I promise I'll come on later and post diagrams and pictures with lines! Really I will! But I've got to go out and clip real horses right now. Yes Jill, we are definitely discussing the slope of the shoulder here, not the chest. The chest and how the neck ties in and all that are all related, but right now according to the topic of this post we are looking strictly at the shoulder.
Remember, horses do not have a shoulder joint like we do. Their scapulas are suspended against the ribcage in a sling of muscles and are designed for one motion only: a gliding rotation on a fore-aft plane. So the structures we must look at to consider what is good or bad conformation in the forehand depends upon how we want the horse to move. There are general guidelines, but what one person may consider a good shoulder because they like a high round movement may not be what someone who desires a horse to reach out long and low would like.
Here, according to Dr. Bennett, are some basic guidelines for what does what.
Of all the joints of the horse's forelimb, only the broad ball-and-socket joint located between the scapula and the humerus is capable of side-to-side movement. This allows the humerus to swing not only from back to front, thus raising or lowering the elbow, but also allows the front limbs to cross each other for lateral movements.
While the shoulder blade governs the entire forelimb, the humerus determines the way in which an individual horse folds and unfolds the elbow, knee, and fetlock joints. Whether long and steep or short and horizontal, the humerus determines the style of a horse's front-end movement in the following ways:
The longer the humerus, the more "scopey" the horse's gait since swinging a long humerous results in a greater arc, or scope, at the elbow end of the bone. Scope is defined as the ability to move the elbow away from the torso, either towards the front, as in a jumper clearing a spread fence, or to the side, as in a dressage horse executing a fluid traversale. Scope enables a cutting horse to crouch in front, spreading or crossing his forelegs as necessary to head the calf. Scope is a very desirable characteristic. It has saved the neck of many a fox hunter, trail rider and three-day eventer, enabling their horses to recover from a mistake after committing to an obstacle....
The shorter the humerus, the more choppy the horse's gaits will be. A horse with a short humerus moves with correspondingly short, stiff strides, has difficulty with lateral work and is frequently dangerous to jump. Choppy-gaited is the opposite of scopey.
The steeper the resting angle of the humerus, the higher the horse can raise his knees. This is of obvious importance in choosing a three-gaited, fine harness or park horse, especially when the owner adheres to a policy of natural trimming and eschews appliances in training and showing. The most spectacular natural action is possessed by horses with the following combination of characteristics: a moderately upright shoulder angle (near 60 degrees); a long, steep humerus; and moderately long cannon bones with high knees.
The same combination of factors produces the hunter-jumper who not only folds nicely but can virtually "put his hooves in his ears" in negotiating a grand-prix course. The only difference between the successful jumper and the three-gaited horse is that the jumper needs to have short cannon bones, low knees and substantial bone to preserve soundness through repeated landings.
The more horizontal the resting angle of the humerus, the less natural ability a horse will have for high action or tight folding. A horizontally orientated humerus guarantees that the horse will be a "grass clipper" -- when asked to jump, he will have difficulty raising his forearms to level and may hang his knees.
...To be considered long, the humerus must be at least 50% as long as the shoulder blade. Then look at the resting angle of the humerus. Ideally, the angle between the line of the humerus and the line of the shoulder will be wider than 90 degrees.
And there you have it according to one person.
Leia