The shoulder consists of the sequence of the blade bone (scapula) and arm (humerus), which join at the "tip of the shoulder': the glen humeral joint. The third point of reference of the shoulder triangle is the elbow corresponding to the end of the ulna (olecrane)
The slope of the shoulder is measured by the angle with the vertical axis of the scapula that is the line that goes from the top of the withers to the tip of the shoulder. This inclination is observed with a horse staying on its 4 members, on flat ground and with the front canon bones vertical (fig.8).
This can be done with a compass of carpenter spotting the two axes, or with two pieces of adhesive tape and a protractor.
It is generally accepted, that an inclined shoulder better predisposes to the natural jump aptitude than a straight shoulder. Even though this rule has exceptions (Deister, Razzia du Poncel...), it can be considered.
The slope of the shoulder will facilitate the movement towards the front of the scapulohumeral join when jumping. The angle formed at this joint by the shoulder blade and the arm is generally included between 90 and 100 °. The higher it is, the more this corner is open, more the arm will be vertical. By closing over the jump, this joint plays an important role in the passage of the fore hand.
The longer are these bones, the more effective and powerfull the lever arm will be (at equal muscle tone). Unlike humans, the horse has no collarbones connecting the shoulder blades, chest and forelimbs: They are connected to the trunk by muscles.
We may think that the size of the shoulder guarantees the solidity of this link, which is particularly important at the time of take-off for jumping.
We therefore think that a large, inclined with a rather open front corner shoulder is an advantage. The limit of this reasoning is that the larger is the shoulder, the higher is the weight of the front end and the more is increased the risk of balance issues.
It is still a matter of interaction: a developed shoulder is a quality for a jumper up to a point. The shoulders of the quarter-horses, hyper developed muscles due to what they are told to, are not compatible with the high jump.
Similarly, if the distance between the tips of the shoulders (chest width) was a power token for draft horses, it can be a disadvantage if it brings too much weight on the front-hand. In this matter, a thin and toned shoulder musculature is preferable to a too thick musculature: Quality is better than quantity!
It is at work and interacting with other criteria that we can appreciate the quality of the shoulder. Its level of looseness, that is how the tip of the shoulder comes forward, in particular at walk and canter, can compensate an excessive verticality.
The form, the flexibility and the strength of the back may also interact positively or negatively. The downhill silhouette can reinforce the perverse effects of a too large shoulder. An uphill horse with a good hindquarters may more easily offset the disadvantages of a small shoulder. Between the small shoulder of Landgraaf offsprings and the powerful shoulder of the heirs of Ibrahim, this is the breeder own decision to appreciate and choose...