A dog's front assembly determines the amount of reach he will have when moving. It also determines -- at least partially -- how clean, correct and efficent his front movement will be. (Front movement is also determined by length of body and the similarity of the dog's front and rear angulation.)
When going over the dog's front assembly, one looks for the following characteristics (among others):
Length of upper arm or humerus: As you can see from the diagram at left, the dog's upper arm (humerus), should actually be slightly longer than the shoulder blade (scapula). Most people assess length of upper arm by using three reference points that can be easily located (even when covered with skin and hair). These reference points are the top of the dog's shoulder (point A in the diagram at left), his point of shoulder (B) and the tip of his elbow (C). Putting your thumb on B, you can reach the top of the shoulder (A) with one of your other fingers. Keeping your hand in the same position (keeping the same spread between thumb and finger), swing down to the tip of the elbow (C). The second distance measured -- B to C -- is somewhat shorter than the true length of the upper arm; but if the upper arm is long enough, the distance from B to A should be the same as the distance from B to C. If the distance from B to C is shorter than the distance from B to A, the upper arm is too short. This common structural problem can result in less reach when moving (an inability to swing the leg forward the optimum amount in order to cover the most ground). It can also result in inefficent movement, such as lifting the front feet too far off the ground with each stride (the front feet should just clear the ground as the dog moves -- anything more is wasted motion).
Layback of shoulder: When going over a dog's front, most people estimate shoulder layback by placing the thumb at the point of shoulder (point B in the diagram) and one finger at the top of the dog's shoulder (point A) and figuring the angle of that line (B-A) off the vertical (line E-B). Using this method, you can see that the dog in the diagram has a shoulder layback of 30 degrees. Many standards, including the Sheltie standard, call for a shoulder layback of 45 degrees. This angle was derived from what the standard-writers knew about horse anatomy, where well-laidback shoulders make for a smoother ride. Unfortunately, good front assemblies in dogs and horses are very different. The front assembly a rigid-backed horse needs for optimal gait (the comfort of a rider being the primary consideration) has little relationship to the correct front assembly in a dog, which has a flexible back and is not ridden. Research has shown that the best-moving trotting dogs have an average shoulder layback of 28 degrees, plus or minus 5 degrees. Galloping dogs (sighthounds) have considerably steeper shoulders (more like 15-20 degrees), while diggers (such as Dachshunds) have greater layback than trotters. Shelties are trotting dogs.
Insufficient layback -- sometimes referred to as "a straight shoulder" -- is a widespread problem in Shelties. When moving, dogs with insufficient layback lack reach. This can lead to other movement faults, especially if the rear is better angulated than the front -- a common problem -- causing the drive from the rear to be better than the reach in the front. Such dogs will have a longer stride in the rear than in the front and will have to find a way to compensate. One possible compensation is to lift the front feet too far off the ground with each stride. This is wasted motion, but it enables such a dog to "get out of his own way." The dog may also fail to single-track (feet converge toward a center line) in front, so that his rear feet -- which are single-tracking with good, long strides -- can swing between his lifting, non-single-tracking, short-strided front feet. (Some people call this type of a movement a "reverse tricycle" -- i.e. two "wheels" in front, one behind.)