You will find some old equitation texts, some as recent as the 1980s, that state that "gait" in horses is a sign of lameness. Those texts are wrong. But they are still out there and some folks still believe it.
In fact there are some lameness's that will
cause a trotter to "gait." One of the jobs of a vet. Is to distinguish between lameness and soundness. If there's a question get an evaluation. It's likely the "gait indicates lameness" error comes from poor diagnosis from either vets or laymen.
The more lateral a gait the higher the energy level of the horse must be to maintain it.
Here’s a test, first suggested by Dr. Deb Bennett, to demonstrate the relative energy demands of a lateral vs. diagonal gait.
On a carpeted floor (for human comfort) get down on all fours. Now “walk” using the same footfall pattern as the horse. Do this for a minute or two to get the “feel” for the movement. Note the energy level you are required to expend.
Now “trot” using diagonal pairs. Note the effort level involved. Note that when you are on a lateral pair you have natural “balance” and don’t need to do much to maintain it. Don’t try for a moment of suspension; that won’t be necessary for this experiment!!!
Now “pace” using lateral pairs. Note the effort involved. Note that when you are on a lateral pair you must intentionally shift your weight to that pair or you’d fall over. Note that you are clearly expending more energy than you did at the “trot.”
Is the extra energy required “bad?” No, it’s not. It’s just gravity being balanced by equine biomechanics. Gravity is not just a Good Idea, it’s the Law. So we have to deal with it. In a trotting horse, with the moment of suspension, we deal with gravity by either posting or properly sitting the trot. The rider’s body does the work. In the pace the horse’s body does the work so the rider’s body doesn’t have to. This means the rider must pay particular attention to the strength, fitness, and soundness of the horse’s body.
The Good News is that most gaited breeds have been selected for conformations and temperaments that permit the horse to effectively use their body. The person that tries to teach a trotter to gait is asking for something Nature did not intend. That thing will demand the horse expend energy in ways its conformation was not designed to deliver. Sometimes it can work, but it’s really putting the horse to the test.
On the other hand asking a gaited horse to trot is the mirror image of the problem. Here you’re asking the horse to perform a much lower energy gait. This still has risks as the conformation of the gaited horse might not be well suited to the “moment of suspension” the trot demands. Done for a narrow purpose (i.e., helping a very laterally gaited horse learn to canter) it’s not necessarily a bad thing. Over the long haul I’m not so sure it’s a good idea.
Because a human is not a horse this test is not perfect. It does, however, effectively demonstrate some aspects of the differences between the trot and a gait. A more diagonal gait is easier on the horse but more demanding on the rider. The mirror image is also true. This suggests that first thing a gaited rider must do to improve gait is to improve the strength and fitness level of the horse (not alter the hoof angles, use a different bit, add an action device, etc.).