1. Plantigrade: heavy animals usually put their heels down to support their weight.
2. Unguligrade: large animals with hooves.
3. Digitigrade is usually reserved for lighter animals. It means to walk on the toes.
Most heavy animals walk on four legs. There are some exceptions. Some really heavy birds move or moved on two legs. Moas are one example. It is quite clear that an ostrich is a very effective bird running on two legs.
The study of such things is called "comparative foot morphology".
- Griffin T.M; Main R.P; Farley C.T. 2004. Biomechanics of quadrupedal walking: how do four-legged animals achieve inverted pendulum-like movements?. J Exp Biol. 207 (20): 3545–3558. doi:10.1242/jeb.01177. PMID 15339951
- Weissengruber G.E; & Forstenpointer G. 2004. Shock absorbers and more: design principles of the lower hind limbs in African elephants (Loxodonta Africana). J. Morphology 260: 339.