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.
The saving grace is that horses with a horizontal hip, if it's not excessively short, have plenty of space for strong, supportive muscling through the loins.
A deeply sloped hip, combined with a high stifle, will result in a closed angle. If the horse also possesses long hind legs, he'll perform a low, long, sweeping stride in back that resembles a walk. This, combined with the more open angle described for the forequarters, indicates a horse that can perform a running walk.
A horizontally oriented hip, combined with a lower stifle, results in an open angle, giving the horse more lift than length of stride behind. Combine this type of hindquarter with a closed angle in the forequarters, and you're likely looking at a horse with a fox trot type of gait.
Again, if the angle between the hip and thigh is in the 90-degree range, then the length and height of stride will be moderate. Similar angles in front would lead one to suspect the horse will perform a nice, easy, racking gait.
Your Horse's Timing
The gait a horse prefers depends also on whether he tends to be diagonal, square, or laterally oriented. In general, how a horse is coupled does much to determine his natural, "raw" orientation.
However, this will become less a consideration as the rider consistently "works the walk" to help develop the gait with good timing, in proper form. (Good timing is anything between a slightly lateral rack/running walk to the fox trot. Pace and step pace gaits are destructive to a horse's body, and therefore should be avoided.)
You can determine whether your horse is loosely or tightly coupled by running your hand down his spine and marking the last vertebra that you can feel with your fingers. Now, with the flat edge of your fingers pressing firmly, run your hand back behind this point until you feel a "shelf," where his loin joins into his back. This is the lumbosacral (L-S) joint.
If the L-S area is about three inches (on an average-sized horse), then your horse is likely to be very square. This means he can move in both pace and trot form, or he'll consistently move in a regular, evenly timed four-beat gait. If it's the former, you'll have an easy job of developing a smooth, four-beat gait in good form. Either way, he's a strongly natural gaited animal.
If this space is greater than three inches, then your horse is loosely coupled and likely to be lateral (pacey), or move with a lot of "swing." Such horses tend to move in a level or hollowed-out frame. Collection is a challenge; therefore, they tend to go in a front-heavy fashion.
However, it's entirely possible to condition most of these horses to become acceptably square and lighter-moving animals - and important for their long-term health to do so. A horse that's very loosely coupled will usually move slightly on the lateral in a rack or running walk gait.
If the L-S area is less than three inches, then the horse is said to be tightly coupled. Sometimes, the area is so densely muscled that it's impossible to feel the L-S joint. This usually indicates a tightly coupled horse, as well. Tightly coupled horses tend to be diagonally oriented, regardless of the gait they may perform.
Back, Neck, and Head
While your horse's back, neck and head don't directly determine his gait, they do influence it in various ways. A short back results in more overstride, because there's less real estate that the rear legs need to cover.
Your horse's head and neck affect his ability to give to your hand, how much natural head nod/shake is expressed, and how light he is on his forehand. For example, a horse with a short-to-medium back and neck will express more head nod or shake than a rangier-built horse.
Your horse uses his head and neck for balance as he takes a forward stride. The shorter his body and neck, the deeper he has to nod to achieve that balance. A horse with a high headset will tend to shake, rather than nod, his head.
In general, the more laid back the shoulder, the higher the headset. The fox trotter's laid-back shoulder, therefore, contributes both to a longer front-stride length and the gait's signature head shake.