leg is to provide a device that can provide the functionality of an entire leg structure using only a single spring. This allows for the most efficient use of energy and the least loss of energy. The typical losses in a legged system are negative work and leg sweep. Negative work happens when an actuator applies force opposite from its motion and absorbs energy from the system accidentally. This can be eliminated by a design that avoids articulation altogether. Leg sweep losses come from the need to accelerate the foot to match the speed of the ground moving past it, and it is kept minimal by using a very light leg that does not hold much inertia that needs to be fought against. Terrain Complications The process involved in planning the device's ability to move around is very limited by the available information regarding terrain. In many of the "bow leg" experiments that are conducted (including the ones referenced here), the planner is assuming that they already know both the geometry and friction coefficient of the terrain. In actual practice, either of these vital factors can be incorrectly measured or guessed at, causing many issues in the device's ability to traverse the terrain. However, only small parts of the terrain ever come into contact with the foot, therefore the necessary calculations are minimized, and false positives can be accepted in certain scenarios. The issues are effectively doubled when it is necessary to take into consideration the possibility of an obstacle above the device, as well as the ones that it must traverse below. References Robot legs