When gaited-horse owners purchase a particular breed, they tend to expect the horse to naturally perform the gait considered standard for that breed. We assume, for example, that a Tennessee Walking Horse will perform a correct, natural running walk, and a Missouri Fox Trotting Horse will execute an easy fox trot.
This doesn't necessarily hold true. The gait that the horse will perform naturally isn't based on his breed, but rather is determined by his conformation, as I'll explain here. Like all living creatures, function follows form.
It's imperative to consider your horse's conformation before deciding which gait he "should" perform under saddle. It'll be easier for you, and far better for your horse, to work with his natural abilities than to try to "make the gait" by anything other than safe, natural, easy-to-master fitting and riding techniques.
Note: While I refer to the gaits in general terms, these principles apply to all kinds of gaited horses. (For a description of each gait mentioned, see "Gaits Made Simple," Smooth Talk, September/October '06.)
A horse's shoulder and arm work together to determine the motion of his forequarters. His shoulder is measured from the front edge of his scapula, just in front of the withers, down to his point-of-shoulder. (See Photo #1, Line #1.) (On a photo of your horse, you can mark these spots with a grease pencil, then draw a line between them.) His arm is measured from the point-of-shoulder to the point-of-elbow. (See Photo #1, Line #2.)
The arm's angle in relation to the shoulder helps determine how much lift there will be in front. An open angle indicates a high lifting step, while a closed angle indicates a low stride. If you can imagine a protractor open to various angles, you can see how an open angle provides more room for a horse's leg to lift.
If you extend an imaginary line down the line of the shoulder to the ground (see Photo #1, Line #3), you'll see how long a horse's length of stride will be. A horse with a deeply sloped shoulder will have a greater length of stride than one with a steep, upright shoulder.
A horse's arm length is also an important consideration; a horse with a long, vertically oriented arm will have greater scope, or range of motion, and less concussion to the leg, than a horse possessing a short, horizontally oriented arm.
A long, laid-back shoulder combined with a horizontal arm points to a horse with a long, low, sweeping stride - the ideal fox trotting "walk in front" look. A steep, upright shoulder, combined with a long, vertically oriented arm, results in a horse with a shorter length stride, but with lots of lift and "rolling" action. This is the typical "trot in front" look of a running walk.
If a horse has approximately 45-degree angles at both the shoulder and arm, resulting in a 90-degree angle between them, his stride will have average length and lift.
The rule of thumb regarding open and closed angles in the forequarters holds equally true for a horse's hindquarters; that is, an open angle means greater lift of stride, while a closed angle suggests a lower, "walking" type of stride.
The hip is measured from the top point-of-hip to the point-of-buttock. (The true point-of-buttock is determined by marking the outermost point of the horse's buttock.) The thigh runs from the point-of-buttock to the stifle joint, which is the uppermost joint of the horse's back leg. An indentation in the flesh indicates where the stifle joint is located.
A long, deeply sloped hip suggests that a horse can drop down his hind end far enough to carry his weight over his hindquarters, round up through his loins and back, and collect on the bridle.
On the other hand, a horse with a very horizontal hip - especially if it's relatively short and/or higher than his withers - will limit this ability. If this is the case with your horse, work diligently to help him attain good form while working in gait to avoid him eventually becoming swaybacked or otherwise unsound.