Colloquia

Newtonian physics implies that running is impossible for sufficiently large animals. There are two main factors that influence this:

1. An animal's strength/weight ratio decreases with size, hence a sufficiently large animal will be liable to injury if it attempts a gallop.

2. The time required for an animal to move its limbs increases with size, but the time an animal can remain in the air (while running) does not scale with linear dimension. Therefore there is some size beyond which an animal has "run out of time" and cannot take advantage of a running gait. These aspects of the biomechanics of locomotion bear on the interesting questions of determining the speeds of extinct species, as well as how varying gravity affects locomotion.