Extention of the Gaits
The USDF Glossary of Judging Terms describes the extention of all gaits as “stretching and lengthening of the outline and stride of the horse, and in trot and canter, an increased phase of suspension. The horse covers as much ground as possible with each stride maintaining nearly the same tempo and relatively uphill balance.
In walk, a pace which shows the maximum length of the stride and stretch and oscillation of the neck. The hind feet touch the ground clearly in front of the prints of the forefeet.”
Suspension is the period of time when none of the limbs are in contact with the ground. This is also the aerial phase. The canter and gallop have one aerial phase per stride, the trot has two per stride and the walk does not have any aerial phase.