A biped is an animal that travels across surfaces supported by two legs.
Bipedal locomotion is walking, running, and standing on two legs (as opposed to four). It is a process requiring complex interaction of mechanical and control-system characteristics. Humans are capable of performing such motion because their spines are s-curved and their heels are round. Whether such a system has evolved in an animal species or been engineered in a robot, many of the same issues are inevitably involved.
Energy-efficient means of standing bipedally involve constant adjustment of balance, and of course these must avoid overcorrection.
Efficient walking complicates these issues, as it entails tipping slightly off-balance forward and to the side, and correcting balance with the right timing.
Running is an inherently continuous process, in contrast to walking; a bipedal creature or device, when efficiently running, is in a constant state of falling forward, that is maintained as relatively smooth motion only by repeatedly "catching oneself" with, again, the right timing, but in the case of running only delaying the nearly inevitable fall for the duration of another step.
The phenomenon of "tripping" is also informative in this regard. One popular way to think of it is as having one's leg pulled out from under them. In fact, however, merely stopping the movement of one leg of a walker, and merely slowing one leg of a runner, is sufficient to amount to tripping them. They were already falling, and preventing the tripped leg from aborting that fall is sufficient to literally "drop them like a sack of dirt".
Engineers who study bipedal walking describe it as a repeatedly interrupted fall.
Animals and humans
Many animals, including humans, have evolved bipedalism, with anatomical adaptations constituting the required mechanical systems and neurological adaptations the control-system ones...............
