Quadrupedalism

Quadrupedalism is a form of terrestrial locomotion where a tetrapod animal uses all four limbs (legs) to bear weight, walk, and run. An animal or machine that usually maintains a four-legged posture and moves using all four limbs is said to be a quadruped (from Latin "quattuor" for "four", and "pes", "pedis" for "foot"). Most quadrupeds are terrestrial vertebrates, including mammals and reptiles, though some are largely aquatic such as turtles, amphibians, and pinnipeds.
Bipedal tetrapods such as some birds (such as the shoebill) sometimes use their wings to right themselves after lunging at prey.
Quadrupeds vs. tetrapods.
Although the words "quadruped" and "tetrapod" are both derived from terms meaning "four-footed", they have distinct meanings. A tetrapod is any member of the taxonomic unit Tetrapoda (which is defined by descent from a specific four-limbed ancestor), whereas a quadruped actually uses four limbs for locomotion. Not all tetrapods are quadrupeds and not all quadrupeds are tetrapods.
The distinction between quadrupeds and tetrapods is important in evolutionary biology, particularly in the context of tetrapods whose limbs have adapted to other roles (e.g., hands in the case of humans, wings in the case of birds, and fins in the case of whales). All of these animals are tetrapods, but none is a quadruped. Even snakes, whose limbs have become vestigial or lost entirely, are nevertheless tetrapods.
Most quadrupedal animals are tetrapods, but with a few exceptions. For instance, among the insects, the praying mantis is a quadruped.
In humans.
In July 2005, in rural Turkey, scientists discovered five Kurdish siblings who had learned to walk naturally on their hands and feet. Unlike chimpanzees, which ambulate on their knuckles, the Kurdish siblings walked on their palms, allowing them to preserve the dexterity of their fingers.
Many people, especially practitioners of parkour and freerunning and Georges Hébert's natural method, find benefit in quadrupedal movements to build full body strength. Kenichi Ito is a Japanese man famous for speed running on four limbs. Quadrupedalism is sometimes referred to as being on all fours, and is observed in crawling, especially by infants.
Quadrupedal robots.
BigDog is a dynamically stable quadruped robot created in 2005 by Boston Dynamics with Foster-Miller, the NASA Jet Propulsion Laboratory, and the Harvard University Concord Field Station.
Also by NASA JPL, in collaboration with University of California, Santa Barbara Robotics Lab, is RoboSimian, with emphasis on stability and deliberation. It has been demonstrated at the DARPA Robotics Challenge.
Pronograde posture.
A related concept to quadrupedalism is pronogrady, or having a horizontal posture of the trunk. Although nearly all quadrupedal animals are pronograde, bipedal animals also have that posture, including many living birds and extinct dinosaurs.
Nonhuman apes with orthograde (vertical) backs may walk quadrupedally in what is called knuckle-walking.