The walk is a four time gait, with each leg landing before the next foot leaves the ground. The trot is a two time gait, with diagonal pairs of legs landing and pushing off at the same time, as seen below.
The pace occurs where the legs on one side of the body land and push off at the same time (see below). Dogs can pace in walk or trot (a walk type pace or a trot type pace). Some breeds of dog are more likely to pace than others, for instance labradors and retrievers are dogs with a natural pacing gait. Dogs with long legs and short backs, especially crossbreeds, will pace to prevent the back feet from stepping on or interferring with the front feet in trot.
The pace is an efficiant gait, many owners may observe their dogs pacing towards the end of a long walk, or if the dog have been particularly energetic whilst playing.
The reason a dog will pace when this happens is that when trotting there is considerable rotational movement through the middle of the back. If the muscles here are sore the dog will pace to avoid that rotational movement, stabilising and splinting the centre of the back.
As long as there is no problem with arthritis in the spine or a problem with the vertebral discs then getting your dogs back treated by a specialist, such as a McTimoney therapist, can help relive this discomfort. After treatment the dog is less likely to pace (although there may be an adaptation period while the dog realises that trotting no longer causes discomfort) and the dog will appear to have much more energy.