In the extended trot the horse reaches as far under the belly as he can with
his hind legs and brings the forelegs correspondingly forward. However,
should the foreleg reach beyond the nose before touching the ground, he
would have to withdraw the leg to touch down. This is incorrect, and is
called "flinging" the front legs - it usually indicates that the hindlegs
are not bearing weight and pushing off.
There should not be anymore suspension in the extended trot that there is in
the normal trot; the strides simply get longer (as should the entire frame
of the horse). When judging dressage, we look for the tempo and rhthym to
stay the same.
The trot is indeed a "gait of choice" by all riding disciplines up to a
point. The reason is that because a front and a back leg are always on the
ground, the horse is in "balance" (that being a relative term) with a rider.
It is considered by some to be a Man-made gait, since in the field the horse
prefers to walk or canter. Because humans have a hard time learning to move
correctly with the canter, that gait can be more difficult for the horse to
balance.
However, the trot done for long periods of time becomes a back-stiffening
gait, and thus the horse can develop a sore back. Even fatigue in the back
will cause the horse to stiffen and this in turn causes him to block the
hindquarters. The canter can keep the back loose, and in fact the canter
will improve the trot, but the trot will never improve the canter.
Beverley Fields