption, independent of the location of the person over the treadmill. The design also allows for bilateral surface stiffness regulation (i.e. both legs, independently) that is an extremely helpful criterium in studying the locomotion mechanics and eventually gaining valuable insights into best rehabilitation strategies of mobility impaired patients. In order to show the proof of concept, we are conducting experiments to show the effect of surface stiffness regulation on the metabolic cost and gait of a healthy subject.