(adj.) 1. An animal or being that walks primarily on the heel or ball of a complex foot or paw, as in humans. 2. Term used to describe the human leg and lower body structure, which is much unlike the plantigrade design of most other mammals. Though humans may walk in a digitigrade fasion, it is most common and natural for them to walk on their heels.
Animals that are plantigrade walk in a flat-footed manner. This is a relatively slow way of walking since most of the foot comes in contact with the ground. Most reptiles (like alligators), amphibians (like frogs) and early mammals (plus people and bears) are plantigrade. (Compare with digitigrade.)